Pleasures of the telescope
by Garrett Putman Serviss · English · Gutenberg #28752 · 326,318 chars · public domain (US)
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Title: Pleasures of the telescope
Author: Garrett Putman Serviss
Release date: May 10, 2009 [eBook #28752]
Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK PLEASURES OF THE TELESCOPE ***
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PLEASURES OF THE
TELESCOPE
AN ILLUSTRATED GUIDE FOR AMATEUR ASTRONOMERS
AND A POPULAR DESCRIPTION OF THE CHIEF
WONDERS OF THE HEAVENS FOR
GENERAL READERS
BY
GARRETT P. SERVISS
AUTHOR OF ASTRONOMY WITH AN OPERA-GLASS
"This being made, He yearned for worlds to make
From other chaos out beyond our night--
For to create is still God's prime delight.
The large moon, all alone, sailed her dark lake,
And the first tides were moving to her might;
Then Darkness trembled, and began to quake
Big with the birth of stars, and when He spake
A million worlds leapt into radiant light."
LLOYD MIFFLIN.
_WITH MANY ILLUSTRATIONS_
NEW YORK D. APPLETON AND COMPANY 1901
COPYRIGHT, 1901,
BY D. APPLETON AND COMPANY.
PREFACE
By the introduction of a complete series of star maps, drawn specially
for the use of the amateur and distributed through the body of the work,
thus facilitating consultation, it is believed that this book makes a
step in advance of its predecessors. The maps show all of the stars
visible to the naked eye in the regions of sky represented, and, in
addition, some stars that can only be seen with optical aid. The latter
have been placed in the maps as guide posts in the telescopic field to
assist those who are searching for faint and inconspicuous objects
referred to in the text. As the book was not written for those who
possess the equipment of an observatory, with telescopes driven by
clockwork and provided with graduated circles, right ascensions and
declinations are not given. All of the telescopic phenomena described
are, however, represented in the maps. Star clusters are indicated by a
conventional symbol, and nebulæ by a little white circle; while a small
cross serves to mark the places where notable new stars have appeared.
The relative magnitudes of the stars are approximately shown by the
dimensions of their symbols in the maps, the smaller stars being
represented by white dots and the larger by star-shaped figures.
In regard to binary stars, it should be remembered that, in many cases,
their distances and angles of position change so rapidly that any
statement concerning them remains valid only for a few years at the
most. There is also much confusion among the measurements announced by
different authorities. In general, the most recent measurements
obtainable in 1900 are given in the text, but the observer who wishes to
study close and rapid binaries will do well to revise his information
about them as frequently as possible. An excellent list of double stars
kept up to date, will be found in the annual Companion to the
Observatory, published in London.
In the lunar charts the plan of inserting the names of the principal
formations has been preferred to that usually followed, of indicating
them only by numbers, accompanied by a key list. Even in the most
detailed charts of the moon only a part of what is visible with
telescopes can be shown, and the representation, at best, must be merely
approximate. It is simply a question of what to include and what to
omit; and in the present case the probable needs of the amateur observer
have governed the selection--readiness and convenience of reference
being the chief aim.
It should, perhaps, be said here that the various chapters composing
this book--like those of "Astronomy with an Opera-glass"--were, in their
original form, with the single exception of Chapter IX, published in
Appletons' Popular Science Monthly. The author, it is needless to say,
was much gratified by the expressed wish of many readers that these
scattered papers should be revised and collected in a more permanent
form. As bearing upon the general subject of the book, a chapter has
been added, at the end, treating on the question of the existence of
planets among the stars. This also first appeared in the periodical
above mentioned.
In conclusion, the author wishes for his readers as great a pleasure in
the use of the telescope as he himself has enjoyed.
G. P. S.
BOROUGH OF BROOKLYN, NEW YORK, _January, 1901_.
CONTENTS
CHAPTER I PAGE
THE SELECTION AND TESTING OF A GLASS 1
How to get a good telescope--Difference between reflectors and
refractors--How a telescope is made achromatic--The way to test
a telescope on stars.
CHAPTER II
IN THE STARRY HEAVENS 19
Orion and its wonders, Lepus, Canis Major, Argo, Monoceros,
Canis Minor, and the Head of Hydra.
CHAPTER III
FROM GEMINI TO LEO AND ROUND ABOUT 38
The zodiacal constellations Gemini, Cancer, and Leo, and their
neighbors Auriga, the Lynx, Hydra, Sextans, and Coma Berenices.
CHAPTER IV
VIRGO AND HER NEIGHBORS 57
Crater and Corvus, Hydra, Virgo, the "Field of the Nebulæ,"
Libra, Boötes, and the great Arcturus, Canes Venatici, and
Corona Borealis.
CHAPTER V
IN SUMMER STAR-LANDS 75
Scorpio and its red-green gem, Ophiuchus, Sagittarius, Scutum
Sobieskii, Capricornus, Serpens, Hercules, Draco, Aquila, and
Delphinus.
CHAPTER VI
FROM LYRA TO ERIDANUS 97
Lyra and its brilliant Vega, Cygnus, Vulpecula, Aquarius,
Equuleus, Pegasus, Cetus, and Eridanus.
CHAPTER VII
PISCES, ARIES, TAURUS, AND THE NORTHERN MARS 117
The first double star ever discovered, the Pleiades and their
photographic wonders, the Royal Family of the Sky, Andromeda,
Cassiopeia, Perseus and Cepheus, Ursa Major, Camelopardalus,
Ursa Minor, and the Pole Star.
CHAPTER VIII
SCENES ON THE PLANETS 139
Jupiter, its belts and its moons--Saturn, the ringed
planet--Saturn's moons and Roche's limit--Mars and its white
polar caps and so-called seas and continents--Venus and her
atmosphere--The peculiar rotations of Venus and Mercury.
CHAPTER IX
THE MOUNTAINS AND PLAINS OF THE MOON AND THE SPECTACLES OF THE
SUN 156
Peculiarities of the lunar landscapes--The so-called seas, the
craters, the ring mountains, the inclosed plains, the mountain
ranges, Tycho's mysterious streaks, and other lunar features
described--How to view the sun and its spots.
CHAPTER X
ARE THERE PLANETS AMONG THE STARS? 183
Significance of Dr. See's observations--Why our telescopes do
not show planets circling around distant suns--Reasons for
thinking that such planets may exist--The bearing of stellar
evolution on the question.
PLEASURES OF THE TELESCOPE
CHAPTER I
THE SELECTION AND TESTING OF A GLASS
"O telescope, instrument of much knowledge, more precious than any
scepter! Is not he who holds thee in his hand made king and lord of
the works of God?"--JOHN KEPLER.
If the pure and elevated pleasure to be derived from the possession and
use of a good telescope of three, four, five, or six inches aperture
were generally known, I am certain that no instrument of science would
be more commonly found in the homes of intelligent people. The writer,
when a boy, discovered unexpected powers in a pocket telescope not more
than fourteen inches long when extended, and magnifying ten or twelve
times. It became his dream, which was afterward realized, to possess a
more powerful telescope, a real astronomical glass, with which he could
see the beauties of the double stars, the craters of the moon, the spots
on the sun, the belts and satellites of Jupiter, the rings of Saturn,
the extraordinary shapes of the nebulæ, the crowds of stars in the Milky
Way, and the great stellar clusters. And now he would do what he can to
persuade others, who perhaps are not aware how near at hand it lies, to
look for themselves into the wonder-world of the astronomers.
There is only one way in which you can be sure of getting a good
telescope. First, decide how large a glass you are to have, then go to a
maker of established reputation, fix upon the price you are willing to
pay--remembering that good work is never cheap--and finally see that the
instrument furnished to you answers the proper tests for a telescope of
its size. There are telescopes and telescopes. Occasionally a rare
combination of perfect homogeneity in the material, complete harmony
between the two kinds of glass of which the objective is composed, and
lens surfaces whose curves are absolutely right, produces a telescope
whose owner would part with his last dollar sooner than with it. Such
treasures of the lens-maker's art can not, perhaps, be commanded at
will, yet, they are turned out with increasing frequency, and the best
artists are generally able, at all times, to approximate so closely to
perfection that any shortcoming may be disregarded.
In what is said above I refer, of course, to the refracting telescope,
which is the form of instrument that I should recommend to all amateurs
in preference to the reflector. But, before proceeding further, it may
be well to recall briefly the principal points of difference between
these two kinds of telescopes. The purpose of a telescope of either
description is, first, to form an image of the object looked at by
concentrating at a focus the rays of light proceeding from that object.
The refractor achieves this by means of a carefully shaped lens, called
the object glass, or objective. The reflector, on the other hand, forms
the image at the focus of a concave mirror.
[Illustration: IMAGE AT THE FOCUS OF A LENS.]
A very pretty little experiment, which illustrates these two methods of
forming an optical image, and, by way of corollary, exemplifies the
essential difference between refracting and reflecting telescopes, may
be performed by any one who possesses a reading glass and a magnifying
hand mirror. In a room that is not too brightly illuminated pin a sheet
of white paper on the wall opposite to a window that, by preference,
should face the north, or away from the position of the sun. Taking
first the reading glass, hold it between the window and the wall
parallel to the sheet of paper, and a foot or more distant from the
latter. By moving it to and fro a little you will be able to find a
distance, corresponding to the focal length of the lens, at which a
picture of the window is formed on the paper. This picture, or image,
will be upside down, because the rays of light cross at the focus. By
moving the glass a little closer to the wall you will cause the picture
of the window to become indistinct, while a beautiful image of the
houses, trees, or other objects of the outdoor world beyond, will be
formed upon the paper. We thus learn that the distance of the image from
the lens varies with the distance of the object whose image is formed.
In precisely a similar manner an image is formed at the focus of the
object glass of a refracting telescope.
[Illustration: IMAGE AT THE FOCUS OF A CONCAVE MIRROR.]
Take next your magnifying or concave mirror, and detaching the sheet of
paper from the wall, hold it nearly in front of the mirror between the
latter and the window. When you have adjusted the distance to the focal
length of the mirror, you will see an image of the window projected upon
the paper, and by varying the distance, as before, you will be able to
produce, at will, pictures of nearer or more remote objects. It is in
this way that images are formed at the focus of the mirror of a
reflecting telescope.
Now, you will have observed that the chief apparent difference between
these two methods of forming an image of distant objects is that in the
first case the rays of light, passing through the transparent lens, are
brought to a focus on the side opposite to that where the real object
is, while in the second case the rays, being reflected from the
brilliant surface of the opaque mirror, come to a focus on the same side
as that on which the object itself is. From this follows the most
striking difference in the method of using refracting and reflecting
telescopes. In the refractor the observer looks toward the object; in
the reflector he looks away from it. Sir William Herschel made his great
discoveries with his back to the sky. He used reflecting telescopes.
This principle, again, can be readily illustrated by means of our simple
experiment with a reading glass and a magnifying mirror. Hold the
reading glass between the eye and a distant object with one hand, and
with the other hand place a smaller lens such as a pocket magnifier,
near the eye, and in line with the reading glass. Move the two carefully
until they are at a distance apart equal to the sum of the focal lengths
of the lenses, and you will see a magnified image of the distant object.
In other words, you have constructed a simple refracting telescope. Then
take the magnifying mirror, and, turning your back to the object to be
looked at, use the small lens as before--that is to say, hold it between
your eye and the mirror, so that its distance from the latter is equal
to the sum of the focal lengths of the mirror and the lens, and you will
see again a magnified image of the distant object. This time it is a
reflecting telescope that you hold in your hands.
The magnification of the image reminds us of the second purpose which is
subserved by a telescope. A telescope, whether refracting or reflecting,
consists of two essential parts, the first being a lens, or a mirror, to
form an image, and the second a microscope, called an eyepiece, to
magnify the image. The same eyepieces will serve for either the
reflector or the refractor. But in order that the magnification may be
effective, and serve to reveal what could not be seen without it, the
image itself must be as nearly perfect as possible; this requires that
every ray of light that forms the image shall be brought to a point in
the image precisely corresponding to that from which it emanates in the
real object. In reflectors this is effected by giving a parabolic form
to the concave surface of the mirror. In refractors there is a twofold
difficulty to be overcome. In the first place, a lens with spherical
surfaces does not bend all the rays that pass through it to a focus at
precisely the same distance. The rays that pass near the outer edge of
the lens have a shorter focus than that of the rays which pass near the
center of the lens; this is called spherical aberration. A similar
phenomenon occurs with a concave mirror whose surface is spherical. In
that case, as we have seen, the difficulty is overcome by giving the
mirror a parabolic instead of a spherical form. In an analogous way the
spherical aberration of a lens can be corrected by altering its curves,
but the second difficulty that arises with a lens is not so easily
disposed of: this is what is called chromatic aberration. It is due to
the fact that the rays belonging to different parts of the spectrum
have different degrees of refrangibility, or, in other words, that they
come to a focus at different distances from the lens; and this is
independent of the form of the lens. The blue rays come to a focus
first, then the yellow, and finally the red. It results from this
scattering of the spectral rays along the axis of the lens that there is
no single and exact focus where all meet, and that the image of a star,
for instance, formed by an ordinary lens, even if the spherical
aberration has been corrected, appears blurred and discolored. There is
no such difficulty with a mirror, because there is in that case no
refraction of the light, and consequently no splitting up of the
elements of the spectrum.
In order to get around the obstacle formed by chromatic aberration it is
necessary to make the object glass of a refractor consist of two lenses,
each composed of a different kind of glass. One of the most interesting
facts in the history of the telescope is that Sir Isaac Newton could see
no hope that chromatic aberration would be overcome, and accordingly
turned his attention to the improvement of the reflecting telescope and
devised a form of that instrument which still goes under his name. And
even after Chester More Hall in 1729, and John Dollond in 1757, had
shown that chromatic aberration could be nearly eliminated by the
combination of a flint-glass lens with one of crown glass, William
Herschel, who began his observations in 1774, devoted his skill entirely
to the making of reflectors, seeing no prospect of much advance in the
power of refractors.
A refracting telescope which has been freed from the effects of
chromatic aberration is called achromatic. The principle upon which its
construction depends is that by combining lenses of different dispersive
power the separation of the spectral colors in the image can be
corrected while the convergence of the rays of light toward a focus is
not destroyed. Flint glass effects a greater dispersion than crown glass
nearly in the ratio of three to two. The chromatic combination consists
of a convex lens of crown backed by a concave, or plano-concave, lens of
flint. When these two lenses are made of focal lengths which are
directly proportional to their dispersions, they give a practically
colorless image at their common focus. The skill of the telescope-maker
and the excellence of his work depend upon the selection of the glasses
to be combined and his manipulation of the curves of the lenses.
[Illustration: ACHROMATIC OBJECT GLASS.
_a_, crown glass; _b_, flint glass.]
Now, the reader may ask, "Since reflectors require no correction for
color dispersion, while that correction is only approximately effected
by the combination of two kinds of lenses and two kinds of glass in a
refractor, why is not the reflector preferable to the refractor?"
The answer is, that the refractor gives more light and better
definition. It is superior in the first respect because a lens transmits
more light than a mirror reflects. Professor Young has remarked that
about eighty-two per cent of the light reaches the eye in a good
refractor, while "in a Newtonian reflector, in average condition, the
percentage seldom exceeds fifty per cent, and more frequently is lower
than higher." The superiority of the refractor in regard to definition
arises from the fact that any distortion at the surface of a mirror
affects the direction of a ray of light three times as much as the same
distortion would do at the surface of a lens. And this applies equally
both to permanent errors of curvature and to temporary distortions
produced by strains and by inequality of temperature. The perfect
achromatism of a reflector is, of course, a great advantage, but the
chromatic aberration of refractors is now so well corrected that their
inferiority in that respect may be disregarded. It must be admitted that
reflectors are cheaper and easier to make, but, on the other hand, they
require more care, and their mirrors frequently need resilvering, while
an object glass with reasonable care never gets seriously out of order,
and will last for many a lifetime.
Enough has now, perhaps, been said about the respective properties of
object glasses and mirrors, but a word should be added concerning
eyepieces. Without a good eyepiece the best telescope will not perform
well. The simplest of all eyepieces is a single double-convex lens. With
such a lens the magnifying power of the telescope is measured by the
ratio of the focal length of the objective to that of the eye lens.
Suppose the first is sixty inches and the latter half an inch; then the
magnifying power will be a hundred and twenty diameters--i. e., the disk
of a planet, for instance, will be enlarged a hundred and twenty times
along each diameter, and its area will be enlarged the square of a
hundred and twenty, or fourteen thousand four hundred times. But in
reckoning magnifying power, diameter, not area, is always considered.
For practical use an eyepiece composed of an ordinary single lens is
seldom advantageous, because good definition can only be obtained in the
center of the field. Lenses made according to special formulæ, however,
and called solid eyepieces, give excellent results, and for high powers
are often to be preferred to any other. The eyepieces usually furnished
with telescopes are, in their essential principles, compound
microscopes, and they are of two descriptions, "positive" and
"negative." The former generally goes under the name of its inventor,
Ramsden, and the latter is named after great Dutch astronomer, Huygens.
The Huygens eyepiece consists of two plano-convex lenses whose focal
lengths are in the ratio of three to one. The smaller lens is placed
next to the eye. Both lenses have their convex surfaces toward the
object glass, and their distance apart is equal to half the sum of their
focal lengths. In this kind of eyepiece the image is formed between the
two lenses, and if the work is properly done such an eyepiece is
achromatic. It is therefore generally preferred for mere seeing
purposes. In the Ramsden eyepiece two plano-convex lenses are also used,
but they are of equal focal length, are placed at a distance apart equal
to two thirds of the focal length of either, and have their convex sides
facing one another. With such an eyepiece the image viewed is beyond the
farther or field lens instead of between the two lenses, and as this
fact renders it easier to adjust wires or lines for measuring purposes
in the focus of the eyepiece, the Ramsden construction is used when a
micrometer is to be employed. In order to ascertain the magnifying power
which an eyepiece gives when applied to a telescope it is necessary to
know the equivalent, or combined, focal length of the two lenses. Two
simple rules, easily remembered, supply the means of ascertaining this.
The equivalent focal length of a negative or Huygens eyepiece is equal
to half the focal length of the larger or field lens. The equivalent
focal length of a positive or Ramsden eyepiece is equal to three fourths
of the focal length of either of the lenses. Having ascertained the
equivalent focal length of the eyepiece, it is only necessary to divide
it into the focal length of the object glass (or mirror) in order to
know the magnifying power of your telescope when that particular
eyepiece is in use.
[Illustration: NEGATIVE EYEPIECE.]
[Illustration: POSITIVE EYEPIECE.]
A first-class object glass (or mirror) will bear a magnifying power of
one hundred to the inch of aperture when the air is in good
condition--that is, if you are looking at stars. If you are viewing the
moon, or a planet, better results will always be obtained with lower
powers--say fifty to the inch at the most. And under ordinary
atmospheric conditions a power of from fifty to seventy-five to the inch
is far better for stars than a higher power. With a five-inch telescope
that would mean from two hundred and fifty to three hundred and
seventy-five diameters, and such powers should only be applied for the
sake of separating very close double stars. As a general rule, the
lowest power that will distinctly show what you desire to see gives the
best results. The experienced observer never uses as high powers as the
beginner does. The number of eyepieces purchased with a telescope should
never be less than three--a very low power--say ten to the inch; a very
high power, seventy-five or one hundred to the inch, for occasional use;
and a medium power--say forty to the inch--for general use. If you can
afford it, get a full battery of eyepieces--six or eight, or a
dozen--for experience shows that different objects require different
powers in order to be best seen, and, moreover, a slight change of power
is frequently a great relief to the eye.
There is one other thing of great importance to be considered in
purchasing a telescope--the mounting. If your glass is not well mounted
on a steady and easily managed stand, you might better have spent your
money for something more useful. I have endured hours of torment while
trying to see stars through a telescope that was shivering in the wind
and dancing to every motion of the bystanders, to say nothing of the
wriggling contortions caused by the application of my own fingers to the
focusing screw. The best of all stands is a solid iron pillar firmly
fastened into a brick or stone pier, sunk at least four feet in the
ground, and surmounted by a well-made equatorial bearing whose polar
axis has been carefully placed in the meridian. It can be readily
protected from the weather by means of a wooden hood or a rubber sheet,
while the tube of the telescope may be kept indoors, being carried out
and placed on its bearing only when observations are to be made. With
such a mounting you can laugh at the observatories with their cumbersome
domes, for the best of all observatories is the open air. But if you
dislike the labor of carrying and adjusting the tube every time it is
used, and are both fond of and able to procure luxuries, then, after
all, perhaps, you had better have the observatory, dome, draughts and
all.
The next best thing in the way of a mounting is a portable tripod stand.
This may be furnished either with an equatorial bearing for the
telescope, or an altazimuth arrangement which permits both up-and-down
and horizontal motions. The latter is cheaper than the equatorial and
proportionately inferior in usefulness and convenience. The essential
principle of the equatorial bearing is motion about two axes placed at
right angles to one another. When the polar axis is in the meridian, and
inclined at an angle equal to the latitude of the place, the telescope
can be moved about the two axes in such a way as to point to any quarter
of the sky, and the motion of a star, arising from the earthy rotation,
can be followed hour after hour without disturbing the instrument. When
thus mounted, the telescope may be driven by clockwork, or by hand with
the aid of a screw geared to a handle carrying a universal joint.
And now for testing the telescope. It has already been remarked that the
excellence of a telescope depends upon the perfection of the image
formed at the focus of the objective. In what follows I have only a
refractor in mind, although the same principles would apply to a
reflector. With a little practice anybody who has a correct eye can form
a fair judgment of the excellence of a telescopic image. Suppose we have
our telescope steadily mounted out of doors (if you value your peace of
mind you will not try to use a telescope pointed out of a window,
especially in winter), and suppose we begin our observations with the
pole star, employing a magnifying power of sixty or seventy to the inch.
Our first object is to see if the optician has given us a good glass. If
the air is not reasonably steady we had better postpone our experiment
to another night, because we shall find that the star as seen in the
telescope flickers and "boils," and behaves in so extraordinary a
fashion that there is no more definition in the image than there is
steadiness in a bluebottle buzzing on a window pane. But if the night is
a fine one the star image will be quiescent, and then we may note the
following particulars: The real image is a minute bright disk, about one
second of arc in diameter if we are using a four-and-a-half or five-inch
telescope, and surrounded by one very thin ring of light, and the
fragments, so to speak, of one or possibly two similar rings a little
farther from the disk, and visible, perhaps, only by glimpses. These
"diffraction rings" arise from the undulatory nature of light, and their
distance apart as well as the diameter of the central disk depend upon
the length of the waves of light. If the telescope is a really good one,
and both object glass and eyepiece are properly adjusted, the disk will
be perfectly round, slightly softer at the edge, but otherwise equally
bright throughout; and the ring or rings surrounding it will be exactly
concentric, and not brighter on one side than on another. Even if our
telescope were only two inches or two inches and a half in aperture we
should at once notice a little bluish star, the mere ghost of a star in
a small telescope, hovering near the polar star. It is the celebrated
"companion," but we shall see it again when we have more time to study
it. Now let us put the star out of focus by turning the focusing screw.
Suppose we turn it in such a way that the eyepiece moves slightly
outside the focus, or away from the object glass. Very beautiful
phenomena immediately begin to make their appearance. A slight motion
outward causes the little disk to expand perceptibly, and just as this
expansion commences, a bright-red point appears at the precise center of
the disk. But, the outward motion continuing, this red center
disappears, and is replaced by a blue center, which gradually expands
into a sort of flare over the middle of the disk. The disk itself has in
the mean time enlarged into a series of concentric bright rings,
graduated in luminosity with beautiful precision from center toward
circumference. The outermost ring is considerably brighter, however,
than it would be if the same gradation applied to it as applies to the
inner rings, and it is surrounded, moreover, on its outer edge by a
slight flare which tends to increase its apparent width. Next let us
return to the focus and then move the eyepiece gradually inside the
focal point or plane. Once more the star disk expands into a series of
circles, and, if we except the color phenomena noticed outside the
focus, these circles are precisely like those seen before in
arrangement, in size, and in brightness. If they were not the same, we
should pronounce the telescope to be imperfect. There is one other
difference, however, besides the absence of the blue central flare, and
that is a faint reddish edging around the outer ring when the expansion
inside the focus is not carried very far. Upon continuing to move the
eyepiece inside or outside the focus we observe that the system of rings
becomes larger, while the rings themselves rapidly increase in number,
becoming at the same time individually thinner and fainter.
[Illustration: THE STAR IMAGE.]
By studying the appearance of the star disk when in focus and of the
rings when out of focus on either side, an experienced eye can readily
detect any fault that a telescope may have. The amateur, of course, can
only learn to do this by considerable practice. Any glaring and serious
fault, however, will easily make itself manifest. Suppose, for example,
we observe that the image of a star instead of being perfectly round is
oblong, and that a similar defect appears in the form of the rings when
the eyepiece is put out of focus. We know at once that something is
wrong; but the trouble may lie either in the object glass, in the
eyepiece, in the eye of the observer himself, or in the adjustment of
the lenses in the tube. A careful examination of the image and the
out-of-focus circles will enable us to determine with which of these
sources of error we have to deal. If the star image when in focus has a
sort of wing on one side, and if the rings out of focus expand
eccentrically, appearing wider and larger on one side than on the other,
being at the same time brightest on the least expanded side, then the
object glass is probably not at right angles to the axis of the tube and
requires readjustment. That part of the object glass on the side where
the rings appear most expanded and faintest needs to be pushed slightly
inward. This can be effected by means of counterscrews placed for that
purpose in or around the cell. But it, after we have got the object
glass properly squared to the axis of the tube or the line of sight, the
image and the ring system in and out of focus still appear oblong, the
fault of astigmatism must exist either in the objective, the eyepiece,
or the eye. The chances are very great that it is the eye itself that is
at fault. We may be certain of this if we find, on turning the head so
as to look into the telescope with the eye in different positions, that
the oblong image turns with the head of the observer, keeping its major
axis continually in the same relative position with respect to the eye.
The remedy then is to consult an oculist and get a pair of cylindrical
eyeglasses. If the oblong image does not turn round with the eye, but
does turn when the eyepiece is twisted round, then the astigmatism is in
the latter. If, finally, it does not follow either the eye or the
eyepiece, it is the objective that is at fault.
But instead of being oblong, the image and the rings may be misshapen in
some other way. If they are three-cornered, it is probable that the
object glass is subjected to undue pressure in its cell. This, if the
telescope has been brought out on a cool night from a warm room, may
arise from the unequal contraction of the metal work and the glass as
they cool off. In fact, no good star image can be got while a telescope
is assuming the temperature of the surrounding atmosphere. Even the air
inclosed in the tube is capable of making much trouble until its
temperature has sunk to the level of that outside. Half an hour at least
is required for a telescope to adjust itself to out-of-door temperature,
except in the summer time, and it is better to allow an hour or two for
such adjustment in cold weather. Any irregularity in the shape of the
rings which persists after the lenses have been accurately adjusted and
the telescope has properly cooled may be ascribed to imperfections, such
as veins or spots of unequal density in the glass forming the objective.
[Illustration: THE OUT-OF-FOCUS RINGS.
1, Correct figure; 2 and 3, spherical aberration.]
The spherical aberration of an object glass may be undercorrected or
overcorrected. In the former case the central rings inside the focus
will appear faint and the outer ones unduly strong, while outside the
focus the central rings will be too bright and the outer ones too
feeble. But if the aberration is overcorrected the central rings will be
overbright inside the focus and abnormally faint outside the focus.
[Illustration: TWO VIEWS OF MARS IN 1892.
The smaller with a three-and-three-eighths-inch telescope; the larger
with a nine-inch.]
Assuming that we have a telescope in which no obvious fault is
discernible, the next thing is to test its powers in actual work. In
what is to follow I shall endeavor to describe some of the principal
objects in the heavens from which the amateur observer may expect to
derive pleasure and instruction, and which may at the same time serve
as tests of the excellence of his telescope. No one should be deterred
or discouraged in the study of celestial objects by the apparent
insignificance of his means of observation. The accompanying pictures of
the planet Mars may serve as an indication of the fact that a small
telescope is frequently capable of doing work that appears by no means
contemptible when placed side by side with that of the greater
instruments of the observatories.
CHAPTER II
IN THE STARRY HEAVENS
"Now constellations, Muse, and signs rehearse;
In order let them sparkle in thy verse."--MANILIUS.
Let us imagine ourselves the happy possessors of three properly mounted
telescopes of five, four, and three inches aperture, respectively. A
fine midwinter evening has come along, the air is clear, cool, and
steady, and the heavens, of that almost invisible violet which is
reserved for the lovers of celestial scenery, are spangled with stars
that hardly twinkle. We need not disturb our minds about a few thin
clouds here and there floating lazily in the high air; they announce a
change of weather, but they will not trouble us to-night.
Which way shall we look? Our eyes will answer the question for us.
However we may direct them, they instinctively return to the south, and
are lifted to behold Orion in his glory, now near the meridian and
midway to the zenith, with Taurus shaking the glittering Pleiades before
him, and Canis Major with the flaming Dog Star following at his heels.
Not only is Orion the most brilliant of all constellations to the casual
star-gazer, but it contains the richest mines that the delver for
telescopic treasures can anywhere discover. We could not have made a
better beginning, for here within a space of a few square degrees we
have a wonderful variety of double stars and multiple stars, so close
and delicate as to test the powers of the best telescopes, besides a
profusion of star-clusters and nebulæ, including one of the supreme
marvels of space, the Great Nebula in the Sword.
[Illustration: MAP NO. 1.]
Our star map No. 1 will serve as a guide to the objects which we are
about to inspect. Let us begin operations with our smallest telescope,
the three-inch. I may remark here that, just as the lowest magnifying
power that will clearly reveal the object looked for gives ordinarily
better results than a higher power, so the smallest telescope that is
competent to show what one wishes to see is likely to yield more
satisfaction, as far as that particular object is concerned, than a
larger glass. The larger the object glass and the higher the power, the
greater are the atmospheric difficulties. A small telescope will perform
very well on a night when a large one is helpless.
Turn the glass upon beta (Rigel), the white first-magnitude star in
Orion's left foot. Observe whether the image with a high power is clear,
sharp, and free from irregular wisps of stray light. Look at the rings
in and out of focus, and if you are satisfied with the performance, try
for the companion. A good three-inch is certain to show it, except in a
bad state of the atmosphere, and even then an expert can see it, at
least by glimpses. The companion is of the ninth magnitude, some say the
eighth, and the distance is about 9.5", angle of position (hereafter
designated by p.) 199°.[1] Its color is blue, in decided contrast with
the white light of its great primary. Sir John Herschel, however, saw
the companion red, as others have done. These differences are doubtless
due to imperfections of the eye or the telescope. In 1871 Burnham
believed he had discovered that the companion was an exceedingly close
double star. No one except Burnham himself succeeded in dividing it, and
he could only do so at times. Afterward, when he was at Mount Hamilton,
he tried in vain to split it with the great thirty-six-inch telescope,
in 1889, 1890, and 1891. His want of success induced him to suggest that
the component stars were in rapid motion, and so, although he admitted
that it might not be double after all, he advised that it should be
watched for a few years longer. His confidence was justified, for in
1898 Aitken, with the Lick telescope, saw and measured the distance of
the extremely minute companion--distance 0.17", p. 177°.
[1] The angle of position measures the inclination to the meridian of a
line drawn between the principal star and its companion; in other words,
it shows in what direction from the primary we must look for the
companion. It is reckoned from 0° up to 360°, beginning at the north
point and passing around by east through south and west to north again.
Thus, if the angle of position is 0° or 360°, the companion is on the
north side of the primary; if the angle is 90°, the companion is to the
east; if 180°, to the south; if 270°, to the west, and so for
intermediate angles. It must be remembered, however, that in the field
of the telescope the top is south and the bottom north, unless a prism
is used, when directions become complicated. East and west can be
readily identified by noticing the motion of a star through the field;
it moves toward the west and from the east.
Rigel has been suspected of a slight degree of variability. It is
evidently a star of enormous actual magnitude, for its parallax escapes
trustworthy measurement. It can only be ranked among the very first of
the light-givers of the visible universe. Spectroscopically it belongs
to a peculiar type which has very few representatives among the bright
stars, and which has been thus described: "Spectra in which the hydrogen
lines and the few metallic lines all appear to be of equal breadth and
sharp definition." Rigel shows a line which some believe to represent
magnesium; but while it has iron lines in its spectrum, it exhibits no
evidence of the existence of any such cloud of volatilized iron as that
which helps to envelop the sun.
For another test of what the three-inch will do turn to zeta, the lower,
or left-hand, star in the Belt. This is a triple, the magnitudes being
second, sixth, and tenth. The sixth-magnitude star is about 2.5" from
the primary, p. 149°, and has a very peculiar color, hard to describe.
It requires careful focusing to get a satisfactory view of this star
with a three-inch telescope. Use magnifying powers up to two hundred and
fifty diameters. With our four-inch the star is much easier, and the
five-inch shows it readily with a power of one hundred. The
tenth-magnitude companion is distant 56", p. 8°, and may be glimpsed
with the three-inch. Upon the whole, we shall find that we get more
pleasing views of zeta Orionis with the four-inch glass.
Just to the left of zeta, and in the same field of view with a very low
power, is a remarkable nebula bearing the catalogue number 1227. We must
use our five-inch on this with a low power, but with zeta out of the
field in order to avoid its glare. The nebula is exceedingly faint, and
we can be satisfied if we see it simply as a hazy spot, although with
much larger telescopes it has appeared at least half a degree broad.
Tempel saw several centers of condensation in it, and traced three or
four broad nebulous streams, one of which decidedly suggested spiral
motion.
The upper star in the Belt, delta, is double; distance, 53", p. 360°;
magnitudes, second and seventh very nearly; colors, white and green or
blue. This, of course, is an easy object for the three-inch with a low
magnifying power. It would be useless to look for the two fainter
companions of delta, discovered by Burnham, even with our five-inch
glass. But we shall probably need the five-inch for our next attempt,
and it will be well to put on a high power, say three hundred diameters.
The star to be examined is the little brilliant dangling below the
right-hand end of the Belt, toward Rigel. It appears on the map as eta.
Spare no pains in getting an accurate focus, for here is something worth
looking at, and unless you have a trained eye you will not easily see
it. The star is double, magnitudes third and sixth, and the distance
from center to center barely exceeds 1", p. 87°. A little tremulousness
of the atmosphere for a moment conceals the smaller star, although its
presence is manifest from the peculiar jutting of light on one side of
the image of the primary. But in an instant the disturbing undulations
pass, the air steadies, the image shrinks and sharpens, and two points
of piercing brightness, almost touching one another, dart into sight,
the more brilliant one being surrounded by an evanescent circle, a tiny
ripple of light, which, as it runs round the star and then recedes,
alternately embraces and releases the smaller companion. The wash of the
light-waves in the atmosphere provokes many expressions of impatience
from the astronomer, but it is often a beautiful phenomenon
nevertheless.
Between eta and delta is a fifth-magnitude double star, Sigma 725, which
is worth a moment's attention. The primary, of a reddish color, has a
very faint star, eleventh magnitude, at a distance of 12.7", p. 88°.
Still retaining the five-inch in use, we may next turn to the other end
of the Belt, where, just under zeta, we perceive the fourth-magnitude
star sigma. He must be a person of indifferent mind who, after looking
with unassisted eyes at the modest glimmering of this little star, can
see it as the telescope reveals it without a thrill of wonder and a cry
of pleasure. The glass, as by a touch of magic, changes it from one into
eight or ten stars. There are two quadruple sets three and a half
minutes of arc apart. The first set exhibits a variety of beautiful
colors. The largest star, of fourth magnitude, is pale gray; the second
in rank, seventh magnitude, distance 42", p. 61°, presents a singular
red, "grape-red" Webb calls it; the third, eighth magnitude, distance
12", p. 84°, is blue; and the fourth, eleventh magnitude, distance 12",
p. 236°, is apparently white. Burnham has doubled the fourth-magnitude
star, distance 0.23". The second group of four stars consists of three
of the eighth to ninth magnitude, arranged in a minute triangle with a
much fainter star near them. Between the two quadruple sets careful
gazing reveals two other very faint stars. While the five-inch gives a
more satisfactory view of this wonderful multiple star than any smaller
telescope can do, the four-inch and even the three-inch would have shown
it to us as a very beautiful object. However we look at them, there is
an appearance of association among these stars, shining with their
contrasted colors and their various degrees of brilliance, which is
significant of the diversity of conditions and circumstances under which
the suns and worlds beyond the solar walk exist.
From sigma let us drop down to see the wonders of Orion's Sword
displayed just beneath. We can use with advantage any one of our three
telescopes; but since we are going to look at a nebula, it is fortunate
that we have a glass so large as five inches aperture. It will reveal
interesting things that escape the smaller instruments, because it
grasps more than one and a half times as much light as the four-inch,
and nearly three times as much as the three-inch; and in dealing with
nebulæ a plenty of light is the chief thing to be desired. The middle
star in the Sword is theta, and is surrounded by the celebrated Nebula
of Orion. The telescope shows theta separated into four stars arranged
at the corners of an irregular square, and shining in a black gap in the
nebula. These four stars are collectively named the Trapezium. The
brightest is of the sixth magnitude, the others are of the seventh,
seven and a half, and eighth magnitudes respectively. The radiant mist
about them has a faint greenish tinge, while the four stars, together
with three others at no great distance, which follow a fold of the
nebula like a row of buttons on a coat, always appear to me to show an
extraordinary liveliness of radiance, as if the strange haze served to
set them off.
[Illustration: THE TRAPEZIUM WITH THE FIFTH AND SIXTH STARS.]
Our three-inch would have shown the four stars of the Trapezium
perfectly well, and the four-inch would have revealed a fifth star, very
faint, outside a line joining the smallest of the four and its nearest
neighbor. But the five-inch goes a step farther and enables us, with
steady gazing to see even a sixth star, of only the twelfth magnitude,
just outside the Trapezium, near the brightest member of the quartet.
The Lick telescope has disclosed one or two other minute points of light
associated with the Trapezium. But more interesting than the Trapezium
is the vast cloud, full of strange shapes, surrounding it. Nowhere else
in the heavens is the architecture of a nebula so clearly displayed. It
is an unfinished temple whose gigantic dimensions, while exalting the
imagination, proclaim the omnipotence of its builder. But though
unfinished it is not abandoned. The work of creation is proceeding
within its precincts. There are stars apparently completed, shining like
gems just dropped from the hand of the polisher, and around them are
masses, eddies, currents, and swirls of nebulous matter yet to be
condensed, compacted, and constructed into suns. It is an education in
the nebular theory of the universe merely to look at this spot with a
good telescope. If we do not gaze at it long and wistfully, and return
to it many times with unflagging interest, we may be certain that there
is not the making of an astronomer in us.
Before quitting the Orion nebula do not fail to notice an
eighth-magnitude star, a short distance northeast of the Great Nebula,
and nearly opposite the broad opening in the latter that leads in toward
the gap occupied by the Trapezium. This star is plainly enveloped in
nebulosity, that is unquestionably connected with the larger mass of
which it appears to form a satellite.
At the lower end of the Sword is the star iota, somewhat under the third
magnitude. Our three-inch will show that it has a bluish companion of
seventh or eighth magnitude, at a little more than 11" distance, p.
142°, and the larger apertures will reveal a third star, of tenth
magnitude, and reddish in color, distance 49", p. 103°. Close by iota we
find the little double star Sigma 747, whose components are of five and
a half and six and a half magnitudes respectively, and separated 36", p.
223°. Above the uppermost star in the Sword is a small star cluster, No.
1184, which derives a special interest from the fact that it incloses a
delicate double star, Sigma 750, whose larger component is of the sixth
magnitude, while the smaller is of the ninth, and the distance is only
4.3", p. 59°. We may try the four-inch on this object.
Having looked at alpha (Betelgeuse), the great topaz star on Orion's
right shoulder, and admired the splendor of its color, we may turn the
four-inch upon the star Sigma 795, frequently referred to by its number
as "52 Orionis." It consists of one star of the sixth and another of
sixth and a half magnitude, only 1.5" apart, p. 200°. Having separated
them with a power of two hundred and fifty diameters on the four-inch,
we may try them with a high power on the three-inch. We shall only
succeed this time if our glass is of first-rate quality and the air is
perfectly steady.
The star lambda in Orion's head presents an easy conquest for the
three-inch, as it consists of a light-yellow star of magnitude three and
a half and a reddish companion of the sixth magnitude; distance 4", p.
43°. There is also a twelfth-magnitude star at 27", p. 183°, and a tenth
or eleventh magnitude one at 149", p. 278°. These are tests for the
five-inch, and we must not be disappointed if we do not succeed in
seeing the smaller one even with that aperture.
Other objects in Orion, to be found with the aid of our map, are: Sigma
627, a double star, magnitude six and a half and seven, distance 21", p.
260°; Omicron Sigma 98, otherwise named iota Orionis, double, magnitude
six and seven, distance 1", p. 180°, requires five-inch glass; Sigma
652, double, magnitudes six and a half and eight, distance 1.7", p.
184°; rho, double, magnitudes five and eight and a half, the latter
blue, distance 7", p. 62°, may be tried with a three-inch; tau, triple
star, magnitudes four, ten and a half, and eleven, distances 36", p.
249°, and 36", p. 60°. Burnham discovered that the ten-and-a-half
magnitude star is again double, distance 4", p. 50°. There is not much
satisfaction in attempting tau Orionis with telescopes of ordinary
apertures; Sigma 629 otherwise _m_ Orionis, double, magnitudes five and
a half (greenish) and seven, distance 31.7", p. 28°, a pretty object;
Sigma 728, otherwise A 32, double, magnitudes five and seven, distance,
0.5" or less, p. 206°, a rapid binary,[2] which is at present too close
for ordinary telescopes, although it was once within their reach; Sigma
729, double, magnitudes six and eight, distance 2", p. 26°, the smaller
star pale blue--try it with a four-inch, but five-inch is better; Sigma
816, double, magnitudes six and half and eight and a half, distance 4",
p. 289°; psi 2, double, magnitudes five and a half and eleven, distance
3", or a little less, p. 322°; 905, star cluster, contains about twenty
stars from the eighth to the eleventh magnitude; 1267, nebula, faint,
containing a triple star of the eighth magnitude, two of whose
components are 51" apart, while the third is only 1.7" from its
companion, p. 85°; 1376, star cluster, small and crowded; 1361, star
cluster, triangular shape, containing thirty stars, seventh to tenth
magnitudes, one of which is a double, distance 2.4".
[2] The term "binary" is used to describe double stars which are in
motion about their common center of gravity.
Let us now leave the inviting star-fields of Orion and take a glance at
the little constellation of Lepus, crouching at the feet of the mythical
giant. We may begin with a new kind of object, the celebrated red
variable R Leporis (map No. 1). This star varies from the sixth or
seventh magnitude to magnitude eight and a half in a period of four
hundred and twenty-four days. Hind's picturesque description of its
color has frequently been quoted. He said it is "of the most intense
crimson, resembling a blood-drop on the black ground of the sky." It is
important to remember that this star is reddest when faintest, so that
if we chance to see it near its maximum of brightness it will not
impress us as being crimson at all, but rather a dull, coppery red. Its
spectrum indicates that it is smothered with absorbing vapors, a sun
near extinction which, at intervals, experiences an accession of energy
and bursts through its stifling envelope with explosive radiance, only
to faint and sink once more. It is well to use our largest aperture in
examining this star.
We may also employ the five-inch for an inspection of the double star
iota, whose chief component of the fifth magnitude is beautifully tinged
with green. The smaller companion is very faint, eleventh magnitude, and
the distance is about 13", p. 337°.
Another fine double in Lepus is kappa, to be found just below iota; the
components are of the fifth and eighth magnitudes, pale yellow and blue
respectively, distance 2.5", p. 360°; the third-magnitude star alpha has
a tenth-magnitude companion at a distance of 35", p. 156°, and its
neighbor beta (map No. 2), according to Burnham, is attended by three
eleventh-magnitude stars, two of which are at distances of 206", p. 75°,
and 240", p. 58°, respectively, while the third is less than 3" from
beta, p. 288°; the star gamma (map No. 2) is a wide double, the distance
being 94", and the magnitudes four and eight. The star numbered 45 is a
remarkable multiple, but the components are too faint to possess much
interest for those who are not armed with very powerful telescopes.
[Illustration: MAP NO. 2.]
From Lepus we pass to Canis Major (map No. 2). There is no hope of our
being able to see the companion of alpha (Sirius), at present (1901),
even with our five-inch. Discovered by Alvan Clark with an eighteen-inch
telescope in 1862, when its distance was 10" from the center of Sirius,
this ninth-magnitude star has since been swallowed up in the blaze of
its great primary. At first, it slightly increased its distance, and
from 1868 until 1879 most of the measures made by different observers
considerably exceeded 11". Then it began to close up, and in 1890 the
distance scarcely exceeded 4". Burnham was the last to catch sight of it
with the Lick telescope in that year. After that no human eye saw it
until 1896, when it was rediscovered at the Lick Observatory. Since
then the distance has gradually increased to nearly 5". According to
Burnham, its periodic time is about fifty-three years, and its nearest
approach to Sirius should have taken place in the middle of 1892. Later
calculations reduce the periodic time to forty-eight or forty-nine
years. If we can not see the companion of the Dog Star with our
instruments, we can at least, while admiring the splendor of that
dazzling orb, reflect with profit upon the fact that although the
companion is ten thousand times less bright than Sirius, it is half as
massive as its brilliant neighbor. Imagine a subluminous body half as
ponderous as the sun to be set revolving round it somewhere between
Uranus and Neptune. Remember that that body would possess one hundred
and sixty-five thousand times the gravitating energy of the earth, and
that five hundred and twenty Jupiters would be required to equal its
power of attraction, and then consider the consequences to our
easy-going planets! Plainly the solar system is not cut according to the
Sirian fashion. We shall hardly find a more remarkable coupling of
celestial bodies until we come, on another evening, to a star that
began, ages ago, to amaze the thoughtful and inspire the superstitious
with dread--the wonderful Algol in Perseus.
We may remark in passing that Sirius is the brightest representative of
the great spectroscopic type I, which includes more than half of all the
stars yet studied, and which is characterized by a white or bluish-white
color, and a spectrum possessing few or at best faint metallic lines,
but remarkably broad, black, and intense lines of hydrogen. The
inference is that Sirius is surrounded by an enormous atmosphere of
hydrogen, and that the intensity of its radiation is greater, surface
for surface, than that of the sun. There is historical evidence to
support the assertion, improbable in itself, that Sirius, within
eighteen hundred years, has changed color from red to white.
With either of our telescopes we shall have a feast for the eye when we
turn the glass upon the star cluster No. 1454, some four degrees south
of Sirius. Look for a red star near the center. Observe the curving rows
so suggestive of design, or rather of the process by which this cluster
was evolved out of a pre-existing nebula. You will recall the winding
streams in the Great Nebula of Orion. Another star cluster worth a
moment's attention is No. 1479, above and to the left of Sirius. We had
better use the five-inch for this, as many of the stars are very faint.
Not far away we find the double star , whose components are of the fifth
and eighth magnitudes, distance 2.8", p. 343°. The small star is pale
blue. Cluster No. 1512 is a pleasing object with our largest aperture.
In No. 1511 we have a faint nebula remarkable for the rows of minute
stars in and near it. The star gamma is an irregular variable. In 1670
it is said to have almost disappeared, while at the beginning of the
eighteenth century it was more than twice as bright as it is to-day. The
reddish star delta is also probably variable. In my "Astronomy with an
Opera Glass" will be found a cut showing a singular array of small stars
partly encircling delta. These are widely scattered by a telescope, even
with the lowest power.
Eastward from Canis Major we find some of the stars of Argo Navis. Sigma
1097, of the sixth magnitude, has two minute companions at 20" distance,
p. 311° and 312°. The large star is itself double, but the distance,
0.8", p. 166°, places it beyond our reach. According to Burnham, there
is yet a fourth faint star at 31", p. 40°. Some three degrees and a half
below and to the left of the star just examined is a beautiful star
cluster, No. 1551. Nos. 1564, 1571, and 1630 are other star clusters
well worth examination. A planetary nebula is included in 1564. With
very powerful telescopes this nebula has been seen ring-shaped. Sigma
1146, otherwise known as 5 Navis, is a pretty double, colors pale yellow
and blue, magnitudes five and seven, distance 3.25", p. 19°. Our
three-inch will suffice for this.
[Illustration: MAP NO. 3.]
North of Canis Major and Argo we find Monoceros and Canis Minor (map No.
3). The stars forming the western end of Monoceros are depicted on map
No. 1. We shall begin with these. The most interesting and beautiful is
11, a fine triple star, magnitudes five, six, and seven, distances 7.4",
p. 131°, and 2.7", p. 103°. Sir William Herschel regarded this as one of
the most beautiful sights in the heavens. It is a good object to try our
three-inch on, although it should not be difficult for such an aperture.
The star 4 is also a triple, magnitudes six, ten, and eleven, distances
3.4", p. 178°, and 10", p. 244°. We should glance at the star 5 to
admire its fine orange color. In 8 we find a golden fifth-magnitude
star, combined with a blue or lilac star of the seventh magnitude,
distance 14", p. 24°. Sigma 938 is a difficult double, magnitudes six
and a half and twelve, distance 10", p. 210°. Sigma 921 is double,
magnitudes six and a half and eight, distance 16", p. 4°. At the spot
marked on the map 1424 we find an interesting cluster containing one
star of the sixth magnitude.
The remaining stars of Monoceros will be found on map No. 3. The double
and triple stars to be noted are S, or Sigma 950 (which is also a
variable and involved in a faint nebula), magnitudes six and nine,
distance 2.5", p. 206°; Sigma 1183, double, magnitudes five and a half
and eight, distance 31", p. 326°; Sigma 1190, triple, magnitudes five
and a half, ten, and nine, distances 31", p. 105°, and 67", p. 244°.
The clusters are 1465, which has a minute triple star near the center;
1483, one member of whose swarm is red; 1611, very small but rich; and
1637, interesting for the great number of ninth-magnitude stars that it
contains. We should use the five-inch for all of these.
Canis Minor and the Head of Hydra are also contained on map No. 3.
Procyon, alpha of Canis Minor, has several minute stars in the same
field of view. There is, besides, a companion which, although it was
known to exist, no telescope was able to detect until November, 1896. It
must be of immense mass, since its attraction causes perceptible
perturbations in the motion of Procyon. Its magnitude is eight and a
half, distance 4.83", p. 338°. One of the small stars just referred to,
the second one east of Procyon, distant one third of the moon's
diameter, is an interesting double. Our four-inch may separate it, and
the five-inch is certain to do so. The magnitudes are seven and seven
and a half or eight, distance 1.2", p. 133°. This star is variously
named Sigma 1126 and 31 Can. Min. Bode. Star No. 14 is a wide triple,
magnitudes six, seven, and eight, distances 75, p. 65°, and 115", p.
154°.
PROCYON AND ITS NEIGHBORS.
In the Head of Hydra we find Sigma 1245, a double of the sixth and
seventh magnitudes, distance 10.5", p. 25°. The larger star shows a fine
yellow. In epsilon we have a beautiful combination of a yellow with a
blue star, magnitudes four and eight, distance 3.4", p. 198°. Finally,
let us look at theta for a light test with the five-inch. The two stars
composing it are of the fourth and twelfth magnitudes, distance 50", p.
170°.
The brilliant constellations of Gemini and Taurus tempt us next, but
warning clouds are gathering, and we shall do well to house our
telescopes and warm our fingers by the winter fire. There will be other
bright nights, and the stars are lasting.
CHAPTER III
FROM GEMINI TO LEO AND ROUND ABOUT
"If thou wouldst gaze on starry Charioteer,
And hast heard legends of the wondrous Goat,
Vast looming shalt thou find on the Twins' left,
His form bowed forward."--POSTE'S ARATUS.
[Illustration: MAP NO. 4.]
The zodiacal constellations of Gemini, Cancer, and Leo, together with
their neighbors Auriga, the Lynx, Hydra, Sextans, and Coma Berenices,
will furnish an abundance of occupation for our second night at the
telescope. We shall begin, using our three-inch glass, with alpha, the
chief star of Gemini (map No. 4). This is ordinarily known as Castor.
Even an inexperienced eye perceives at once that it is not as bright as
its neighbor Pollux, beta. Whether this fact is to be regarded as
indicating that Castor was brighter than Pollux in 1603, when Bayer
attached their Greek letters, is still an unsettled question. Castor may
or may not be a variable, but it is, at any rate, one of the most
beautiful double stars in the heavens. A power of one hundred is amply
sufficient to separate its components, whose magnitudes are about two
and three, the distance between them being 6", p. 226°. A slight yet
distinct tinge of green, recalling that of the Orion nebula, gives a
peculiar appearance to this couple. Green is one of the rarest colors
among the stars. Castor belongs to the same general spectroscopic type
in which Sirius is found, but its lines of hydrogen are broader than
those seen in the spectrum of the Dog Star. There is reason for
thinking that it may be surrounded with a more extensive atmosphere of
that gaseous metal called hydrogen than any other bright star possesses.
There seems to be no doubt that the components of Castor are in
revolution around their common center of gravity, although the period is
uncertain, varying in different estimates all the way from two hundred
and fifty to one thousand years; the longer estimate is probably not far
from the truth. There is a tenth-magnitude star, distance 73", p. 164°,
which may belong to the same system.
From Castor let us turn to Pollux, at the same time exchanging our
three-inch telescope for the four-inch, or, still better, the five-inch.
Pollux has five faint companions, of which we may expect to see three,
as follows: Tenth magnitude, distance 175", p. 70°; nine and a half
magnitude, distance 206", p. 90°, and ninth magnitude, distance 229", p.
75°. Burnham has seen a star of thirteen and a half magnitude, distance
43", p. 275°, and has divided the tenth-magnitude star into two
components, only 1.4" apart, the smaller being of the thirteenth
magnitude, and situated at the angle 128°. A calculation based on Dr.
Elkin's parallax of 0.068" for Pollux shows that that star may be a
hundredfold more luminous than the sun, while its nearest companion may
be a body smaller than our planet Jupiter, but shining, of course, by
its own light. Its distance from Pollux, however, exceeds that of
Jupiter from the sun in the ratio of about one hundred and thirty to
one.
In the double star pi we shall find a good light test for our three-inch
aperture, the magnitudes being six and eleven, distance 22", p. 212°.
The four-inch will show that kappa is a double, magnitudes four and ten,
distance 6", p. 232°. The smaller star is of a delicate blue color, and
it has been suspected of variability. That it may be variable is
rendered the more probable by the fact that in the immediate
neighborhood of kappa there are three undoubted variables, S, T, and U,
and there appears to be some mysterious law of association which causes
such stars to group themselves in certain regions. None of the variables
just named ever become visible to the naked eye, although they all
undergo great changes of brightness, sinking from the eighth or ninth
magnitude down to the thirteenth or even lower. The variable R, which
lies considerably farther west, is well worth attention because of the
remarkable change of color which it sometimes exhibits. It has been seen
blue, red, and yellow in succession. It varies from between the sixth
and seventh magnitudes to less than the thirteenth in a period of about
two hundred and forty-two days.
Not far away we find a still more curious variable zeta; this is also an
interesting triple star, its principal component being a little under
the third magnitude, while one of the companions is of the seventh
magnitude, distance 90", p. 355°, and the other is of the eleventh
magnitude or less, distance 65", p. 85°. We should hardly expect to see
the fainter companion with the three-inch. The principal star varies
from magnitude three and seven tenths down to magnitude four and a half
in a period of a little more than ten days.
[Illustration: WONDERFUL NEBULA IN GEMINI (1532).]
With the four-or five-inch we get a very pretty sight in delta, which
appears split into a yellow and a purple star, magnitudes three and
eight, distance 7", p. 206°.
Near delta, toward the east, lies one of the strangest of all the
nebulæ. (See the figures 1532 on the map.) Our telescopes will show it
to us only as a minute star surrounded with a nebulous atmosphere, but
its appearance with instruments of the first magnitude is so
astonishing and at the same time so beautiful that I can not refrain
from giving a brief description of it as I saw it in 1893 with the great
Lick telescope. In the center glittered the star, and spread evenly
around it was a circular nebulous disk, pale yet sparkling and
conspicuous. This disk was sharply bordered by a narrow _black_ ring,
and outside the ring the luminous haze of the nebula again appeared,
gradually fading toward the edge to invisibility. The accompanying cut,
which exaggerates the brightness of the nebula as compared with the
star, gives but a faint idea of this most singular object. If its
peculiarities were within the reach of ordinary telescopes, there are
few scenes in the heavens that would be deemed equally admirable.
In the star eta we have another long-period variable, which is also a
double star; unfortunately the companion, being of only the tenth
magnitude and distant less than 1" from its third-magnitude primary, is
beyond the reach of our telescopes. But eta points the way to one of the
finest star clusters in the sky, marked 1360 on the map. The naked eye
perceives that there is something remarkable in that place, and the
opera glass faintly reveals its distant splendors, but the telescope
fairly carries us into its presence. Its stars are innumerable, varying
from the ninth magnitude downward to the last limit of visibility, and
presenting a wonderful array of curves which are highly interesting from
the point of view of the nebular origin of such clusters. Looking
backward in time, with that theory to guide us, we can see spiral lines
of nebulous mist occupying the space that now glitters with interlacing
rows of stars. It is certainly difficult to understand how such lines of
nebula could become knotted with the nuclei of future stars, and then
gradually be absorbed into those stars; and yet, if such a process does
not occur, what is the meaning of that narrow nebulous streak in the
Pleiades along which five or six stars are strung like beads on a
string? The surroundings of this cluster, 1360, as one sweeps over them
with the telescope gradually drawing toward the nucleus, have often
reminded me of the approaches to such a city as London. Thicker and
closer the twinkling points become, until at last, as the observers eye
follows the gorgeous lines of stars trending inward, he seems to be
entering the streets of a brilliantly lighted metropolis.
Other objects in Gemini that we can ill miss are: , double, magnitudes
three and eleven, distance 73", p. 76°, colors yellow and blue; 15,
double, magnitudes six and eight, distance 33", p. 205°; gamma,
remarkable for array of small stars near it; 38, double, magnitudes six
and eight, distance 6.5", p. 162°, colors yellow and blue (very pretty);
lambda, double, magnitudes four and eleven, distance 10", p. 30°, color
of larger star blue--try with the five-inch; epsilon, double, magnitudes
three and nine, distance 110", p. 94°.
From Gemini we pass to Cancer. This constellation has no large stars,
but its great cluster Præsepe (1681 on map No. 4) is easily seen as a
starry cloud with the naked eye. With the telescope it presents the most
brilliant appearance with a very low power. It was one of the first
objects that Galileo turned to when he had completed his telescope, and
he wonderingly counted its stars, of which he enumerated thirty-six, and
made a diagram showing their positions.
The most interesting star in Cancer is zeta, a celebrated triple. The
magnitudes of its components are six, seven, and seven and a half;
distances 1.14", p. 6°, and 5.7", p. 114°. We must use our five-inch
glass in order satisfactorily to separate the two nearest stars. The
gravitational relationship of the three stars is very peculiar. The
nearest pair revolve around their common center in about fifty-eight
years, while the third star revolves with the other two, around a center
common to all three, in a period of six or seven hundred years. But the
movements of the third star are erratic, and inexplicable except upon
the hypothesis advanced by Seeliger, that there is an invisible, or
dark, star near it by whose attraction its motion is perturbed.
In endeavoring to picture the condition of things in zeta Cancri we
might imagine our sun to have a companion sun, a half or a third as
large as itself, and situated within what may be called planetary
distance, circling with it around their center of gravity; while a third
sun, smaller than the second and several times as far away, and
accompanied by a _black_ or non-luminous orb, swings with the first two
around another center of motion. There you would have an entertaining
complication for the inhabitants of a system of planets!
Other objects in Cancer are: Sigma 1223, double star, magnitudes six and
six and a half, distance 5", p. 214°; Sigma 1291, double, magnitudes
both six, distance 1.3", p. 328°--four-inch should split it; iota,
double, magnitudes four and a half and six and a half, distance 30", p.
308°; 66, double magnitudes six and nine, distance 4.8", p. 136°; Sigma
1311, double, magnitudes both about the seventh, distance 7", p. 200°;
1712, star cluster, very beautiful with the five-inch glass.
[Illustration: MAP NO. 5.]
The constellation of Auriga may next command our attention (map No. 5).
The calm beauty of its leading star Capella awakens an admiration that
is not diminished by the rivalry of Orion's brilliants glittering to the
south of it. Although Capella must be an enormously greater sun than
ours, its spectrum bears so much resemblance to the solar spectrum that
a further likeness of condition is suggested. No close telescopic
companion to Capella has been discovered. A ninth-magnitude companion,
distant 159", p. 146°, and two others, one of twelfth magnitude at 78",
p. 317°, the other of thirteenth magnitude at 126", p. 183°, may be
distant satellites of the great star, but not planets in the ordinary
sense, since it is evident that they are self-luminous. It is a
significant fact that most of the first-magnitude stars have faint
companions which are not so distant as altogether to preclude the idea
of physical relationship.
But while Capella has no visible companion, Campbell, of the Lick
Observatory, has lately discovered that it is a conspicuous example of a
peculiar class of binary stars only detected within the closing decade
of the nineteenth century. The nature of these stars, called
spectroscopic binaries, may perhaps best be described while we turn our
attention from Capella to the second star in Auriga beta (Menkalina),
which not only belongs to the same class, but was the first to be
discovered. Neither our telescopes, nor any telescope in existence, can
directly reveal the duplicity of beta Aurigæ to the eye--i. e., we can
not see the two stars composing it, because they are so close that their
light remains inextricably mingled after the highest practicable
magnifying power has been applied in the effort to separate them. But
the spectroscope shows that the star is double and that its components
are in rapid revolution around one another, completing their orbital
swing in the astonishingly short period of _four days_! The combined
mass of the two stars is estimated to be two and a half times the mass
of the sun, and the distance between them, from center to center, is
about eight million miles.
The manner in which the spectroscope revealed the existence of two
stars in beta Aurigæ is a beautiful illustration of the unexpected and,
so to speak, automatic application of an old principle in the discovery
of new facts not looked for. It was noticed at the Harvard Observatory
that the lines in the photographed spectrum of beta Aurigæ (and of a few
other stars to be mentioned later) appeared single in some of the
photographs and double in others. Investigation proved that the lines
were doubled at regular intervals of about two days, and that they
appeared single in the interim. The explanation was not far to seek. It
is known that all stars which are approaching us have their spectral
lines shifted, by virtue of their motion of approach, toward the violet
end of the spectrum, and that, for a similar reason, all stars which are
receding have their lines shifted toward the red end of the spectrum.
Now, suppose two stars to be revolving around one another in a plane
horizontal, or nearly so, to the line of sight. When they are at their
greatest angular distance apart as seen from the earth one of them will
evidently be approaching at the same moment that the other is receding.
The spectral lines of the first will therefore be shifted toward the
violet, and those of the second will be shifted toward the red. Then if
the stars, when at their greatest distance apart, are still so close
that the telescope can not separate them, their light will be combined
in the spectrum; but the spectral lines, being simultaneously shifted in
opposite directions, will necessarily appear to be doubled. As the
revolution of the stars continues, however, it is clear that their
motion will soon cease to be performed in the line of sight, and will
become more and more athwart that line, and as this occurs the spectral
lines will gradually assume their normal position and appear single.
This is the sequence of phenomena in beta Aurigæ. And the same sequence
is found in Capella and in several other more or less conspicuous stars
in various parts of the heavens.
Such facts, like those connecting rows and groups of stars with masses
and spiral lines of nebula are obscure signboards, indicating the
opening of a way which, starting in an unexpected direction, leads deep
into the mysteries of the universe.
Southward from beta we find the star theta, which is a beautiful
quadruple. We shall do best with our five-inch here, although in a fine
condition of the atmosphere the four-inch might suffice. The primary is
of the third magnitude; the first companion is of magnitude seven and a
half, distance 2", p. 5°; the second, of the tenth magnitude, distance
45", p. 292°; and the third, of the tenth magnitude, distance 125", p.
350°.
We should look at the double Sigma 616 with one of our larger apertures
in order to determine for ourselves what the colors of the components
are. There is considerable diversity of opinion on this point. Some say
the larger star is pale red and the smaller light blue; others consider
the color of the larger star to be greenish, and some have even called
it white. The magnitudes are five and nine, distance 6", p. 350°.
Auriga contains several noteworthy clusters which will be found on the
map. The most beautiful of these is 1295, in which about five hundred
stars have been counted.
The position of the new star of 1892, known as Nova Aurigæ, is also
indicated on the map. While this never made a brilliant appearance, it
gave rise to a greater variety of speculative theories than any previous
phenomenon of the kind. Although not recognized until January 24, 1892,
this star, as photographic records prove, was in existence on December
9, 1891. At its brightest it barely exceeded magnitude four and a half,
and its maximum occurred within ten days after its first recognition.
When discovered it was of the fifth magnitude. It was last seen in its
original form with the Lick telescope on April 26th, when it had sunk to
the lowest limit of visibility. To everybody's astonishment it
reappeared in the following August, and on the 17th of that month was
seen shining with the light of a tenth-magnitude star, _but presenting
the spectrum of a nebula!_ Its visual appearance in the great telescope
was now also that of a planetary nebula. Its spectrum during the first
period of its visibility had been carefully studied, so that the means
existed for making a spectroscopic comparison of the phenomenon in its
two phases. During the first period, when only a stellar spectrum was
noticed, remarkable shiftings of the spectral lines occurred, indicating
that two and perhaps three bodies were concerned in the production of
the light of the new star, one of which was approaching the earth, while
the other or the others receded with velocities of several hundred miles
per second! On the revival in the form of a planetary nebula, while the
character of the spectrum had entirely changed, evidences of rapid
motion in the line of sight remained.
But what was the meaning of all this? Evidently a catastrophe of some
kind had occurred out there in space. The idea of a collision involving
the transformation of the energy of motion into that of light and heat
suggests itself at once. But what were the circumstances of the
collision? Did an extinguished sun, flying blindly through space, plunge
into a vast cloud of meteoric particles, and, under the lashing impact
of so many myriads of missiles, break into superficial incandescence,
while the cosmical wrack through which it had driven remained glowing
with nebulous luminosity? Such an explanation has been offered by
Seeliger. Or was Vogel right when he suggested that Nova Aurigæ could
be accounted for by supposing that a wandering dark body had run into
collision with a system of planets surrounding a decrepit sun (and
therefore it is to be hoped uninhabited), and that those planets had
been reduced to vapor and sent spinning by the encounter, the second
outburst of light being caused by an outlying planet of the system
falling a prey to the vagabond destroyer? Or some may prefer the
explanation, based on a theory of Wilsing's, that _two_ great bodies,
partially or wholly opaque and non-luminous at their surfaces, but
liquid hot within, approached one another so closely that the tremendous
strain of their tidal attraction burst their shells asunder so that
their bowels of fire gushed briefly visible, amid a blaze of spouting
vapors. And yet Lockyer thinks that there was no solid or semisolid mass
concerned in the phenomenon at all, but that what occurred was simply
the clash of two immense swarms of meteors that had crossed one
another's track.
Well, where nobody positively knows, everybody has free choice. In the
meantime, look at the spot in the sky where that little star made its
appearance and underwent its marvelous transformation, for, even if you
can see no remains of it there, you will feel your interest in the
problem it has presented, and in the whole subject of astronomy, greatly
heightened and vivified, as the visitor to the field of Waterloo becomes
a lover of history on the spot.
The remaining objects of special interest in Auriga may be briefly
mentioned: 26, triple star, magnitudes five, eight, and eleven,
distances 12", p. 268°, and 26", p. 113°; 14, triple star, magnitudes
five, seven and a half, and eleven, distances 14", p. 224°, and 12.6",
p. 342°, the last difficult for moderate apertures; lambda, double,
magnitudes five and nine, distance 121", p. 13°; epsilon, variable,
generally of third magnitude, but has been seen of only four and a half
magnitude; 41, double, magnitudes five and six, distance 8", p. 354°;
996, 1067, 1119, and 1166, clusters all well worth inspection, 1119
being especially beautiful.
The inconspicuous Lynx furnishes some fine telescopic objects, all
grouped near the northwestern corner of the constellation. Without a
six-inch telescope it would be a waste of time to attack the double star
4, whose components are of sixth and eighth magnitudes, distance 0.8",
p. 103°; but its neighbor, 5, a fine triple, is within our reach, the
magnitudes being six, ten, and eight, distances 30", p. 139°, and 96",
p. 272°. In 12 Lyncis we find one of the most attractive of triple
stars, which in good seeing weather is not beyond the powers of a
three-inch glass, although we shall have a far more satisfactory view of
it with the four-inch. The components are of the sixth, seventh, and
eighth magnitudes, distances 1.4", p. 117°, and 8.7", p. 304°. A
magnifying power which just suffices clearly to separate the disks of
the two nearer stars makes this a fine sight. A beautiful contrast of
colors belongs to the double star 14, but unfortunately the star is at
present very close, the distance between its sixth and seventh magnitude
components not exceeding 0.8", position angle 64°. Sigma 958 is a pretty
double, both stars being of the sixth magnitude, distance 5", p. 257°.
Still finer is Sigma 1009, a double, whose stars are both a little above
the seventh magnitude and nearly equal, distance 3", p. 156°. A low
power suffices to show the three stars in 19, their magnitudes being six
and a half, seven and a half, and eight, distances 15", p. 312°, and
215", p. 358°. Webb describes the two smaller stars as plum-colored.
Plum-colored suns!
At the opposite end of the constellation are two fine doubles, Sigma
1333, magnitudes six and a half and seven, distance 1.4", p. 39°; and
38, magnitudes four and seven, distance 2.9", p. 235°.
Under the guidance of map No. 6 we turn to Leo, which contains one of
the leading gems among the double stars, gamma, whose components, of the
second and fourth magnitudes, are respectively yellow and green, the
green star, according to some observers, having a peculiar tinge of red.
Their distance apart is 3.7", p. 118°, and they are undoubtedly in
revolution about a common center, the probable period being about four
hundred years. The three-inch glass should separate them easily when the
air is steady, and a pleasing sight they are.
The star iota is a closer double, and also very pretty, magnitudes four
and eight, colors lemon and light blue, distance 2.17", p. 53°. Other
doubles are tau, magnitudes five and seven, distance 95", p. 170°; 88,
magnitudes seven and nine, distance 15", p. 320°; 90, triple, magnitudes
six, seven and a half, and ten, distance, 3.5", p. 209°, and 59", p.
234°; 54, magnitudes four and a half and seven, distance 6.2", p. 102°;
and 49, magnitudes six and nine, distance 2.4", p. 158°.
Leo contains a remarkable variable star, R, deep red in color, and
varying in a space of a hundred and forty-four days from the fifth to
the tenth magnitude. It has also several nebulæ, of which only one needs
special mention, No. 1861. This is spindle-shaped, and large telescopes
show that it consists of three nebulæ. The observer with ordinary
instruments finds little to see and little to interest him in these
small, faint nebulæ.
We may just glance at two double stars in the small constellation of
Sextans, situated under Leo. These are: 9, magnitudes seven and eight,
distance 53", p. 292°; and 35, magnitudes six and seven, distance 6.9",
p. 240°.
[Illustration: MAP NO. 6.]
Coma Berenices (map No. 6) includes several interesting objects. Let
us begin with the star 2, a double, of magnitudes six and seven and a
half, distance 3.6", p. 240°. The color of the smaller star is lilac.
This hue, although not extremely uncommon among double stars elsewhere,
recurs again and again, with singular persistence, in this little
constellation. For instance, in the very next star that we look at, 12,
we find a double whose smaller component is _lilac_. The magnitudes in
12 are five and eight, distance 66", p. 168°. So also the wide double
17, magnitudes five and a half and six, distance 145", exhibits a tinge
of _lilac_ in the smaller component; the triple 35, magnitudes five,
eight, and nine, distances 1", p. 77°, and 28.7", p. 124°, has four
colors yellow, _lilac_, and blue, and the double 24, magnitudes five and
six, distance 20", p. 270°, combines an orange with a _lilac_ star, a
very striking and beautiful contrast. We should make a mistake if we
regarded this wonderful distribution of color among the double stars as
accidental. It is manifestly expressive of their physical condition,
although we can not yet decipher its exact meaning.
The binary 42 Comæ Berenicis is too close for ordinary telescopes, but
it is highly interesting as an intermediate between those pairs which
the telescope is able to separate and those--like beta Aurigæ--which no
magnifying power can divide, but which reveal the fact that they are
double by the periodical splitting of their spectral lines. The orbit in
42 Comæ Berenicis is a very small one, so that even when the components
are at their greatest distance apart they can not be separated by a
five-or six-inch glass. Burnham, using the Lick telescope, in 1890 made
the distance 0.7"; Hall, using the Washington telescope, in 1891 made it
a trifle more than 0.5". He had measured it in 1886 as only 0.27". The
period of revolution is believed to be about twenty-five years.
In Coma Berenices there is an outlying field of the marvelous nebulous
region of Virgo, which we may explore on some future evening. But the
nebulæ in Coma are very faint, and, for an amateur, hardly worth the
trouble required to pick them up. The two clusters included in the map,
2752 and 3453, are bright enough to repay inspection with our largest
aperture.
[Illustration: MAP NO. 7.]
Although Hydra is the largest constellation in the heavens, extending
about seven hours, or 105°, in right ascension, it contains
comparatively few objects of interest, and most of these are in the head
or western end of the constellation, which we examined during our first
night at the telescope. In the central portion of Hydra, represented on
map No. 7, we find its leading star alpha, sometimes called Alphard, or
Cor Hydræ, a bright second-magnitude star that has been suspected of
variability. It has a decided orange tint, and is accompanied, at a
distance of 281", p. 153°, by a greenish tenth-magnitude star. Bu. 339
is a fine double, magnitudes eight and nine and a half, distance 1.3",
p. 216°. The planetary nebula 2102 is about 1' in diameter, pale blue in
color, and worth looking at, because it is brighter than most objects of
its class. Tempel and Secchi have given wonderful descriptions of it,
both finding multitudes of stars intermingled with nebulous matter.
For a last glimpse at celestial splendors for the night, let us turn to
the rich cluster 1630, in Argo, just above the place where the stream of
the Milky Way--here bright in mid-channel and shallowing toward the
shores--separates into two or three currents before disappearing behind
the horizon. It is by no means as brilliant as some of the star clusters
we have seen, but it gains in beauty and impressiveness from the
presence of one bright star that seems to captain a host of inferior
luminaries.
CHAPTER IV
VIRGO AND HER NEIGHBORS
... "that region
Where still by night is seen
The Virgin goddess near to bright Boötes."--POSTE'S ARATUS.
[Illustration: MAP NO. 8.]
Following the order of right ascension, we come next to the little
constellations Crater and Corvus, which may be described as standing on
the curves of Hydra (map No. 8). Beginning with Crater, let us look
first at alpha, a yellow fourth-magnitude star, near which is a
celebrated red variable R. With a low power we can see both alpha and R
in the same field of view, like a very wide double. There is a third
star of ninth magnitude, and bluish in color, near R on the side toward
alpha. R is variable both in color and light. When reddest, it has been
described as "scarlet," "crimson," and "blood-colored"; when palest, it
is a deep orange-red. Its light variation has a period the precise
length of which is not yet known. The cycle of change is included
between the eighth and ninth magnitudes.
While our three-inch telescope suffices to show R, it is better to use
the five-inch, because of the faintness of the star. When the color is
well seen, the contrast with alpha is very pleasing.
There is hardly anything else in Crater to interest us, and we pass over
the border into Corvus, and go at once to its chief attraction, the star
delta. The components of this beautiful double are of magnitudes three
and eight; distance 24", p. 211°; colors yellow and purple.
The night being dark and clear, we take the five-inch and turn it on the
nebula 3128, which the map shows just under the border of Corvus in the
edge of Hydra. Herschel believed he had resolved this into stars. It is
a faint object and small, not exceeding one eighth of the moon's
diameter.
Farther east in Hydra, as indicated near the left-hand edge of map No.
8, is a somewhat remarkable variable, R Hydræ. This star occasionally
reaches magnitude three and a half, while at minimum it is not much
above the tenth magnitude. Its period is about four hundred and
twenty-five days.
[Illustration: MAP NO. 9.]
While we have been examining these comparatively barren regions, glad to
find one or two colored doubles to relieve the monotony of the search, a
glittering white star has frequently drawn our eyes eastward and upward.
It is Spica, the great gem of Virgo, and, yielding to its attraction, we
now enter the richer constellation over which it presides (map No. 9).
Except for its beauty, which every one must admire, Spica, or alpha
Virginis, has no special claim upon our attention. Some evidence has
been obtained that, like beta Aurigæ and Capella, it revolves with an
invisible companion of great mass in an orbit only six million miles in
diameter. Spica's spectrum resembles that of Sirius. The faint star
which our larger apertures show about 6' northeast of Spica is of the
tenth magnitude.
Sweeping westward, we come upon Sigma 1669, a pretty little double with
nearly equal components of about the sixth magnitude, distance 5.6", p.
124°. But our interest is not fully aroused until we reach gamma, a star
with a history. The components of this celebrated binary are both
nearly of the third magnitude, distance about 5.8", p. 150°. They
revolve around their common center in something less than two hundred
years. According to some authorities, the period is one hundred and
seventy years, but it is not yet certainly ascertained. It was noticed
about the beginning of the seventeenth century that gamma Virginis was
double. In 1836 the stars were so close together that no telescope then
in existence was able to separate them, although it is said that the
disk into which they had merged was elongated at Pulkowa. In a few years
they became easily separable once more. If the
one-hundred-and-seventy-year period is correct, they should continue to
get farther apart until about 1921. According to Asaph Hall, their
greatest apparent distance is 6.3", and their least apparent distance
0.5"; consequently, they will never again close up beyond the separating
power of existing telescopes.
There is a great charm in watching this pair of stars even with a
three-inch telescope--not so much on account of what is seen, although
they are very beautiful, as on account of what we know they are doing.
It is no slight thing to behold two distant stars obeying the law that
makes a stone fall to the ground and compels the earth to swing round
the sun.
In theta we discover a fine triple, magnitudes four and a half, nine,
and ten; distances 7", p. 345°, and 65", p. 295°. The ninth-magnitude
star has been described as "violet," but such designations of color are
often misleading when the star is very faint. On the other hand it
should not be assumed that a certain color does not exist because the
observer can not perceive it, for experience shows that there is a wide
difference among observers in the power of the eye to distinguish color.
I have known persons who could not perceive the difference of hue in
some of the most beautifully contrasted colored doubles to be found in
the sky. I am acquainted with an astronomer of long experience in the
use of telescopes, whose eye is so deficient in color sense that he
denies that there are any decided colors among the stars. Such persons
miss one of the finest pleasures of the telescope. In examining theta
Virginis we shall do best to use our largest aperture, viz., the
five-inch. Yet Webb records that all three of the stars in this triple
have been seen with a telescope of only three inches aperture. The
amateur must remember in such cases how much depends upon practice as
well as upon the condition of the atmosphere. There are lamentably few
nights in a year when even the best telescope is ideally perfect in
performance, but every night's observation increases the capacity of the
eye, begetting a kind of critical judgment which renders it to some
extent independent of atmospheric vagaries. It will also be found that
the idiosyncrasies of the observer are reflected in his instrument,
which seems to have its fits of excellence, its inspirations so to
speak, while at other times it behaves as if all its wonderful powers
had departed.
Another double that perhaps we had better not try with less than four
inches aperture is 84 Virginis. The magnitudes are six and nine;
distance, 3.5", p. 233°. Colors yellow and blue. Sigma 1846 is a
fifth-magnitude star with a tenth-magnitude companion, distance only 4",
p. 108°. Use the five-inch.
And now we approach something that is truly marvelous, the "Field of the
Nebulæ." This strange region, lying mostly in the constellation Virgo,
is roughly outlined by the stars beta, eta, gamma, delta, and epsilon,
which form two sides of a square some 15° across. It extends, however,
for some distance into Coma Berenices, while outlying nebulæ belonging
to it are also to be found in the eastern part of Leo. Unfortunately
for those who expect only brilliant revelations when they look through a
telescope, this throng of nebulæ consists of small and inconspicuous
wisps as ill defined as bits of thistle-down floating high in the air.
There are more than three hundred of them all told, but even the
brightest are faint objects when seen with the largest of our
telescopes. Why do they congregate thus? That is the question which
lends an interest to the assemblage that no individual member of it
could alone command. It is a mystery, but beyond question it is
explicable. The explanation, however, is yet to be discovered.
The places of only three of the nebulæ are indicated on the map. No.
2806 has been described as resembling in shape a shuttle. Its length is
nearly one third of the moon's diameter. It is brightest near the
center, and has several faint companions. No. 2961 is round, 4' in
diameter, and is accompanied by another round nebula in the same field
of view toward the south. No. 3105 is double, and powerful telescopes
show two more ghostly companions. There is an opportunity for good and
useful work in a careful study of the little nebulæ that swim into view
all over this part of Virgo. Celestial photography has triumphs in store
for itself here.
Scattered over and around the region where the nebulæ are thickest we
find eight or nine variable stars, three of the most remarkable of
which, R, S, and U, may be found on the map. R is very irregular,
sometimes attaining magnitude six and a half, while at other times its
maximum brightness does not exceed that of an eighth-magnitude star. At
minimum it sinks to the tenth or eleventh magnitude. Its period is one
hundred and forty-five days. U varies from magnitude seven or eight down
to magnitude twelve or under and then regains its light, in a period of
about two hundred and seven days. S is interesting for its brilliant red
color. When brightest, it exceeds the sixth magnitude, but at some of
its maxima the magnitude is hardly greater than the eighth. At minimum
it goes below the twelfth magnitude. Period, three hundred and
seventy-six days.
[Illustration: MAP NO. 10.]
Next east of Virgo is Libra, which contains a few notable objects (map
No. 10). The star alpha has a fifth-magnitude companion, distant about
230", which can be easily seen with an opera glass. At the point marked
A on the map is a curious multiple star, sometimes referred to by its
number in Piazzi's catalogues as follows: 212 P. xiv. The two principal
stars are easily seen, their magnitudes being six and seven and a half;
distance 15", p. 290°. Burnham found four other faint companions, for
which it would be useless for us to look. The remarkable thing is that
these faint stars, the nearest of which is distant about 50" from the
largest member of the group and the farthest about 129", do not share,
according to their discoverer, in the rapid proper motion of the two
main stars.
In iota we find a double a little difficult for our three-inch. The
components are of magnitudes four and a half and nine, distance 57", p.
110°. Burnham discovered that the ninth-magnitude star consists of two
of the tenth less than 2" apart, p. 24°.
No astronomer who happens to be engaged in this part of the sky ever
fails, unless his attention is absorbed by something of special
interest, to glance at beta Libræ, which is famous as the only naked-eye
star having a decided green color. The hue is pale, but manifest.[3]
[3] Is the slight green tint perceptible in Sirius variable? I am
sometimes disposed to think it is.
The star is a remarkable variable, belonging to what is called the Algol
type. Its period, according to Chandler, is 2 days 7 hours, 51
minutes, 22.8 seconds. The time occupied by the actual changes is about
twelve hours. At maximum the star is of magnitude five and at minimum of
magnitude 6.2.
[Illustration: MAP NO. 11.]
We may now conveniently turn northward from Virgo in order to explore
Boötes, one of the most interesting of the constellations (map No. 11).
Its leading star alpha, Arcturus, is the brightest in the northern
hemisphere. Its precedence over its rivals Vega and Capella, long in
dispute, has been settled by the Harvard photometry. You notice that the
color of Arcturus, when it has not risen far above the horizon, is a
yellowish red, but when the star is near mid-heaven the color fades to
light yellow. The hue is possibly variable, for it is recorded that in
1852 Arcturus appeared to have nearly lost its color. If it should
eventually turn white, the fact would have an important bearing upon the
question whether Sirius was, as alleged, once a red or flame-colored
star.
But let us sit here in the starlight, for the night is balmy, and talk
about Arcturus, which is perhaps actually the greatest sun within the
range of terrestrial vision. Its parallax is so minute that the
consideration of the tremendous size of this star is a thing that the
imagination can not placidly approach. Calculations, based on its
assumed distance, which show that it _outshines the sun several thousand
times_, may be no exaggeration of the truth! It is easy to make such a
calculation. One of Dr. Elkin's parallaxes for Arcturus is 0.018". That
is to say, the displacement of Arcturus due to the change in the
observer's point of view when he looks at the star first from one side
and then from the other side of the earth's orbit, 186,000,000 miles
across, amounts to only eighteen one-thousandths of a second of arc. We
can appreciate how small that is when we reflect that it is about equal
to the apparent distance between the heads of two pins placed an inch
apart and viewed from a distance of a hundred and eighty miles!
Assuming this estimate of the parallax of Arcturus, let us see how it
will enable us to calculate the probable size or light-giving power of
the star as compared with the sun. The first thing to do is to multiply
the earth's distance from the sun, which may be taken at 93,000,000
miles, by 206,265, the number of seconds of arc in a radian, the base of
circular measure, and then divide the product by the parallax of the
star. Performing the multiplication and division, we get the following:
19,182,645,000,000 / .018 = 1,065,702,500,000,000.
The quotient represents miles! Call it, in round numbers, a thousand
millions of millions of miles. This is about 11,400,000 times the
distance from the earth to the sun.
Now for the second part of the calculation: The amount of light received
on the earth from some of the brighter stars has been experimentally
compared with the amount received from the sun. The results differ
rather widely, but in the case of Arcturus the ratio of the star's light
to sunlight may be taken as about one twenty-five-thousand-millionth--i.
e., 25,000,000,000 stars, each equal to Arcturus, would together shed
upon the earth as much light as the sun does. But we know that light
varies inversely as the square of the distance; for instance, if the sun
were twice as far away as it is, its light would be diminished for us to
a quarter of its present amount. Suppose, then, that we could remove the
earth to a point midway between the sun and Arcturus, we should then be
5,700,000 times as far from the sun as we now are. In order to estimate
how much light the sun would send us from that distance we must square
the number 5,700,000 and then take the result inversely, or as a
fraction. We thus get 1 / 32,490,000,000,000, representing the ratio of
the sun's light at half the distance of Arcturus to that at its real
distance. But while receding from the sun we should be approaching
Arcturus. We should get, in fact, twice as near to that star as we were
before, and therefore its light would be increased for us fourfold. Now,
if the amount of sunlight had not changed, it would exceed the light of
Arcturus only a quarter as much as it did before, or in the ratio of
25,000,000,000 / 4 = 6,250,000,000 to 1. But, as we have seen, the
sunlight would diminish through increase of distance to one
32,490,000,000,000th part of its original amount. Hence its altered
ratio to the light of Arcturus would become 6,250,000,000 to
32,490,000,000,000, or 1 to 5,198.
This means that if the earth were situated midway between the sun and
Arcturus, it would receive 5,198 times as much light from that star as
it would from the sun! It is quite probable, moreover, that the heat of
Arcturus exceeds the solar heat in the same ratio, for the spectroscope
shows that although Arcturus is surrounded with a cloak of metallic
vapors proportionately far more extensive than the sun's, yet, smothered
as the great star seems in some respects to be, it rivals Sirius itself
in the intensity of its radiant energy.
If we suppose the radiation of Arcturus to be the same per unit of
surface as the sun's, it follows that Arcturus exceeds the sun about
375,000 times in volume, and that its diameter is no less than
62,350,000 miles! Imagine the earth and the other planets constituting
the solar system removed to Arcturus and set revolving around it in
orbits of the same forms and sizes as those in which they circle about
the sun. Poor Mercury! For that little planet it would indeed be a jump
from the frying pan into the fire, because, as it rushed to perihelion,
Mercury would plunge more than 2,500,000 miles beneath the surface of
the giant star. Venus and the earth would melt like snowflakes at the
mouth of a furnace. Even far-away Neptune, the remotest member of the
system, would swelter in torrid heat.
But stop! Look at the sky. Observe how small and motionless the disks of
the stars have become. Back to the telescopes at once, for this is a
token that the atmosphere is steady, and that "good seeing" may be
expected. It is fortunate, for we have some delicate work before us. The
very first double star we try in Boötes, Sigma 1772, requires the use of
the four-inch, and the five-inch shows it more satisfactorily. The
magnitudes are sixth and ninth, distance 5", p. 140°. On the other side
of Arcturus we find zeta, a star that we should have had no great
difficulty in separating thirty years ago, but which has now closed up
beyond the reach even of our five-inch. The magnitudes are both fourth,
and the distance less than a quarter of a second; position angle
changing. It is apparently a binary, and if so will some time widen
again, but its period is unknown. The star 279, also known as Sigma
1910, near the southeastern edge of the constellation, is a pretty
double, each component being of the seventh magnitude, distance 4", p.
212°. Just above zeta we come upon pi, an easy double for the
three-inch, magnitudes four and six, distance 6" p. 99°. Next is xi, a
yellow and purple pair, whose magnitudes are respectively five and
seven, distance less than 3", p. 200°. This is undoubtedly a binary with
a period of revolution of about a hundred and thirty years. Its distance
decreased about 1" between 1881 and 1891. It was still decreasing in
1899, when it had become 2.5". The orbital swing is also very apparent
in the change of the position angle.
The telescopic gem of Boötes, and one of "the flowers of the sky," is
epsilon, also known as Mirac. When well seen, as we shall see it
to-night, epsilon Boötis is superb. The magnitudes of its two component
stars are two and a half (according to Hall, three) and six. The
distance is about 2.8", p. 326°. The contrast of colors--bright orange
yellow, set against brilliant emerald green--is magnificent. There are
very few doubles that can be compared with it in this respect. The
three-inch will separate it, but the five-inch enables us best to enjoy
its beauty. It appears to be a binary, but the motion is very slow, and
nothing certain is yet known of its period.
In delta we have a very wide and easy double; magnitudes three and a
half and eight and a half, distance 110", p. 75°. The smaller star has a
lilac hue. We can not hope with any of our instruments to see all of the
three stars contained in , but two of them are easily seen; magnitudes
four and seven, distance 108", p. 172°. The smaller star is again
double; magnitudes seven and eight, distance 0.77", p. 88°. It is
clearly a binary, with a long period. A six-inch telescope that could
separate this star at present would be indeed a treasure. Sigma 1926 is
another object rather beyond our powers, on account of the contrast of
magnitudes. These are six and eight and a half; distance 1.3", p. 256°.
Other doubles are: 44 (Sigma 1909), magnitudes five and six, distance
4.8", p. 240°; 39 (Sigma 1890), magnitudes both nearly six, distance
3.6", p. 45°. Smaller star light red; iota, magnitudes four and a half
and seven and a half, distance 38", p. 33°; kappa, magnitudes five and a
half and eight, distance 12.7", p. 238°. Some observers see a greenish
tinge in the light of the larger star, the smaller one being blue.
There are one or two interesting things to be seen in that part of Canes
Venatici which is represented on map No. 11. The first of these is the
star cluster 3936. This will reward a good look with the five-inch. With
large telescopes as many as one thousand stars have been discerned
packed within its globular outlines.
The star 25 (Sigma 1768) is a close binary with a period estimated at
one hundred and twenty-five years. The magnitudes are six and seven or
eight, distance about 1", p. 137°. We may try for this with the
five-inch, and if we do not succeed in separating the stars we may hope
to do so some time, for the distance between them is increasing.
Although the nebula 3572 is a very wonderful object, we shall leave it
for another evening.
Eastward from Boötes shines the circlet of Corona Borealis, whose form
is so strikingly marked out by the stars that the most careless eye
perceives it at once. Although a very small constellation, it abounds
with interesting objects. We begin our attack with the five-inch on
Sigma 1932, but not too confident that we shall come off victors, for
this binary has been slowly closing for many years. The magnitudes are
six and a half and seven, distance 0.84", p. 150°. Not far distant is
another binary, at present beyond our powers, eta. Here the magnitudes
are both six, distance 0.65", p. 3°. Hall assigns a period of forty
years to this star.
The assemblage of close binaries in this neighborhood is very curious.
Only a few degrees away we find one that is still more remarkable, the
star gamma. What has previously been said about 42 Comæ Berenicis
applies in a measure to this star also. It, too, has a comparatively
small orbit, and its components are never seen widely separated. In 1826
their distance was 0.7"; in 1880 they could not be split; in 1891 the
distance had increased to 0.36", and in 1894 it had become 0.53", p.
123°. But in 1899 Lewis made the distance only 0.43". The period has
been estimated at one hundred years.
While the group of double stars in the southern part of Corona Borealis
consists, as we have seen, of remarkably close binaries, another group
in the northern part of the same constellation comprises stars that are
easily separated. Let us first try zeta. The powers of the three-inch
are amply sufficient in this case. The magnitudes are four and five,
distance 6.3", p. 300°. Colors, white or bluish-white and blue or green.
Next take sigma, whose magnitudes are five and six, distance 4", p.
206°. With the five-inch we may look for a second companion of the tenth
magnitude, distance 54", p. 88°. It is thought highly probable that
sigma is a binary, but its period has simply been guessed at.
Finally, we come to nu, which consists of two very widely separated
stars, nu^1 and nu^2, each of which has a faint companion. With the
five-inch we may be able to see the companion of nu^2, the more
southerly of the pair. The magnitude of the companion is variously given
as tenth and twelfth, distance 137", p. 18°.
With the aid of the map we find the position of the new star of 1866,
which is famous as the first so-called temporary star to which
spectroscopic analysis was applied. When first noticed, on May 12, 1866,
this star was of the second magnitude, fully equaling in brilliancy
alpha, the brightest star of the constellation; but in about two weeks
it fell to the ninth magnitude. Huggins and Miller eagerly studied the
star with the spectroscope, and their results were received with deepest
interest. They concluded that the light of the new star had two
different sources, each giving a spectrum peculiar to itself. One of the
spectra had dark lines and the other bright lines. It will be
remembered that a similar peculiarity was exhibited by the new star in
Auriga in 1893. But the star in Corona did not disappear. It diminished
to magnitude nine and a half or ten, and stopped there; and it is still
visible. In fact, subsequent examination proved that it had been
catalogued at Bonn as a star of magnitude nine and a half in 1855.
Consequently this "blaze star" of 1866 will bear watching in its
decrepitude. Nobody knows but that it may blaze again. Perhaps it is a
sun-like body; perhaps it bears little resemblance to a sun as we
understand such a thing. But whatever it may be, it has proved itself
capable of doing very extraordinary things.
We have no reason to suspect the sun of any latent eccentricities, like
those that have been displayed by "temporary" stars; yet, acting on the
principle which led the old emperor-astrologer Rudolph II to torment his
mind with self-made horoscopes of evil import, let us unscientifically
imagine that the sun _could_ suddenly burst out with several hundred
times its ordinary amount of heat and light, thereby putting us into a
proper condition for spectroscopic examination by curious astronomers in
distant worlds.
But no, after all, it is far pleasanter to keep within the strict
boundaries of science, and not imagine anything of the kind.
CHAPTER V
IN SUMMER STAR-LANDS
"I heard the trailing garments of the night
Sweep through her marble halls,
I saw her sable skirts all fringed with light
From the celestial walls."--H. W. LONGFELLOW.
In the soft air of a summer night, when fireflies are flashing their
lanterns over the fields, the stars do not sparkle and blaze like those
that pierce the frosty skies of winter. The light of Sirius, Aldebaran,
Rigel, and other midwinter brilliants possesses a certain gemlike
hardness and cutting quality, but Antares and Vega, the great summer
stars, and Arcturus, when he hangs westering in a July night, exhibit a
milder radiance, harmonizing with the character of the season. This
difference is, of course, atmospheric in origin, although it may be
partly subjective, depending upon the mental influences of the mutations
of Nature.
[Illustration: MAP NO. 12.]
The constellation Scorpio is nearly as striking in outline as Orion, and
its brightest star, the red Antares (alpha in map No. 12), carries
concealed in its rays a green jewel which, to the eye of the enthusiast
in telescopic recreation, appears more beautiful and inviting each time
that he penetrates to its hiding place.
We shall begin our night's work with this object, and the four-inch
glass will serve our purpose, although the untrained observer would be
more certain of success with the five-inch. A friend of mine has seen
the companion of Antares with a three-inch, but I have never tried the
star with so small an aperture. When the air is steady and the companion
can be well viewed, there is no finer sight among the double stars. The
contrast of colors is beautifully distinct--fire-red and bright green.
The little green star has been seen emerging from behind the moon, ahead
of its ruddy companion. The magnitudes are one and seven and a half or
eight, distance 3", p. 270°. Antares is probably a binary, although its
binary character has not yet been established.
A slight turn of the telescope tube brings us to the star sigma, a wide
double, the smaller component of which is blue or plum-colored;
magnitudes four and nine, distance 20", p. 272°. From sigma we pass to
beta, a very beautiful object, of which the three-inch gives us a
splendid view. Its two components are of magnitudes two and six,
distance 13", p. 30°; colors, white and bluish. It is interesting to
know that the larger star is itself double, although none of the
telescopes we are using can split it. Burnham discovered that it has a
tenth-magnitude companion; distance less than 1", p. 87°.
And now for a triple, which will probably require the use of our largest
glass. Up near the end of the northern prolongation of the constellation
we perceive the star xi. The three-inch shows us that it is double; the
five-inch divides the larger star again. The magnitudes are respectively
five, five and a half, and seven and a half, distances 0.94", p. 215°,
and 7", p. 70°.
A still more remarkable star, although one of its components is beyond
our reach, is nu. With the slightest magnifying this object splits up
into two stars, of magnitudes four and seven, situated rather more than
40" apart. A high power divides the seventh-magnitude companion into
two, each of magnitude six and a half, distance 1.8", p. 42°. But (and
this was another of Burnham's discoveries) the fourth-magnitude star
itself is double, distance 0.8", p. about 0°. The companion in this case
is of magnitude five and a half.
Next we shall need a rather low-power eyepiece and our largest aperture
in order to examine a star cluster, No. 4173, which was especially
admired by Sir William Herschel, who discovered that it was not, as
Messier had supposed, a circular nebula. Herschel regarded it as the
richest mass of stars in the firmament, but with a small telescope it
appears merely as a filmy speck that has sometimes been mistaken for a
comet. In 1860 a new star, between the sixth and seventh magnitude in
brilliance, suddenly appeared directly in or upon the cluster, and the
feeble radiance of the latter was almost extinguished by the superior
light of the stranger. The latter disappeared in less than a month, and
has not been seen again, although it is suspected to be a variable, and,
as such, has been designated with the letter T. Two other known
variables, both very faint, exist in the immediate neighborhood.
According to the opinion that was formerly looked upon with favor, the
variable T, if it is a variable, simply lies in the line of sight
between the earth and the star cluster, and has no actual connection
with the latter. But this opinion may not, after all, be correct, for
Mr. Bailey's observations show that variable stars sometimes exist in
large numbers in clusters, although the variables thus observed are of
short period. The cluster 4183, just west of Antares, is also worth a
glance with the five-inch glass. It is dense, but its stars are very
small, so that to enjoy its beauty we should have to employ a large
telescope. Yet there is a certain attraction in these far-away glimpses
of starry swarms, for they give us some perception of the awful
profundity of space. When the mind is rightly attuned for these
revelations of the telescope, there are no words that can express its
impressions of the overwhelming perspective of the universe.
The southern part of the constellation Ophiuchus is almost inextricably
mingled with Scorpio. We shall, therefore, look next at its attractions,
beginning with the remarkable array of star clusters 4264, 4268, 4269,
and 4270. All of these are small, 2' or 3' in diameter, and globular in
shape. No. 4264 is the largest, and we can see some of the stars
composing it. But these clusters, like those just described in Scorpio,
are more interesting for what they signify than for what they show; and
the interest is not diminished by the fact that their meaning is more or
less of a mystery. Whether they are composed of pygmy suns or of great
solar globes like that one which makes daylight for the earth, their
association in spherical groups is equally suggestive.
There are two other star clusters in Ophiuchus, and within the limits of
map No. 12, both of which are more extensive than those we have just
been looking at. No. 4211 is 5' or 6' in diameter, also globular,
brighter at the center, and surrounded by several comparatively
conspicuous stars. No. 4346 is still larger, about half as broad as the
moon, and many of its scattered stars are of not less than the ninth
magnitude. With a low magnifying power the field of view surrounding the
cluster appears powdered with stars.
There are only two noteworthy doubles in that part of Ophiuchus with
which we are at present concerned: 36, whose magnitudes are five and
seven, distance 4.3", p. 195°, colors yellow and red; and 39, magnitudes
six and seven and a half, distance 12", p. 356°, colors yellow or
orange and blue. The first named is a binary whose period has not been
definitely ascertained.
The variable R has a period a little less than three hundred and three
days. At its brightest it is of magnitude seven or eight, and at minimum
it diminishes to about the twelfth magnitude.
The spot where the new star of 1604 appeared is indicated on the map.
This was, with the exception of Tycho's star in 1572, the brightest
temporary star of which we possess a trustworthy account. It is
frequently referred to as Kepler's star, because Kepler watched it with
considerable attention, but unfortunately he was not as good an observer
as Tycho was. The star was first seen on October 10, 1604, and was then
brighter than Jupiter. It did not, however, equal Venus. It gradually
faded and in March, 1606, disappeared. About twelve degrees northwest of
the place of the star of 1604, and in that part of the constellation
Serpens which is included in map No. 12, we find the location of another
temporary star, that of 1848. It was first noticed by Mr. Hind on April
28th of that year, when its magnitude was not much above the seventh,
and its color was red. It brightened rapidly, until on May 2d it was of
magnitude three and a half. Then it began to fade, but very slowly, and
it has never entirely disappeared. It is now of the twelfth or
thirteenth magnitude.
In passing we may glance with a low power at nu Serpentis, a wide
double, magnitudes four and nine, distance 50", p. 31°, colors
contrasted but uncertain.
Sagittarius and its neighbor, the small but rich constellation Scutum
Sobieskii, attract us next. We shall first deal with the western
portions of these constellations which are represented on Map No. 12.
The star in Sagittarius is a wide triple, magnitudes three and a half,
nine and a half, and ten, distances 40", p. 315°, and 45", p. 114°. But
the chief glory of Sagittarius (and the same statement applies to Scutum
Sobieskii) lies in its assemblage of star clusters. One of these, No.
4361, also known as M 8, is plainly visible to the naked eye as a bright
spot in the Milky Way. We turn our five-inch telescope, armed with a low
magnifying power, upon this subject and enjoy a rare spectacle. As we
allow it to drift through the field we see a group of three
comparatively brilliant stars advancing at the front of a wonderful
train of mingled star clusters and nebulous clouds. A little northwest
of it appears the celebrated trifid nebula, No. 4355 on the map. There
is some evidence that changes have occurred in this nebula since its
discovery in the last century. Barnard has made a beautiful photograph
showing M 8 and the trifid nebula on the same plate, and he remarks that
the former is a far more remarkable object than its more famous
neighbor. Near the eastern border of the principal nebulous cloud there
is a small and very black hole with a star poised on its eastern edge.
This hole and the star are clearly shown in the photograph.
Cluster No. 4397 (M 24) is usually described as resembling, to the naked
eye, a protuberance on the edge of the Milky Way. It is nearly three
times as broad as the moon, and is very rich in minute stars, which are
at just such a degree of visibility that crowds of them continually
appear and disappear while the eye wanders over the field, just as faces
are seen and lost in a vast assemblage of people. This kind of luminous
agitation is not peculiar to M 24, although that cluster exhibits it
better than most others do on account of both the multitude and the
minuteness of its stars.
A slight sweep eastward brings us to yet another meeting place of stars,
the cluster M 25, situated between the variables U and V. This is
brilliant and easily resolved into its components, which include a
number of double stars.
The two neighboring variables just referred to are interesting. U has a
period of about six days and three quarters, and its range of magnitude
runs from the seventh down to below the eighth. V is a somewhat
mysterious star. Chandler removed it from his catalogue of variables
because no change had been observed in its light by either himself,
Sawyer, or Yendell. Quirling, the discoverer of its variability, gave
the range as between magnitudes 7.6 and 8.8. It must, therefore, be
exceedingly erratic in its changes, resembling rather the temporary
stars than the true variables.
In that part of Scutum Sobieskii contained in map No. 12 we find an
interesting double, Sigma 2325, whose magnitudes are six and nine,
distance 12.3", p. 260°, colors white and orange. Sigma 2306 is a
triple, magnitudes seven, eight, and nine, distances 12", p. 220°, and
0.8", p. 68°. The third star is, however, beyond our reach. The colors
of the two larger are respectively yellow and violet.
The star cluster 4400 is about one quarter as broad as the moon, and
easily seen with our smallest aperture.
[Illustration: MAP NO. 13.]
Passing near to the region covered by map No. 13, we find the remaining
portions of the constellations Sagittarius and Scutum Sobieskii. It will
be advisable to finish with the latter first. Glance at the clusters
4426 and 4437. Neither is large, but both are rich in stars. The nebula
4441 is a fine object of its kind. It brightens toward the center, and
Herschel thought he had resolved it into stars. The variable R is
remarkable for its eccentricities. Sometimes it attains nearly the
fourth magnitude, although usually at maximum it is below the fifth,
while at minimum it is occasionally of the sixth and at other times of
the seventh or eighth magnitude. Its period is irregular.
Turning back to Sagittarius, we resume our search for interesting
objects there, and the first that we discover is another star cluster,
for the stars are wonderfully gregarious in this quarter of the heavens.
The number our cluster bears on the map is 4424, corresponding with M 22
in Messier's catalogue. It is very bright, containing many stars of the
tenth and eleventh magnitudes, as well as a swarm of smaller ones. Sir
John Herschel regarded the larger stars in this cluster as possessing a
reddish tint. Possibly there was some peculiarity in his eye that gave
him this impression, for he has described a cluster in the constellation
Toucan in the southern hemisphere as containing a globular mass of
rose-colored stars inclosed in a spherical shell of white stars. Later
observers have confirmed his description of the shape and richness of
this cluster in Toucan, but have been unable to perceive the red hue of
the interior stars.
The eastern expanse of Sagittarius is a poor region compared with the
western end of the constellation, where the wide stream of the Milky Way
like a great river enriches its surroundings. The variables T and R are
of little interest to us, for they never become bright enough to be seen
without the aid of a telescope. In 54 we find, however, an interesting
double, which with larger telescopes than any of ours appears as a
triple. The two stars that we see are of magnitudes six and seven and a
half, distance 45", p. 42°, colors yellow and blue. The third star,
perhaps of thirteenth magnitude, is distant 36", p. 245°.
Retaining map No. 13 as our guide, we examine the western part of the
constellation Capricornus. Its leader alpha is a naked-eye double, the
two stars being a little more than 6' apart. Their magnitudes are three
and four, and both have a yellowish hue. The western star is alpha^1,
and is the fainter of the two. The other is designated as alpha^2. Both
are double. The components of alpha^1 are of magnitudes four and eight
and a half, distance 44", p. 220°. With the Washington twenty-six-inch
telescope a third star of magnitude fourteen has been found at a
distance of 40", p. 182°. In alpha^2 the magnitudes of the components
are three and ten and a half, distance 7.4", p. 150°. The smaller star
has a companion of the twelfth or thirteenth magnitude, distance 1.2",
p. 240°. This, of course, is hopelessly beyond our reach. Yet another
star of magnitude nine, distance 154", p. 156, we may see easily.
Dropping down to beta, we find it to be a most beautiful and easy
double, possessing finely contrasted colors, gold and blue. The larger
star is of magnitude three, and the smaller, the blue one, of magnitude
six, distance 205", p. 267°. Between them there is a very faint star
which larger telescopes than ours divide into two, each of magnitude
eleven and a half; separated 3", p. 325°.
Still farther south and nearly in a line drawn from alpha through beta
we find a remarkable group of double stars, sigma, pi, rho, and omicron.
The last three form a beautiful little triangle. We begin with sigma,
the faintest of the four. The magnitudes of its components are six and
nine, distance 54", p. 177°. In pi the magnitudes are five and nine,
distance 3.4", p. 145°; in rho, magnitudes five and eight, distance
3.8", p. 177° (a third star of magnitude seven and a half is seen at a
distance of 4', p. 150°); in omicron, magnitudes six and seven, distance
22", p. 240°.
The star cluster 4608 is small, yet, on a moonless night, worth a glance
with the five-inch.
[Illustration: MAP NO. 14.]
We now pass northward to the region covered by map No. 14, including the
remainder of Ophiuchus and Serpens. Beginning with the head of Serpens,
in the upper right-hand corner of the map, we find that beta, of
magnitude three and a half, has a ninth-magnitude companion, distance
30", p. 265°. The larger star is light blue and the smaller one
yellowish. The little star nu is double, magnitudes five and nine,
distance 50", p. 31°, colors contrasted but uncertain. In delta we find
a closer double, magnitudes three and four, distance 3.5", p. 190°. It
is a beautiful object for the three-inch. The leader of the
constellation, alpha, of magnitude two and a half, has a faint companion
of only the twelfth magnitude, distance 60", p. 350°. The small star is
bluish. The variable R has a period about a week short of one year, and
at maximum exceeds the sixth magnitude, although sinking at minimum to
less than the eleventh. Its color is ruddy.
Passing eastward, we turn again into Ophiuchus, and find immediately the
very interesting double, lambda, whose components are of magnitudes four
and six, distance 1", p. 55°. This is a long-period binary, and
notwithstanding the closeness of its stars, our four-inch should
separate them when the seeing is fine. We shall do better, however, to
try with the five-inch. Sigma 2166 consists of two stars of magnitudes
six and seven and a half, distance 27", p. 280°. Sigma 2173 is a double
of quite a different order. The magnitudes of its components are both
six, the distance in 1899 0.98", p. 331°. It is evidently a binary in
rapid motion, as the distance changed from about a quarter of a second
in 1881 to more than a second in 1894. The star tau is a fine triple,
magnitudes five, six, and nine, distances 1.8", p. 254°, and 100", p.
127°. The close pair is a binary system with a long period of
revolution, estimated at about two hundred years. We discover another
group of remarkable doubles in 67, 70, and 73. In the first-named star
the magnitudes are four and eight, distance 55", p. 144°, colors
finely contrasted, pale yellow and red.
Much more interesting, however, is 70, a binary whose components have
completed a revolution since their discovery by Sir William Herschel,
the period being ninety-five years. The magnitudes are four and six, or,
according to Hall, five and six, distance in 1894 2.3"; in 1900, 1.45",
according to Maw. Hall says the apparent distance when the stars are
closest is about 1.7", and when they are widest 6.7". This star is one
of those whose parallax has been calculated with a reasonable degree of
accuracy. Its distance from us is about 1,260,000 times the distance of
the sun, the average distance apart of the two stars is about
2,800,000,000 miles (equal to the distance of Neptune from the sun), and
their combined mass is three times that of the sun. Hall has seen in the
system of 70 Ophiuchi three stars of the thirteenth magnitude or less,
at distances of about 60", 90", and 165" respectively.
The star 73 is also a close double, and beyond our reach. Its magnitudes
are six and seven, distance 0.7", p. 245°. It is, no doubt, a binary.
Three star clusters in Ophiuchus remain to be examined. The first of
these, No. 4256, is partially resolved into stars by the five-inch. No.
4315 is globular, and has a striking environment of bystanding stars. It
is about one quarter as broad as the full moon, and our largest aperture
reveals the faint coruscation of its crowded components. No. 4410 is a
coarser and more scattered star swarm--a fine sight!
Farther toward the east we encounter a part of Serpens again, which
contains just one object worth glancing at, the double theta, whose
stars are of magnitudes four and four and a half, distance 21", p. 104°.
Color, both yellow, the smaller star having the deeper hue.
[Illustration: MAP NO. 15.]
Let us next, with the guidance of map No. 15, enter the rich star fields
of Hercules, and of the head and first coils of Draco. According to
Argelander, Hercules contains more stars visible to the naked eye than
any other constellation, and he makes the number of them one hundred and
fifty-five, nearly two thirds of which are only of the sixth magnitude.
But Heis, who saw more naked-eye stars than Argelander, makes Ursa Major
precisely equal to Hercules in the number of stars, his enumeration
showing two hundred and twenty-seven in each constellation, while,
according to him, Draco follows very closely after, with two hundred and
twenty stars. Yet, on account of the minuteness of the majority of their
stars, neither of these constellations makes by any means as brilliant a
display as does Orion, to which Argelander assigns only one hundred and
fifteen naked-eye stars, and Heis one hundred and thirty-six.
We begin in Hercules with the star kappa, a pretty little double of
magnitudes five and a half and seven, distance 31", p. 10°, colors
yellow and red. Not far away we find, in gamma, a larger star with a
fainter companion, the magnitudes in this case being three and a half
and nine, distance 38", p. 242°, colors white and faint blue or lilac.
One of the most beautiful of double stars is alpha Herculis. The
magnitudes are three and six, distance 4.7", p. 118°, colors orange and
green, very distinct. Variability has been ascribed to each of the stars
in turn. It is not known that they constitute a binary system, because
no certain evidence of motion has been obtained. Another very beautiful
and easily separated double is delta, magnitudes three and eight,
distance 19", p. 175°, colors pale green and purple.
Sweeping northwestward to zeta, we encounter a celebrated binary, to
separate which at present requires the higher powers of a six-inch
glass. The magnitudes are three and six and a half, distance in 1899,
0.6", p. 264°; in 1900, 0.8", p. 239°. The period of revolution is
thirty-five years, and two complete revolutions have been observed. The
apparent distance changes from 0.6" to 1.6". They were at their extreme
distance in 1884.
Two pleasing little doubles are Sigma 2101, magnitudes six and nine,
distance 4", p. 57°, and Sigma 2104, magnitudes six and eight, distance
6", p. 20°. At the northern end of the constellation is 42, a double
that requires the light-grasping power of our largest glass. Its
magnitudes are six and twelve, distance 20", p. 94°. In rho we discover
another distinctly colored double, both stars being greenish or bluish,
with a difference of tone. The magnitudes are four and five and a half,
distance 3.7", p. 309°. But the double 95 is yet more remarkable for the
colors of its stars. Their magnitudes are five and five and a half,
distance 6", p. 262°, colors, according to Webb, "light apple-green and
cherry-red." But other observers have noted different hues, one calling
them both golden yellow. I think Webb's description is more nearly
correct. Sigma 2215 is a very close double, requiring larger telescopes
than those we are working with. Its magnitudes are six and a half and
eight, distance 0.7", p. 300°. It is probably a binary. Sigma 2289 is
also close, but our five-inch will separate it: magnitudes six and
seven, distance 1.2", p. 230°.
Turning to , we have to deal with a triple, one of whose stars is at
present beyond the reach of our instruments. The magnitudes of the two
that we see are four and ten, distance 31", p. 243°. The tenth-magnitude
star is a binary of short period (probably less than fifty years), the
distance of whose components was 2" in 1859, 1" in 1880, 0.34" in 1889,
and 0.54" in 1891, when the position angle was 25°, and rapidly
increasing. The distance is still much less than 1".
For a glance at a planetary nebula we may turn with the five-inch to No.
4234. It is very small and faint, only 8" in diameter, and equal in
brightness to an eighth-magnitude star. Only close gazing shows that it
is not sharply defined like a star, and that it possesses a bluish tint.
Its spectrum is gaseous.
The chief attraction of Hercules we have left for the last, the famous
star cluster between eta and zeta, No. 4230, more commonly known as M
13. On a still evening in the early summer, when the moon is absent and
the quiet that the earth enjoys seems an influence descending from the
brooding stars, the spectacle of this sun cluster in Hercules, viewed
with a telescope of not less than five-inches aperture, captivates the
mind of the most uncontemplative observer. With the Lick telescope I
have watched it resolve into separate stars to its very center--a scene
of marvelous beauty and impressiveness. But smaller instruments reveal
only the in-running star streams and the sprinkling of stellar points
over the main aggregation, which cause it to sparkle like a cloud of
diamond dust transfused with sunbeams. The appearance of flocking
together that those uncountable thousands of stars present calls up at
once a picture of our lone sun separated from its nearest stellar
neighbor by a distance probably a hundred times as great as the entire
diameter of the spherical space within which that multitude is
congregated. It is true that unless we assume what would seem an
unreasonable remoteness for the Hercules cluster, its component stars
must be much smaller bodies than the sun; yet even that fact does not
diminish the wonder of their swarming. Here the imagination must bear
science on its wings, else science can make no progress whatever. It is
an easy step from Hercules to Draco. In the conspicuous diamond-shaped
figure that serves as a guide-board to the head of the latter, the
southernmost star belongs not to Draco but to Hercules. The brightest
star in this figure is gamma, of magnitude two and a half, with an
eleventh-magnitude companion, distant 125", p. 116°. Two stars of
magnitude five compose nu, their distance apart being 62", p. 312°. A
more interesting double is , magnitudes five and five, distance 2.4", p.
158°. Both stars are white, and they present a pretty appearance when
the air is steady. They form a binary system of unknown period. Sigma
2078 (also called 17 Draconis) is a triple, magnitudes six, six and a
half, and six, distances 3.8", p. 116°, and 90", p. 195°. Sigma 1984 is
an easy double, magnitudes six and a half and eight and a half, distance
6.4", p. 276°. The star eta is a very difficult double for even our
largest aperture, on account of the faintness of one of its components.
The magnitudes are two and a half and ten, distance 4.7", p. 140°. Its
near neighbor, Sigma 2054, may be a binary. Its magnitudes are six and
seven, distance 1", p. 0°. In Sigma 2323 we have another triple,
magnitudes five, eight and a half, and seven, distances 3.6", p. 360°,
and 90", p. 22°, colors white, blue, and reddish. A fine double is
epsilon, magnitudes five and eight, distance 3", p. 5°.
The nebula No. 4373 is of a planetary character, and interesting as
occupying the pole of the ecliptic. A few years ago Dr. Holden, with the
Lick telescope, discovered that it is unique in its form. It consists of
a double spiral, drawn out nearly in the line of sight, like the thread
of a screw whose axis lies approximately endwise with respect to the
observer. There is a central star, and another fainter star is involved
in the outer spiral. The form of this object suggests strange ideas as
to its origin. But the details mentioned are far beyond the reach of
our instruments. We shall only see it as a hazy speck. No. 4415 is
another nebula worth glancing at. It is Tuttle's so-called variable
nebula.
[Illustration: MAP NO. 16.]
There are three constellations represented on map No. 16 to which we
shall pay brief visits. First Aquila demands attention. Its doubles may
be summarized as follows: 11, magnitudes five and nine, distance 17.4",
p. 252°; pi, magnitudes six and seven, distance 1.6", p. 122°; 23,
magnitudes six and ten, distance 3.4", p. 12°--requires the five-inch
and good seeing; 57, magnitudes five and six, distance 36", p. 170°;
Sigma 2654, magnitudes six and eight, distance 12", p. 234°; Sigma 2644,
magnitudes six and seven, distance 3.6", p. 208°.
The star eta is an interesting variable between magnitudes three and a
half and 4.7; period, seven days, four hours, fourteen minutes. The
small red variable R changes from magnitude six to magnitude seven and a
half and back again in a period of three hundred and fifty-one days.
Star cluster No. 4440 is a striking object, its stars ranging from the
ninth down to the twelfth magnitude.
Just north of Aquila is the little constellation Sagitta, containing
several interesting doubles and many fine star fields, which may be
discovered by sweeping over it with a low-power eyepiece. The star zeta
is double, magnitudes five and nine, distance 8.6", p. 312°. The larger
star is itself double, but far too close to be split, except with very
large telescopes. In theta we find three components of magnitudes seven,
nine, and eight respectively, distances 11.4", p. 327°, and 70", p.
227°. A wide double is epsilon, magnitudes six and eight, distance 92",
p. 81°. Nebula No. 4572 is planetary.
Turning to Delphinus, we find a very beautiful double in gamma,
magnitudes four and five, distance 11", p. 273°, colors golden and
emerald. The leader alpha, which is not as bright as its neighbor beta,
and which is believed to be irregularly variable, is of magnitude four,
and has a companion of nine and a half magnitude at the distance 35", p.
278°. At a similar distance, 35", p. 335°, beta has an
eleventh-magnitude companion, and the main star is also double, but
excessively close, and much beyond our reach. It is believed to be a
swiftly moving binary, whose stars are never separated widely enough to
be distinguished with common telescopes.
CHAPTER VI
FROM LYRA TO ERIDANUS
"This Orpheus struck when with his wondrous song
He charmed the woods and drew the rocks along."--MANILIUS.
[Illustration: MAP NO. 17.]
We resume our celestial explorations with the little constellation Lyra,
whose chief star, Vega (alpha), has a very good claim to be regarded as
the most beautiful in the sky. The position of this remarkable star is
indicated in map No. 17. Every eye not insensitive to delicate shades of
color perceives at once that Vega is not white, but blue-white. When the
telescope is turned upon the star the color brightens splendidly.
Indeed, some glasses decidedly exaggerate the blueness of Vega, but the
effect is so beautiful that one can easily forgive the optical
imperfection which produces it. With our four-inch we look for the
well-known companion of Vega, a tenth-magnitude star, also of a blue
color deeper than the hue of its great neighbor. The distance is 50", p.
158°. Under the most favorable circumstances it might be glimpsed with
the three-inch, but, upon the whole, I should regard it as too severe a
test for so small an aperture.
Vega is one of those stars which evidently are not only enormously
larger than the sun (one estimate makes the ratio in this case nine
hundred to one), but whose physical condition, as far as the
spectroscope reveals it, is very different from that of our ruling orb.
Like Sirius, Vega displays the lines of hydrogen most conspicuously, and
it is probably a much hotter as well as a much more voluminous body
than the sun.
Close by, toward the east, two fourth-magnitude stars form a little
triangle with Vega. Both are interesting objects for the telescope, and
the northern one, epsilon, has few rivals in this respect. Let us first
look at it with an opera glass. The slight magnifying power of such an
instrument divides the star into two twinkling points. They are about
two and a quarter minutes of arc apart, and exceptionally sharp-sighted
persons are able to see them divided with the naked eye. Now take the
three-inch telescope and look at them, with a moderate power. Each of
the two stars revealed by the opera glass appears double, and a fifth
star of the ninth magnitude is seen on one side of an imaginary line
joining the two pairs. The northern-most pair is named epsilon_1, the
magnitudes being fifth and sixth, distance 3", p. 15°. The other pair is
epsilon_2, magnitudes fifth and sixth, distance 2.3", p. 133°. Each pair
is apparently a binary; but the period of revolution is unknown. Some
have guessed a thousand years for one pair, and two thousand for the
other. Another guess gives epsilon_1 a period of one thousand years, and
epsilon_2 a period of eight hundred years. Hall, in his double-star
observations, simply says of each, "A slow motion."
Purely by guesswork a period has also been assigned to the two pairs in
a supposed revolution around their common center, the time named being
about a million years. It is not known, however, that such a motion
exists. Manifestly it could not be ascertained within the brief period
during which scientific observations of these stars have been made. The
importance of the element of time in the study of stellar motions is
frequently overlooked, though not, of course, by those who are engaged
in such work. The sun, for instance, and many of the stars are known
to be moving in what appear to be straight lines in space, but
observations extending over thousands of years would probably show that
these motions are in curved paths, and perhaps in closed orbits.
If now in turn we take our four-inch glass, we shall see something else
in this strange family group of epsilon Lyræ. Between epsilon_1 and
epsilon_2, and placed one on each side of the joining line, appear two
exceedingly faint specks of light, which Sir John Herschel made famous
under the name of the _debillissima_. They are of the twelfth or
thirteenth magnitude, and possibly variable to a slight degree. If you
can not see them at first, turn your eye toward one side of the field of
view, and thus, by bringing their images upon a more sensitive part of
the retina, you may glimpse them. The sight is not much, yet it will
repay you, as every glance into the depths of the universe does.
The other fourth-magnitude star near Vega is zeta, a wide double,
magnitudes fourth and sixth, distance 44", p. 150°. Below we find beta,
another very interesting star, since it is both a multiple and an
eccentric variable. It has four companions, three of which we can easily
see with our three-inch; the fourth calls for the five-inch; the
magnitudes are respectively four, seven or under, eight, eight and a
half, and eleven; distances 45", p. 150°; 65", p. 320°; 85", p. 20°; and
46", p. 248°. The primary, beta, varies from about magnitude three and a
half to magnitude four and a half, the period being twelve days,
twenty-one hours, forty-six minutes, and fifty-eight seconds. Two
unequal maxima and minima occur within this period. In the spectrum of
this star some of the hydrogen lines and the D_3 line (the latter
representing helium, a constituent of the sun and of some of the stars,
which, until its recent discovery in a few rare minerals was not known
to exist on the earth) are bright, but they vary in visibility.
Moreover, dark lines due to hydrogen also appear in its spectrum
simultaneously with the bright lines of that element. Then, too, the
bright lines are sometimes seen double. Professor Pickering's
explanation is that beta Lyræ probably consists of two stars, which,
like the two composing beta Aurigæ, are too close to be separated with
any telescope now existing, and that the body which gives the bright
lines is revolving in a circle in a period of about twelve days and
twenty-two hours around the body which gives the dark lines. He has also
suggested that the appearances could be accounted for by supposing a
body like our sun to be rotating in twelve days and twenty-two hours,
and having attached to it an enormous protuberance extending over more
than one hundred and eighty degrees of longitude, so that when one end
of it was approaching us with the rotation of the star the other end
would be receding, and a splitting of the spectral lines at certain
periods would be the consequence. "The variation in light," he adds,
"may be caused by the visibility of a larger or smaller portion of this
protuberance."
Unfortunate star, doomed to carry its parasitical burden of hydrogen and
helium, like Sindbad in the clasp of the Old Man of the Sea! Surely, the
human imagination is never so wonderful as when it bears an astronomer
on its wings. Yet it must be admitted that the facts in this case are
well calculated to summon the genius of hypothesis. And the puzzle is
hardly simplified by Bélopolsky's observation that the body in beta Lyræ
giving dark hydrogen lines shows those lines also split at certain
times. It has been calculated, from a study of the phenomena noted
above, that the bright-line star in beta Lyræ is situated at a distance
of about fifteen million miles from the center of gravity of the
curiously complicated system of which it forms a part.
We have not yet exhausted the wonders of Lyra. On a line from beta to
gamma, and about one third of the distance from the former to the
latter, is the celebrated Ring Nebula, indicated on the map by the
number 4447. We need all the light we can get to see this object well,
and so, although the three-inch will show it, we shall use the
five-inch. Beginning with a power of one hundred diameters, which
exhibits it as a minute elliptical ring, rather misty, very soft and
delicate, and yet distinct, we increase the magnification first to two
hundred and finally to three hundred, in order to distinguish a little
better some of the details of its shape. Upon the whole, however, we
find that the lowest power that clearly brings out the ring gives the
most satisfactory view. The circumference of the ring is greater than
that of the planet Jupiter. Its ellipticity is conspicuous, the length
of the longer axis being 78" and that of the shorter 60". Closely
following the nebula as it moves through the field of view, our
five-inch telescope reveals a faint star of the eleventh or twelfth
magnitude, which is suspected of variability. The largest instruments,
like the Washington and the Lick glasses, have shown perhaps a dozen
other stars apparently connected with the nebula. A beautiful sparkling
effect which the nebula presents was once thought to be an indication
that it was really composed of a circle of stars, but the spectroscope
shows that its constitution is gaseous. Just in the middle of the open
ring is a feeble star, a mere spark in the most powerful telescope. But
when the Ring Nebula is photographed--and this is seen beautifully in
the photographs made with the Crossley reflector on Mount Hamilton by
the late Prof. J. E. Keeler--this excessively faint star imprints its
image boldly as a large bright blur, encircled by the nebulous ring,
which itself appears to consist of a series of intertwisted spirals.
Not far away we find a difficult double star, 17, whose components are
of magnitudes six and ten or eleven, distance 3.7", p. 325°.
From Lyra we pass to Cygnus, which, lying in one of the richest parts of
the Milky Way, is a very interesting constellation for the possessor of
a telescope. Its general outlines are plainly marked for the naked eye
by the figure of a cross more than twenty degrees in length lying along
the axis of the Milky Way. The foot of the cross is indicated by the
star beta, also known as Albireo, one of the most charming of all the
double stars. The three-inch amply suffices to reveal the beauty of this
object, whose components present as sharp a contrast of light yellow and
deep blue as it would be possible to produce artificially with the
purest pigments. The magnitudes are three and seven, distance 34.6", p.
55°. No motion has been detected indicating that these stars are
connected in orbital revolution, yet no one can look at them without
feeling that they are intimately related to one another. It is a sight
to which one returns again and again, always with undiminished pleasure.
The most inexperienced observer admires its beauty, and after an hour
spent with doubtful results in trying to interest a tyro in double stars
it is always with a sense of assured success that one turns the
telescope to beta Cygni.
Following up the beam of the imaginary cross along the current of the
Milky Way, every square degree of which is here worth long gazing into,
we come to a pair of stars which contend for the name-letter chi. On our
map the letter is attached to the southernmost of the two, a variable of
long period--four hundred and six days--whose changes of brilliance lie
between magnitudes four and thirteen, but which exhibits much
irregularity in its maxima. The other star, not named but easily
recognized in the map, is sometimes called 17. It is an attractive
double whose colors faintly reproduce those of beta. The magnitudes are
five and eight, distance 26", p. 73°. Where the two arms of the cross
meet is gamma, whose remarkable _cortége_ of small stars running in
curved streams should not be missed. Use the lowest magnifying power.
At the extremity of the western arm of the cross is delta, a close
double, difficult for telescopes of moderate aperture on account of the
difference in the magnitudes of the components. We may succeed in
dividing it with the five-inch. The magnitudes are three and eight,
distance 1.5", p. 310°. It is regarded as a binary of long and as yet
unascertained period.
In omicron^2 we find a star of magnitude four and orange in color,
having two blue companions, the first of magnitude seven and a half,
distance 107", p. 174°, and the second of magnitude five and a half,
distance 358", p. 324°. Farther north is psi, which presents to us the
combination of a white five-and-a-half-magnitude star with a lilac star
of magnitude seven and a half. The distance is 3", p. 184°. A very
pretty sight.
We now pass to the extremity of the other arm of the cross, near which
lies the beautiful little double 49, whose components are of magnitudes
six and eight, distance 2.8", p. 50°. The colors are yellow and blue,
conspicuous and finely contrasted. A neighboring double of similar hues
is 52, in which the magnitudes are four and nine, distance 6", p. 60°.
Sweeping a little way northward we come upon an interesting binary,
lambda, which is unfortunately beyond the dividing power of our largest
glass. A good seven-inch or seven-and-a-half-inch should split it under
favorable circumstances. Its magnitudes are six and seven, distance
0.66", p. 74°.
The next step carries us to a very famous object, 61 Cygni, long known
as the nearest star in the northern hemisphere of the heavens. It is a
double which our three-inch will readily divide, the magnitudes being
both six, distance 21", p. 122°. The distance of 61 Cygni, according to
Hall's parallax of 0.27", is about 70,000,000,000,000 miles. There is
some question whether or not it is a binary, for, while the twin stars
are both moving in the same direction in space with comparative
rapidity, yet conclusive evidence of orbital motion is lacking. When one
has noticed the contrast in apparent size between this comparatively
near-by star, which the naked eye only detects with considerable
difficulty, and some of its brilliant neighbors whose distance is so
great as to be immeasurable with our present means, no better proof will
be needed of the fact that the faintness of a star is not necessarily an
indication of remoteness.
We may prepare our eyes for a beautiful exhibition of contrasted colors
once more in the star . This is really a quadruple, although only two of
its components are close and conspicuous. The magnitudes are five, six,
seven and a half, and twelve; distances 2.4", p. 121°; 208", p. 56°; and
35", p. 264°. The color of the largest star is white and that of its
nearest companion blue; the star of magnitude seven and a half is also
blue.
The star cluster 4681 is a fine sight with our largest glass. In the map
we find the place marked where the new star of 1876 made its appearance.
This was first noticed on November 24, 1876, when it shone with the
brilliance of a star of magnitude three and a half. Its spectrum was
carefully studied, especially by Vogel, and the very interesting changes
that it underwent were noted. Within a year the star had faded to less
than the tenth magnitude, and its spectrum had completely changed in
appearance, and had come to bear a close resemblance to that of a
planetary nebula. This has been quoted as a possible instance of a
celestial collision through whose effects the solid colliding masses
were vaporized and expanded into a nebula. At present the star is very
faint and can only be seen with the most powerful telescopes. Compare
with the case of Nova Aurigæ, previously discussed.
Underneath Cygnus we notice the small constellation Vulpecula. It
contains a few objects worthy of attention, the first being the nebula
4532, the "dumb-bell nebula" of Lord Rosse. With the four-inch, and
better with the five-inch, we are able to perceive that it consists of
two close-lying tufts of misty light. Many stars surround it, and large
telescopes show them scattered between the two main masses of the
nebula. The Lick photographs show that its structure is spiral. The star
11 points out the place where a new star of the third magnitude appeared
in 1670. Sigma 2695 is a close double, magnitudes six and eight,
distance 0.96", p. 78°.
[Illustration: MAP NO. 18.]
We turn to map No. 18, and, beginning at the western end of the
constellation Aquarius, we find the variable T, which ranges between
magnitudes seven and thirteen in a period of about two hundred and three
days. Its near neighbor Sigma 2729 is a very close double, beyond the
separating power of our five-inch, the magnitudes being six and seven,
distance 0.6", p. 176°. Sigma 2745, also known as 12 Aquarii, is a good
double for the three-inch. Its magnitudes are six and eight, distance
2.8", p. 190°. In zeta we discover a beauty. It is a slow binary of
magnitudes four and four, distance 3.1", p. 321°. According to some
observers both stars have a greenish tinge. The star 41 is a wider
double, magnitudes six and eight, distance 5", p. 115°, colors yellow
and blue. The uncommon stellar contrast of white with light garnet is
exhibited by tau, magnitudes six and nine, distance 27", p. 115°. Yellow
and blue occur again conspicuously in psi, magnitudes four and a half
and eight and a half, distance 50", p. 310°. Rose and emerald have been
recorded as the colors exhibited in Sigma 2998, whose magnitudes are
five and seven, distance 1.3", p. 346°.
The variables S and R are both red. The former ranges between magnitudes
eight and twelve, period two hundred and eighty days, and the latter
between magnitudes six and eleven, period about three hundred and ninety
days.
The nebula 4628 is Rosse's "Saturn nebula," so called because with his
great telescope it presented the appearance of a nebulous model of the
planet Saturn. With our five-inch we see it simply as a planetary
nebula. We may also glance at another nebula, 4678, which appears
circular and is pinned with a little star at the edge.
The small constellation Equuleus contains a surprisingly large number of
interesting objects. Sigma 2735 is a rather close double, magnitudes six
and eight, distance 1.8", p. 287°. Sigma 2737 (the first star to the
left of Sigma 2735, the name having accidentally been omitted from the
map) is a beautiful triple, although the two closest stars, of
magnitudes six and seven, can not be separated by our instruments. Their
distance in 1886 was 0.78", p. 286°, and they had then been closing
rapidly since 1884, when the distance was 1.26". The third star, of
magnitude eight, is distant 11", p. 75°. Sigma 2744 consists of two
stars, magnitudes six and seven, distance 1.4", p. 1.67°. It is probably
a binary. Sigma 2742 is wider double, magnitudes both six, distance
2.6", p. 225°. Another triple, one of whose components is beyond our
reach, is gamma. Here the magnitudes are fifth, twelfth, and sixth,
distances 2", p. 274° and 366". It would also be useless for us to try
to separate delta, but it is interesting to remember that this is one of
the closest of known double stars, the magnitudes being fourth and
fifth, distance 0.4", p. 198°. These data are from Hall's measurements
in 1887. The star is, no doubt, a binary. With the five-inch we may
detect one and perhaps two of the companion stars in the quadruple beta.
The magnitudes are five, ten, and two eleven, distances 67", p. 309°;
86", p. 276°; and 6.5", p. 15°. The close pair is comprised in the
tenth-magnitude star.
[Illustration: MAP NO. 19.]
Map No. 19 introduces us to the constellation Pegasus, which is
comparatively barren to the naked eye, and by no means rich in
telescopic phenomena. The star epsilon, of magnitude two and a half, has
a blue companion of the eighth magnitude, distance 138", p. 324°; colors
yellow and violet. A curious experiment that may be tried with this star
is described by Webb, who ascribes the discovery of the phenomenon to
Sir John Herschel. When near the meridian the small star in epsilon
appears, in the telescope, underneath the large one. If now the tube of
the telescope be slightly swung from side to side the small star will
appear to describe a pendulumlike movement with respect to the large
one. The explanation suggested is that the comparative faintness of the
small star causes its light to affect the retina of the eye less quickly
than does that of its brighter companion, and, in consequence, the
reversal of its apparent motion with the swinging of the telescope is
not perceived so soon.
The third-magnitude star eta has a companion of magnitude ten and a
half, distance 90", p. 340°. The star beta, of the second magnitude, and
reddish, is variable to the extent of half a magnitude in an irregular
period, and gamma, of magnitude two and a half, has an
eleventh-magnitude companion, distance 162", p. 285°.
[Illustration: MAP NO. 20.]
Our interest is revived on turning, with the guidance of map No. 20,
from the comparative poverty of Pegasus to the spacious constellation
Cetus. The first double star that we meet in this constellation is 26,
whose components are of magnitudes six and nine, distance 16.4", p.
252°; colors, topaz and lilac. Not far away is the closer double 42,
composed of a sixth and a seventh magnitude star, distance 1.25", p.
350°. The four-inch is capable of splitting this star, but we shall do
better to use the five-inch. In passing we may glance at the
tenth-magnitude companion to eta, distance 225", p. 304°. Another wide
pair is found in zeta, magnitudes three and nine, distance 185", p. 40°.
The next step brings us to the wonderful variable omicron, or Mira,
whose changes have been watched for three centuries, the first observer
of the variability of the star having been David Fabricius in 1596. Not
only is the range of variability very great, but the period is
remarkably irregular. In the time of Hevelius, Mira was once invisible
for four years. When brightest, the star is of about the second
magnitude, and when faintest, of the ninth magnitude, but at maximum it
seldom exhibits the greatest brilliance that it has on a few occasions
shown itself capable of attaining. Ordinarily it begins to fade after
reaching the fourth or fifth magnitude. The period averages about three
hundred and thirty-one days, but is irregularly variable to the extent
of twenty-five days. Its color is red, and its spectrum shows bright
lines, which it is believed disappear when the star sinks to a minimum.
Among the various theories proposed to account for such changes as these
the most probable appears to be that which ascribes them to some cause
analogous to that operating in the production of sun spots. The
outburst of light, however, as pointed out by Scheiner, should be
regarded as corresponding to the maximum and not the minimum stage of
sun-spot activity. According to this view, the star is to be regarded as
possessing an extensive atmosphere of hydrogen, which, during the
maximum, is upheaved into enormous prominences, and the brilliance of
the light from these prominences suffices to swamp the photospheric
light, so that in the spectrum the hydrogen lines appear bright instead
of dark.
It is not possible to suppose that Mira can be the center of a system of
habitable planets, no matter what we may think of the more constant
stars in that regard, because its radiation manifestly increases more
than six hundred fold, and then falls off again to an equal extent once
in every ten or eleven months. I have met people who can not believe
that the Almighty would make a sun and then allow its energies "to go to
waste," by not supplying it with a family of worlds. But I imagine that
if they had to live within the precincts of Mira Ceti they would cry out
for exemption from their own law of stellar utility.
The most beautiful double star in Cetus is gamma, magnitudes three and
seven, distance 3", p. 288°; hues, straw-color and blue. The leading
star alpha, of magnitude two and a half, has a distant blue companion
three magnitudes fainter, and between them are two minute stars, the
southernmost of which is a double, magnitudes both eleven, distance 10",
p. 225°.
The variable S ranges between magnitudes seven and twelve in a somewhat
irregular period of about eleven months, while R ranges between the
seventh and the thirteenth magnitudes in a period of one hundred and
sixty-seven days.
[Illustration: MAP NO. 21.]
The constellation Eridanus, represented in map No. 21, contains a few
fine double stars, one of the most interesting of which is 12, a rather
close binary. The magnitudes are four and eight, distance 2", p. 327°.
We shall take the five-inch for this, and a steady atmosphere and sharp
seeing will be necessary on account of the wide difference in the
brightness of the component stars. Amateurs frequently fail to make due
allowance for the effect of such difference. When the limit of
separating power for a telescope of a particular aperture is set at 1"
or 2", as the case may be, it is assumed that the stars composing the
doubles on which the test is made shall be of nearly the same magnitude,
or at least that they shall not differ by more than one or two
magnitudes at the most. The stray light surrounding a comparatively
bright star tends to conceal a faint companion, although the telescope
may perfectly separate them so far as the stellar disks are concerned.
Then, too, I have observed in my own experience that a very faint and
close double is more difficult than a brighter pair not more widely
separated, usually on account of the defect of light, and this is true
even when the components of the faint double are of equal magnitude.
Sigma 470, otherwise known as 32 Eridani, is a superb object on account
of the colors of its components, the larger star being a rich topaz and
the smaller an ultramarine; while the difference in magnitude is not as
great as in many of the colored doubles. The magnitudes are five and
seven, distance 6.7", p. 348°. The star gamma, of magnitude two and a
half, has a tenth-magnitude companion, distant 51", p. 238°. Sigma 516,
also called 39 Eridani, consists of two stars of magnitudes six and
nine, distance 6.4", p. 150°; colors, yellow and blue. The supposed
binary character of this star has not yet been established.
In omicron^2 we come upon an interesting triple star, two of whose
components at any rate we can easily see. The largest component is of
the fourth magnitude. At a distance of 82", p. 105°, we find a
tenth-magnitude companion. This companion is itself double, the
magnitudes of its components being ten and eleven, distance 2.6", p.
98°. Hall says of these stars that they "form a remarkable system." He
has also observed a fourth star of the twelfth magnitude, distant 45"
from the largest star, p. 85°. This is apparently unconnected with the
others, although it is only half as distant as the tenth-magnitude
component is from the primary. Sigma 590 is interesting because of the
similarity of its two components in size, both being of about the
seventh magnitude, distance 10", p. 318°.
Finally, we turn to the nebula 826. This is planetary in form and
inconspicuous, but Lassell has described it as presenting a most
extraordinary appearance with his great reflector--a circular nebula
lying upon another fainter and larger nebula of a similar shape, and
having a star in its center. Yet it may possibly be an immensely distant
star cluster instead of a nebula, since its spectrum does not appear to
be gaseous.
CHAPTER VII
PISCES, ARIES, TAURUS, AND THE NORTHERN STARS
"Now sing we stormy skies when Autumn weighs
The year, and adds to nights and shortens days,
And suns declining shine with feeble rays."--DRYDEN'S VIRGIL.
[Illustration: MAP NO. 22.]
The eastern end of Pisces, represented in map No. 22, includes most of
the interesting telescopic objects that the constellation contains. We
begin our exploration at the star numbered 55, a double that is very
beautiful when viewed with the three-inch glass. The components are of
magnitudes five and eight, distance 6.6", p. 192°. The larger star is
yellow and the smaller deep blue. The star 65, while lacking the
peculiar charm of contrasted colors so finely displayed in 55, possesses
an attraction in the equality of its components which are both of the
sixth magnitude and milk-white. The distance is 4.5", p. 118°. In 66 we
find a swift binary whose components are at present far too close for
any except the largest telescopes. The distance in 1894 was only 0.36",
p. 329°. The magnitudes are six and seven. In contrast with this
excessively close double is psi, whose components are both of magnitude
five and a half, distance 30", p. 160°. Dropping down to 77 we come upon
another very wide and pleasing double, magnitudes six and seven,
distance 33", p. 82°, colors white and lilac or pale blue. Hardly less
beautiful is zeta magnitudes five and six, distance 24", p. 64°. Finest
of all is alpha, which exhibits a remarkable color contrast, the larger
star being greenish and the smaller blue. The magnitudes are four and
five, distance 3", p. 320°. This star is a binary, but the motion is
slow. The variable R ranges between magnitudes seven and thirteen,
period three hundred and forty-four days.
The constellation Aries contains several beautiful doubles, all but one
of which are easy for our smallest aperture. The most striking of these
is gamma, which is historically interesting as the first double star
discovered. The discovery was made by Robert Hooke in 1664 by accident,
while he was following the comet of that year with his telescope. He
expressed great surprise on noticing that the glass divided the star,
and remarked that he had not met with a like instance in all the
heavens. His observations could not have been very extensive or very
carefully conducted, for there are many double stars much wider than
gamma Arietis which Hooke could certainly have separated if he had
examined them. The magnitudes of the components of gamma are four and
four and a half, or, according to Hall, both four; distance 8.5", p.
180°. A few degrees above gamma, passing by beta, is a wide double
lambda, magnitudes five and eight, distance 37", p. 45°, colors white
and lilac or violet. Three stars are to be seen in 14: magnitudes five
and a half, ten, and nine, distances 83", p. 36°, and 106", p. 278°,
colors white, blue, and lilac. The star 30 is a very pretty double,
magnitudes six and seven, distance 38.6", p. 273°. Sigma 289 consists of
a topaz star combined with a sapphire, magnitudes six and nine, distance
28.5", p. 0°. The fourth-magnitude star 41 has several faint companions.
The magnitudes of two of these are eleven and nine, distances 34", p.
203°, and 130", p. 230°. We discover another triple in pi, magnitudes
five, eight, and eleven, distances 3.24", p. 122°, and 25", p. 110°. The
double mentioned above as being too close for our three-inch glass is
epsilon, which, however, can be divided with the four-inch, although the
five-inch will serve us better. The magnitudes are five and a half and
six, distance 1.26", p. 202°. The star 52 has two companions, one of
which is so close that our instruments can not separate it, while the
other is too faint to be visible in the light of its brilliant neighbor
without the aid of a very powerful telescope.
[Illustration: MAP NO. 23.]
We are now about to enter one of the most magnificent regions in the
sky, which is hardly less attractive to the naked eye than Orion, and
which men must have admired from the beginning of their history on the
earth, the constellation Taurus (map No. 23). Two groups of stars
especially distinguish Taurus, the Hyades and the Pleiades, and both are
exceedingly interesting when viewed with the lowest magnifying powers of
our telescopes.
We shall begin with a little star just west of the Pleiades, Sigma 412,
also called 7 Tauri. This is a triple, but we can see it only as a
double, the third star being exceedingly close to the primary. The
magnitudes are six and a half, seven, and ten, distances 0.3", p. 216°,
and 22", p. 62°. In the Pleiades we naturally turn to the brightest star
eta, or Alcyone, famous for having once been regarded as the central sun
around which our sun and a multitude of other luminaries were supposed
to revolve, and picturesque on account of the little triangle of small
stars near it which the least telescopic assistance enables us to see.
One may derive much pleasure from a study of the various groupings of
stars in the Pleiades. Photography has demonstrated, what had long been
suspected from occasional glimpses revealed by the telescope, that this
celebrated cluster of stars is intermingled with curious forms of
nebulæ. The nebulous matter appears in festoons, apparently attached to
some of the larger stars, such as Alcyone, Merope, and Maia, and in
long, narrow, straight lines, the most remarkable of which, a faintly
luminous thread starting midway between Maia and Alcyone and running
eastward some 40', is beaded with seven or eight stars. The width of
this strange nebulous streak is, on an average, 3" or 4", and there is,
perhaps, no more wonderful phenomenon anywhere in celestial space.
Unfortunately, no telescope is able to show it, and all our knowledge
about it is based upon photographs. It might be supposed that it was a
nebulous disk seen edgewise, but for the fact that at the largest star
involved in its course it bends sharply about 10° out of its former
direction, and for the additional fact that it seems to take its origin
from a curved offshoot of the intricate nebulous mass surrounding Maia.
Exactly at the point where this curve is transformed into a straight
line shines a small star! In view of all the facts the idea does not
seem to be very far-fetched that in the Pleiades we behold an assemblage
of suns, large and small, formed by the gradual condensation of a
nebula, and in which evolution has gone on far beyond the stage
represented by the Orion nebula, where also a group of stars may be in
process of formation out of nebulous matter. If we look a little farther
along this line of development, we may perceive in such a stellar
assemblage as the cluster in Hercules, a still later phase wherein all
the originally scattered material has, perhaps, been absorbed into the
starry nuclei.
[Illustration: THE CHIEF STARS IN THE PLEIADES.]
The yellow star Sigma 430 has two companions: magnitudes six, nine, and
nine and a half, distances 26", p. 55°, and 39", p. 302°. The star 30 of
the fifth magnitude has a companion of the ninth magnitude, distance 9",
p. 58°, colors emerald and purple, faint. An interesting variable, of
the type of Algol, is lambda, which at maximum is of magnitude three and
four tenths and at minimum of magnitude four and two tenths. Its period
from one maximum to the next is about three days and twenty-three hours,
but the actual changes occupy only about ten hours, and it loses light
more swiftly than it regains it. A combination of red and blue is
presented by Phi (mistakenly marked on map No. 23 as psi). The
magnitudes are six and eight, distance 56", p. 242°. A double of similar
magnitudes is chi, distance 19", p. 25°. Between the two stars which the
naked eye sees in kappa is a minute pair, each of less than the eleventh
magnitude, distance 5", p. 324°. Another naked-eye double is formed by
theta^1 and theta^2, in the Hyades. The magnitudes are five and five and
a half, distance about 5' 37".
The leading star of Taurus, Aldebaran (alpha), is celebrated for its
reddish color. The precise hue is rather uncertain, but Aldebaran is not
orange as Betelgeuse in Orion is, and no correct eye can for an instant
confuse the colors of these two stars, although many persons seem to be
unable to detect the very plain difference between them in this respect.
Aldebaran has been called "rose-red," and it would be an interesting
occupation for an amateur to determine, with the aid of some proper
color scale, the precise hue of this star, and of the many other stars
which exhibit chromatic idiosyncrasy. Aldebaran is further interesting
as being a standard first-magnitude star. With the four-inch glass we
see without difficulty the tenth-magnitude companion following Aldebaran
at a distance of 114", p. 35°. There is an almost inexplicable charm
about these faint attendants of bright stars, which is quite different
from the interest attaching to a close and nearly equal pair. The
impression of physical relationship is never lacking though it may be
deceptive, and this awakens a lively appreciation of the vast
differences of magnitude that exist among the different suns of space.
The actual size and might of this great red sun form an attractive
subject for contemplation. As it appears to our eyes Aldebaran gives one
twenty-five-thousand-millionth as much light as the sun, but if we were
placed midway between them the star would outshine the sun in the ratio
of not less than 160 to 1. And yet, gigantic as it is, Aldebaran is
possibly a pygmy in comparison with Arcturus, whose possible dimensions
were discussed in the chapter relating to Boötes. Although Aldebaran is
known to possess several of the metallic elements that exist in the sun,
its spectrum differs widely from the solar spectrum in some respects,
and more closely resembles that of Arcturus.
Other interesting objects in Taurus are sigma, divisible with the naked
eye, magnitudes five and five and a half, distance 7'; Sigma 674,
double, magnitudes six and nine, distance 10.5", p. 147°; Sigma 716,
double, magnitudes six and seven, distance 5", p. 200°--a pleasing
sight; tau, triple, magnitudes four, ten and a half, and eleven,
distances 36", p. 249°, and 36", p. 60°--the ten-and-a-half-magnitude
star is itself double, as discovered by Burnham; star cluster No. 1030,
not quite as broad as the moon, and containing some stars as large as
the eleventh magnitude; and nebula No. 1157, the so-called "Crab nebula"
of Lord Rosse, which our glasses will show only as a misty patch of
faint light, although large telescopes reveal in it a very curious
structure.
[Illustration: MAP NO. 24.]
We now turn to the cluster of circumpolar constellations sometimes
called the Royal Family, in allusion to the well-known story of the
Ethiopian king Cepheus and his queen Cassiopeia, whose daughter
Andromeda was exposed on the seashore to be devoured by a monster, but
who was saved by the hero Perseus. All these mythologic personages are
represented in the constellations that we are about to study.[4] We
begin with Andromeda (map No. 24). The leading star alpha marks one
corner of the great square of Pegasus. The first star of telescopic
interest that we find in Andromeda is , a double difficult on account of
the faintness of the smaller component. The magnitudes are four and
eleven, distance 49", p. 110°. A few degrees north of the naked eye
detects a glimmering point where lies the Great Nebula in Andromeda.
This is indicated on the map by the number 116. With either of our three
telescopes it is an interesting object, but of course it is advisable to
use our largest glass in order to get as much light as possible. All
that we can see is a long, shuttle-shaped nebulous object, having a
brighter point near the center. Many stars are scattered over the field
in its neighborhood, but the nebula itself, although its spectrum is
peculiar in resembling that of a faint star, is evidently a gaseous or
at any rate a meteoritic mass, since photographs show it to be composed
of a series of imperfectly separated spirals surrounding a vast central
condensation. This peculiarity of the Andromeda nebula, which is
invisible with telescopes although conspicuous in the photographs, has,
since its discovery a few years ago, given a great impetus to
speculation concerning the transformation of nebulæ into stars and star
clusters. No one can look at a good photograph of this wonderful
phenomenon without noticing its resemblance to the ideal state of things
which, according to the nebular hypothesis, must once have existed in
the solar system. It is to be remembered, however, that there is
probably sufficient material in the Andromeda nebula to make a system
many times, perhaps hundreds or thousands of times, as extensive as that
of which our sun is the center. If one contemplates this nebula only
long enough to get a clear perception of the fact that creation was not
ended when, according to the Mosaic history, God, having in six days
finished "the heavens and the earth and all the host of them," rested
from all his work, a good blow will have been dealt for the cause of
truth. Systems far vaster than ours are now in the bud, and long before
they have bloomed, ambitious man, who once dreamed that all these things
were created to serve him, will probably have vanished with the
extinguishment of the little star whose radiant energy made his life and
his achievements briefly possible.
[4] For further details on this subject see Astronomy with an
Opera-glass.
In August, 1885, a new star of magnitude six and a half made its
appearance suddenly near the center of the Andromeda nebula. Within one
year it had disappeared, having gradually dwindled until the great
Washington telescope, then the largest in use, no longer showed it. That
this was a phenomenon connected with the nebula is most probable, but
just what occurred to produce it nobody knows. The observed appearances
might have been produced by a collision, and no better hypothesis has
yet been suggested to account for them.
Near the opposite end of the constellation from alpha we find the most
interesting of triple stars in gamma. The two larger components of this
beautiful star are of magnitudes three and six, distance 10", colors
golden yellow and deep blue. The three-inch shows them finely. The
smaller star is itself double, its companion being of magnitude eight,
distance when discovered in 1842 0.5", color bluish green. A few years
ago this third star got so close to its primary that it was invisible
even with the highest powers of the great Lick telescope, but at present
it is widening again. In October, 1893, I had the pleasure of looking at
gamma Andromedæ with the Lick telescope, and at that time it was
possible just to separate the third star. The angle seemed too small for
certain measurement, but a single setting of the micrometer by Mr.
Barnard, to whose kindness I was indebted for my view of the star, gave
0.17" as the approximate distance. In 1900 the distance had increased to
0.4", p. 115°. The brilliance of color contrast between the two larger
stars of gamma Andromedæ is hardly inferior to that exhibited in beta
Cygni, so that this star may be regarded as one of the most picturesque
of stellar objects for small telescopes.
Other pleasing objects in this constellation are the binary star 36,
magnitudes six and six and a half, distance 1", p. 17°--the two stars
are slowly closing and the five-inch glass is required to separate them:
the richly colored variable R, which fades from magnitude five and a
half to invisibility, and then recovers its light in a period of about
four hundred and five days; and the bright star cluster 457, which
covers a space about equal to the area of the full moon.
Just south of the eastern end of Andromeda is the small constellation
Triangulum, or the Triangles, containing two interesting objects. One of
these is the beautiful little double 6, magnitudes five and six,
distance 3.8", p. 77°, colors yellow and blue; and the other, the nebula
352, which equals in extent the star cluster in Andromeda described
above, but nevertheless appears very faint with our largest glass. Its
faintness, however, is not an indication of insignificance, for to very
powerful telescopes it exhibits a wonderful system of nuclei and
spirals--another bit of chaos that is yielding by age-long steps to the
influence of demiurgic forces.
A richer constellation than Andromeda, both for naked-eye and telescopic
observation, is Perseus, which is especially remarkable for its star
clusters. Two of these, 512 and 521, constitute the celebrated double
cluster, sometimes called the Sword-hand of Perseus, and also chi
Persei. To the smallest telescope this aggregation of stars, ranging in
magnitude from six and a half to fourteen, and grouped about two
neighboring centers, presents a marvelous appearance. As an educative
object for those unaccustomed to celestial observations it may be
compared among star clusters to beta Cygni among double stars, for the
most indifferent spectator is struck with wonder in viewing it. All the
other clusters in Perseus represented on the map are worth examining,
although none of them calls for special mention, except perhaps 584,
where we may distinguish at least a hundred separate stars within an
area less than one quarter as expansive as the face of the moon.
Among the double stars of Perseus we note first eta, whose components
are of magnitudes four and eight, distance 28", colors white and pale
blue. The double epsilon is especially interesting on account of an
alleged change of color from blue to red which the smaller star
undergoes coincidently with a variation of brightness. The magnitudes
are three and eight, distance 9", p. 9°. An interesting multiple is
zeta, two of whose stars at least we can see. The magnitudes are three,
nine, ten, and ten, distances 13", p. 207°, 90", and 112".
The chief attraction in Perseus is the changeful and wonderful beta, or
Algol, the great typical star among the short-period variables. During
the greater part of its period this star is of magnitude two and two
tenths, but for a very short time, following a rapid loss of light, it
remains at magnitude three and seven tenths. The difference, one
magnitude and a half, corresponds to an actual difference in brightness
in the ratio of 3.75 to 1. The entire loss of light during the
declension occupies only four hours and a half. The star remains at its
faintest for a few minutes only before a perceptible gain of light
occurs, and the return to maximum is as rapid as was the preceding
decline. The period from one minimum to the next is two days twenty
hours forty-eight minutes fifty-three seconds, with an irregularity
amounting to a few seconds in a year. The Arabs named the star Algol, or
the Demon, on account of its eccentricity which did not escape their
attention; and when Goodricke, in 1782, applied a scientific method of
observation to it, the real cause of its variations was suggested by
him, but his explanation failed of general acceptance until its truth
was established by Prof. E. C. Pickering in 1880. This explanation gives
us a wonderful insight into stellar constitution. According to it, Algol
possesses a companion as large as the sun, but invisible, both because
of its proximity to that star and because it yields no light, and
revolving in a plane horizontal to our line of sight. The period of
revolution is identical with the period of Algol's cycle of variation,
and the diminution of light is caused by the interposition of the dark
body as it sweeps along that part of its orbit lying between our point
of view and the disk of Algol. In other words, once in every two days
twenty hours and forty-nine minutes Algol, as seen from the earth,
undergoes a partial eclipse.
In consequence of the great comparative mass of its dark companion,
Algol itself moves in an orbit around their common center with a
velocity quite sufficient to be detected by the shifting of the lines in
its spectrum. By means of data thus obtained the mass, size, and
distance apart of Algol and its singular comrade have been inferred. The
diameter of Algol is believed to be about 1,125,000 miles, that of the
dark body about 840,000 miles, and the mean distance from center to
center 3,230,000 miles. The density of both the light and the dark star
is slight compared with that of the sun, so that their combined mass is
only two thirds as great as the sun's.
Mention has been made of a slight irregularity in Algol's period of
variation. Basing his calculations upon this inequality, Dr. Chandler
has put forward the hypothesis that there is another invisible body
connected with Algol, and situated at a distance from it of about
1,800,000,000 miles, and that around this body, which is far more
massive than the others, Algol and its companions revolve in a period of
one hundred and thirty years! Dr. Chandler has earned the right to have
his hypotheses regarded with respect, even when they are as
extraordinary as that which has just been described. It needs no
indulgence of the imagination to lend interest to Algol; the simple
facts are sufficient. How did that bright star fall in with its black
neighbors? Or were they created together?
[Illustration: MAP NO. 25.]
Passing to the region covered by map No. 25, our eyes are caught by the
curious figure, formed by the five brightest stars of the constellation
Cassiopeia, somewhat resembling the letter W. Like Perseus, this is a
rich constellation, both in star clusters and double stars. Among the
latter we select as our first example sigma, in which we find a
combination of color that is at once very unusual and very
striking--green and blue. The magnitudes are five and seven, distance
3", p. 324°. Another beautiful colored double is eta, whose magnitudes
are four and seven and a half, distance 5", p. 200°, colors white and
purple. This is one of the comparatively small number of stars the
measure of whose distance has been attempted, and a keen sense of the
uncertainty of such measures is conveyed by the fact that authorities of
apparently equal weight place eta Cassiopeiæ at such discordant
distances as 124,000,000,000,000 miles, 70,000,000,000,000 miles, and
42,000,000,000,000 miles. It will be observed that the difference
between the greatest and the least of these estimates is about double
the entire distance given by the latter. The same thing is practically
true of the various attempts to ascertain the distance of the other
stars which have a perceptible parallax, even those which are evidently
the nearest. In some cases the later measures increase the distance, in
other cases they diminish it; in no case is there anything like a
complete accord. Yet of course we are not to infer that it is hopeless
to learn anything about the distances of the stars. With all their
uncertainties and disagreements the few parallaxes we possess have laid
a good foundation for a knowledge of the dimensions of at least the
nearer parts of the universe.
We find an interesting triple in psi, the magnitudes of the larger
components being four and a half and eight and a half, distance 30". The
smaller star has a nine-and-a-half-magnitude companion, distance 3". A
more beautiful triple is iota, magnitudes four, seven, and eight,
distances 2", p. 256°, and 7.5", p. 112°. Cassiopeia contains many
star clusters, three of which are indicated in the map. Of these 392 is
perhaps the most interesting, as it includes stars of many magnitudes,
among which are a red one of the eighth magnitude, and a ninth-magnitude
double whose components are 8" apart. Not far from the star kappa we
find the spot where the most brilliant temporary star on record made its
appearance on November 11, 1572. Tycho Brahe studied this phenomenon
during the entire period of its visibility, which lasted until March,
1574. It burst out suddenly with overpowering splendor, far outshining
every fixed star, and even equaling Venus at her brightest. In a very
short time it began to fade, regularly diminishing in brightness, and at
the same time undergoing changes of color, ending in red, until it
disappeared. It has never been seen since, and the suspicion once
entertained that it was a variable with a period considerably exceeding
three hundred years has not been confirmed. There is a tenth-magnitude
star near the place given by Tycho as that occupied by the stranger.
Many other faint stars are scattered about, however, and Tycho's
measures were not sufficiently exact to enable us to identify the
precise position of his star. If the phenomenon was due to a collision,
no reappearance of the star is to be expected.
Camelopardalus is a very inconspicuous constellation, yet it furnishes
considerable occupation for the telescope. Sigma 390, of magnitude five,
has a companion of magnitude nine and a half, distance 15", 160°. Sigma
385, also of the fifth magnitude, has a ninth-magnitude companion,
distance only 2.4", p. 160°. According to some observers, the larger
star is yellow and the smaller white. The star 1 is a very pretty
double, magnitudes both six, distance 10.4". Its neighbor 2 of magnitude
six has an eighth-magnitude companion, distance 1.7", p. 278°. The star
7 of magnitude five is also double, the companion of magnitude eight
being distant only 1.2". A glance at star cluster 940, which shows a
slight central condensation, completes our work in Camelopardalus, and
we turn to Ursa Major, represented in map No. 26. Here there are many
interesting doubles and triples. Beginning with iota we find at once
occupation for our largest glass. The magnitudes are three and ten,
distance 10", p. 357°. In the double star 23 the magnitudes are four and
nine, distance 23", p. 272°. A more pleasing object is sigma^2, a
greenish fifth-magnitude star which has an eighth-magnitude companion,
distance 2.6", p. 245°. A good double for our four-inch glass is xi,
whose magnitudes are four and five, distance 1.87", p. 183°. This is a
binary with a period of revolution of about sixty years, and is
interesting as the first binary star whose orbit was determined. Savary
calculated it in 1828. Near by is nu, a difficult double, magnitudes
four and ten and a half, distance 7", p. 147°. In 57 we find again an
easy double magnitudes six and eight, distance 5.5", p. 4°. Another
similar double is 65, magnitudes six and eight, distance 3.9", p. 38°. A
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