STARS AND NEBULÆ
209. STELLAR COLORS.--We have already seen that one star differs from
another in respect of color as well as brightness, and the diligent
student of the sky will not fail to observe for himself how the luster
of Sirius and Rigel is more nearly a pure white than is that of any
other stars in the heavens, while at the other end of the scale
[a] Orionis and Aldebaran are strongly ruddy, and Antares presents an
even deeper tone of red. Between these extremes the light of every star
shows a mixture of the rainbow hues, in which a very pale yellow is the
predominant color, shading off, as we have seen, to white at one end of
the scale and red at the other. There are no green stars, or blue stars,
or violet stars, save in one exceptional class of cases--viz., where the
two components of a double star are of very different brightness, it is
quite the usual thing for them to have different colors, and then,
almost without exception, the color of the fainter star lies nearer to
the violet end of the spectrum than does the color of the bright one,
and sometimes shows a distinctly blue or green hue. A fine type of such
double star is [b] Cygni, in which the components are respectively
yellow and blue, and the yellow star furnishes eight times as much light
as the blue one.
The exception which double stars thus make to the general rule of
stellar colors, yellow and red, but no color of shorter wave length, has
never been satisfactorily explained, but the rule itself presents no
difficulties. Each star is an incandescent body, giving off radiant
energy of every wave length within the limits of the visible spectrum,
and, indeed, far beyond these limits. If this radiant energy could come
unhindered to our eyes every star would appear white, but they are all
surrounded by atmospheres--analogous to the chromosphere and reversing
layer of the sun--which absorb a portion of their radiant energy and,
like the earth's atmosphere, take a heavier toll from the violet than
from the red end of the spectrum. The greater the absorption in the
star's atmosphere, therefore, the feebler and the ruddier will be its
light, and corresponding to this the red stars are as a class fainter
than the white ones.
210. CHEMISTRY OF THE STARS.--The spectroscope is pre-eminently the
instrument to deal with this absorption of light in the stellar
atmospheres, just as it deals with that absorption in the sun's
atmosphere to which are due the dark lines of the solar spectrum,
although the faintness of starlight, compared with that of the sun,
presents a serious obstacle to its use. Despite this difficulty most of
the lucid stars and many of the telescopic ones have been studied with
the spectroscope and found to be similar to the sun and the earth as
respects the material of which they are made. Such familiar chemical
elements as hydrogen and iron, carbon, sodium, and calcium are scattered
broadcast throughout the visible universe, and while it would be
unwarranted by the present state of knowledge to say that the stars
contain nothing not found in the earth and the sun, it is evident that
in a broad way their substance is like rather than unlike that composing
the solar system, and is subject to the same physical and chemical laws
which obtain here. Galileo and Newton extended to the heavens the
terrestrial sciences of mathematics and mechanics, but it remained to
the nineteenth century to show that the physics and chemistry of the sky
are like the physics and chemistry of the earth.
211. STELLAR SPECTRA.--When the spectra of great numbers of stars are
compared one with another, it is found that they bear some relation to
the colors of the stars, as, indeed, we should expect, since spectrum
and color are both produced by the stellar atmospheres, and it is found
useful to classify these spectra into three types, as follows:
_Type I. Sirian stars._--Speaking generally, the stars which are white
or very faintly tinged with yellow, furnish spectra like that of Sirius,
from which they take their name, or that of [b] Aurigæ (Fig. 124), which
is a continuous spectrum, especially rich in energy of short wave
length--i. e., violet and ultraviolet light, and is crossed by a
relatively small number of heavy dark lines corresponding to the
spectrum of hydrogen. Sometimes, however, these lines are much fainter
than is here shown, and we find associated with them still other faint
ones pointing to the presence of other metallic substances in the star's
atmosphere. These metallic lines are not always present, and sometimes
even the hydrogen lines themselves are lacking, but the spectrum is
always rich in violet and ultraviolet light.
Since with increasing temperature a body emits a continually increasing
proportion of energy of short wave length (§ 118), the richness of these
spectra in such energy points to a very high temperature in these stars,
probably surpassing in some considerable measure that of the sun. Stars
with this type of spectrum are more numerous than all others combined,
but next to them in point of numbers stands--
_Type II. Solar stars._--To this type of spectrum belong the yellow
stars, which show spectra like that of the sun, or of Pollux (Fig. 125).
These are not so rich in violet light as are those of Type I, but in
complexity of spectrum and in the number of their absorption lines they
far surpass the Sirian stars. They are supposed to be at a lower
temperature than the Sirian stars, and a much larger number of chemical
elements seems present and active in the reversing layer of their
atmospheres. The strong resemblance which these spectra bear to that of
the sun, together with the fact that most of the sun's stellar neighbors
have spectra of this type, justify us in ranking both them and it as
members of one class, called _solar stars_.
_Type III. Red stars._--A small number of stars show spectra comparable
with that of [a] Herculis (Fig. 134), in which the blue and the violet
part of the spectrum is almost obliterated, and the remaining yellow and
red parts show not only dark lines, but also numerous broad dark bands,
sharp at one edge, and gradually fading out at the other. It is this
_selective absorption_, extinguishing the blue and leaving the red end
of the spectrum, which produces the ruddy color of these stars, while
the bands in their spectra "are characteristic of chemical combinations,
and their presence ... proves that at certain elevations in the
atmospheres of these stars the temperature has sunk so low that chemical
combinations can be formed and maintained" (Scheiner-Frost). One of the
chemical compounds here indicated is a hydrocarbon similar to that found
in comets. In the white and yellow stars the temperatures are so high
that the same chemical elements, although present, can not unite one
with another to form compound substances.
[Illustration: FIG. 134.--The spectrum of [a] Herculis.--ESPIN.]
Most of the variable stars are red and have spectra of the third type;
but this does not hold true for the eclipse variables like Algol, all of
which are white stars with spectra of the first type. The ordinary
variable star is therefore one with a dense atmosphere of relatively low
temperature and complex structure, which produces the prevailing red
color of these stars by absorbing the major part of their radiant
energy of short wave length while allowing the longer, red waves to
escape. Although their exact nature is not understood, there can be
little doubt that the fluctuation in the light of these stars is due to
processes taking place within the star itself, but whether above or
below its photosphere is still uncertain.
212. CLASSES OF STARS.--There is no hard-and-fast dividing line between
these types of stellar spectra, but the change from one to another is by
insensible gradations, like the transition from youth to manhood and
from manhood to old age, and along the line of transition are to be
found numberless peculiarities and varieties of spectra not enumerated
above--e. g., a few stars show not only dark absorption lines in their
spectra but bright lines as well, which, like those in Fig. 48, point to
the presence of incandescent vapors, even in the outer parts of their
atmospheres. Among the lucid stars about 75 per cent have spectra of the
first type, 23 per cent are of the second type, 1 per cent of the third
type, and the remaining 1 per cent are peculiar or of doubtful
classification. Among the telescopic stars it is probable that much the
same distribution holds, but in the present state of knowledge it is not
prudent to speak with entire confidence upon this point.
That the great number of stars whose spectra have been studied should
admit of a classification so simple as the above, is an impressive fact
which, when supplemented by the further fact of a gradual transition
from one type of spectrum to the next, leaves little room for doubt that
in the stars we have an innumerable throng of individuals belonging to
the same species but in different stages of development, and that the
sun is only one of these individuals, of something less than medium size
and in a stage of development which is not at all peculiar, since it is
shared by nearly a fourth of all the stars.
213. STAR CLUSTERS.--In previous chapters we have noted the Pleiades and
Præsepe as star clusters visible to the naked eye, and to them we may
add the Hyades, near Aldebaran, and the little constellation Coma
Berenices. But more impressive than any of these, although visible only
in a telescope, is the splendid cluster in Hercules, whose appearance in
a telescope of moderate size is shown in Fig. 135, while Fig. 136 is a
photograph of the same cluster taken with a very large reflecting
telescope. This is only a type of many telescopic clusters which are
scattered over the sky, and which are made up of stars packed so closely
together as to become indistinguishable, one from another, at the center
of the cluster. Within an area which could be covered by a third of the
full moon's face are crowded in this cluster more than five thousand
stars which are unquestionably close neighbors, but whose apparent
nearness to each other is doubtless due to their great distance from us.
It is quite probable that even at the center of this cluster, where more
than a thousand stars are included within a radius of 160", the actual
distances separating adjoining stars are much greater than that
separating earth and sun, but far less than that separating the sun from
its nearest stellar neighbor.
[Illustration: FIG 135.--Star cluster in Hercules.]
An interesting discovery of recent date, made by Professor Bailey in
photographing star clusters, is that some few of them, which are
especially rich in stars, contain an extraordinary number of variable
stars, mostly very faint and of short period. Two clusters, one in the
northern and one in the southern hemisphere, contain each more than a
hundred variables, and an even more extraordinary case is presented by
a cluster, called Messier 5, not far from the star [a] Serpentis,
which contains no less than sixty-three variables, all about of the
fourteenth magnitude, all having light periods which differ but little
from half a day, all having light curves of about the same shape, and
all having a range of brightness from maximum to minimum of about one
magnitude. An extraordinary set of coincidences which "points
unmistakably to a common origin and cause of variability."
[Illustration: FIG. 136.--Star cluster in Hercules.--KEELER.]
[Illustration: FIG. 137.--The Andromeda nebula as seen in a very small
telescope.]
[Illustration: FIG. 138.--The Andromeda nebula and Holmes's comet.
Photographed by BARNARD.]
[Illustration: FIG. 139.--A drawing of the Andromeda nebula.]
[Illustration: FIG. 140.--A photograph of the Andromeda
nebula.--ROBERTS.]
214. NEBULÆ.--Returning to Fig. 136, we note that its background has a
hazy appearance, and that at its center the stars can no longer be
distinguished, but blend one with another so as to appear like a bright
cloud. The outer part of the cluster is _resolved_ into stars, while in
the picture the inner portion is not so resolved, although in the
original photographic plate the individual stars can be distinguished to
the very center of the cluster. In many cases, however, this is not
possible, and we have an _irresolvable cluster_ which it is customary to
call a _nebula_ (Latin, _little cloud_).
The most conspicuous example of this in the northern heavens is the
great nebula in Andromeda (R. A. 0^{h} 37^{m}, Dec. + 41°), which may be
seen with the naked eye as a faint patch of foggy light. Look for it.
This appears in an opera glass or very small telescope not unlike Fig.
137, which is reproduced from a sketch. Fig. 138 is from a photograph of
the same object showing essentially the same shape as in the preceding
figure, but bringing out more detail. Note the two small nebulæ
adjoining the large one, and at the bottom of the picture an object
which might easily be taken for another nebula but which is in fact a
tailless comet that chanced to be passing that part of the sky when the
picture was taken. Fig. 139 is from another drawing of this nebula,
although it is hardly to be recognized as a representation of the same
thing; but its characteristic feature, the two dark streaks near the
center of the picture, is justified in part by Fig. 140, which is from a
photograph made with a large reflecting telescope.
[Illustration: FIG. 141.--Types of nebulæ.]
A comparison of these several representations of the same thing will
serve to illustrate the vagueness of its outlines, and how much the
impressions to be derived from nebulæ depend upon the telescopes
employed and upon the observer's own prepossessions. The differences
among the pictures can not be due to any change in the nebula itself,
for half a century ago it was sketched much as shown in the latest of
them (Fig. 140).
[Illustration: FIG. 142.--The Trifid nebula.--KEELER.]
215. TYPICAL NEBULÆ.--Some of the fantastic forms which nebulæ present
in the telescope are shown on a small scale in Fig. 141, but in recent
years astronomers have learned to place little reliance upon drawings
such as these, which are now almost entirely supplanted by photographs
made with long exposures in powerful telescopes. One of the most
exquisite of these modern photographs is that of the Trifid nebula in
Sagittarius (Fig. 142). Note especially the dark lanes that give to this
nebula its name, Trifid, and which run through its brightest parts,
breaking it into seemingly independent sections. The area of the sky
shown in this cut is about 15 per cent less than that covered by the
full moon.
[Illustration: FIG. 143.--A nebula in Cygnus.--KEELER.]
Fig. 143 shows a very different type of nebula, found in the
constellation Cygnus, which appears made up of filaments closely
intertwined, and stretches across the sky for a distance considerably
greater than the moon's diameter.
[Illustration: FIG. 144.--Spiral nebula in Canes Venatici.--KEELER.]
A much smaller but equally striking nebula is that in the constellation
Canes Venatici (Fig. 144), which shows a most extraordinary spiral
structure, as if the stars composing it were flowing in along curved
lines toward a center of condensation. The diameter of the circular part
of this nebula, omitting the projection toward the bottom of the
picture, is about five minutes of arc, a sixth part of the diameter of
the moon, and its thickness is probably very small compared with its
breadth, perhaps not much exceeding the width of the spiral streams
which compose it. Note how the bright stars that appear within the area
of this nebula fall on the streams of nebulous matter as if they were
part of them. This characteristic grouping of the stars, which is
followed in many other nebulæ, shows that they are really part and
parcel of the nebula and not merely on line with it. Fig. 145 shows how
a great nebula is associated with the star [r] Ophiuchi.
[Illustration: FIG. 145.--Great nebula about the star [r]
Ophiuchi.--BARNARD.]
Probably the most impressive of all nebulæ is the great one in Orion
(Fig. 146), whose position is shown on the star map between Rigel and
[z] Orionis. Look for it with an opera glass or even with the unaided
eye. This is sometimes called an _amorphous_--i. e., shapeless--nebula,
because it presents no definite form which the eye can grasp and little
trace of structure or organization. It is "without form and void" at
least in its central portions, although on its edges curved filaments
may be traced streaming away from the brighter parts of the central
region. This nebula, as shown in Fig. 146, covers an area about equal
to that of the full moon, without counting as any part of this the
companion nebula shown at one side, but photographs made with suitable
exposures show that faint outlying parts of the nebula extend in curved
lines over the larger part of the constellation Orion. Indeed, over a
large part of the entire sky the background is faintly covered with
nebulous light whose brighter portions, if each were counted as a
separate nebula, would carry the total number of such objects well into
the hundreds of thousands.
[Illustration: FIG. 146.--The Orion nebula.]
The Pleiades (Plate IV) present a case of a resolvable star cluster
projected against such a nebulous background whose varying intensity
should be noted in the figure. A part of this nebulous matter is shown
in wisps extending from one star to the next, after the fashion of a
bridge, and leaving little doubt that the nebula is actually a part of
the cluster and not merely a background for it.
[Illustration: THE PLEIADES (AFTER A PHOTOGRAPH)]
Fig. 147 shows a series of so-called double nebulæ perhaps comparable
with double stars, although the most recent photographic work seems to
indicate that they are really faint spiral nebulæ in which only the
brightest parts are shown by the telescope.
According to Keeler, the spiral is the prevailing type of nebulæ, and
while Fig. 144 presents the most perfect example of such a nebula, the
student should not fail to note that the Andromeda nebula (Fig. 140)
shows distinct traces of a spiral structure, only here we do not see its
true shape, the nebula being turned nearly edgewise toward us so that
its presumably circular outline is foreshortened into a narrow ellipse.
[Illustration: FIG. 147.--Double nebulæ. HERSCHEL.]
Another type of nebula of some consequence presents in the telescope
round disks like those of Uranus or Neptune, and this appearance has
given them the name _planetary nebulæ_. The comet in Fig. 138, if
smaller, would represent fairly well the nebulæ of this type. Sometimes
a planetary nebula has a star at its center, and sometimes it appears
hollow, like a smoke ring, and is then called a ring nebula. The most
famous of these is in the constellation Lyra, not far from Vega.
216. SPECTRA OF NEBULÆ.--A star cluster, like the one in Hercules,
shows, of course, stellar spectra, and even when irresolvable the
spectrum is a continuous one, testifying to the presence of stars,
although they stand too close together to be separately seen. But in a
certain number of nebulæ the spectrum is altogether different, a
discontinuous one containing only a few bright lines, showing that here
the nebular light comes from glowing gases which are subject to no
considerable pressure. The planetary nebulæ all have spectra of this
kind and make up about half of all the known gaseous nebulæ. It is
worthy of note that a century ago Sir William Herschel had observed a
green shimmer in the light of certain nebulæ which led him to believe
that they were "not of a starry nature," a conclusion which has been
abundantly confirmed by the spectroscope. The green shimmer is, in fact,
caused by a line in the green part of the spectrum that is always
present and is always the brightest part of the spectrum of gaseous
nebulæ.
In faint nebulæ this line constitutes the whole of their visible
spectrum, but in brighter ones two or three other and fainter lines are
usually associated with it, and a very bright nebula, like that in
Orion, may show a considerable number of extra lines, but for the most
part they can not be identified in the spectrum of any terrestrial
substances. An exception to this is found in the hydrogen lines, which
are well marked in most spectra of gaseous nebulæ, and there are
indications of one or two other known substances.
217. DENSITY OF NEBULÆ.--It is known from laboratory experiments that
diminishing the pressure to which an incandescent gas is subject,
diminishes the number of lines contained in its spectrum, and we may
surmise from the very simple character and few lines of these nebular
spectra that the gas which produces them has a very small density. But
this is far from showing that the nebula itself is correspondingly
attenuated, for we must not assume that this shining gas is all that
exists in the nebula; so far as telescope or camera are concerned, there
may be associated with it any amount of dark matter which can not be
seen because it sends to us no light. It is easy to think in this
connection of meteoric dust or the stuff of which comets are made, for
these seem to be scattered broadcast on every side of the solar system
and may, perchance, extend out to the region of the nebulæ.
But, whatever may be associated in the nebula with the glowing gas which
we see, the total amount of matter, invisible as well as visible, must
be very small, or rather its average density must be very small, for the
space occupied by such a nebula as that of Orion is so great that if the
average density of its matter were equal to that of air the resulting
mass by its attraction would exert a sensible effect upon the motion of
the sun through space. The brighter parts of this nebula as seen from
the earth subtend an angle of about half a degree, and while we know
nothing of its distance from us, it is easy to see that the farther it
is away the greater must be its real dimensions, and that this increase
of bulk and mass with increasing distance will just compensate the
diminishing intensity of gravity at great distances, so that for a given
angular diameter--e. g., half a degree--the force with which this nebula
attracts the sun depends upon its density but not at all upon its
distance. Now, the nebula must attract the sun in some degree, and must
tend to move it and the planets in an orbit about the attracting center
so that year after year we should see the nebula from slightly different
points of view, and this changed point of view should produce a change
in the apparent direction of the nebula from us--i. e., a proper motion,
whose amount would depend upon the attracting force, and therefore upon
the density of the attracting matter. Observations of the Orion nebula
show that its proper motion is wholly inappreciable, certainly far less
than half a second of arc per year, and corresponding to this amount of
proper motion the mean density of the nebula must be some millions of
times (10^{10} according to Ranyard) less than that of air at sea
level--i. e., the average density throughout the nebula is comparable
with that of those upper parts of the earth's atmosphere in which
meteors first become visible.
218. MOTION OF NEBULÆ.--The extreme minuteness of their proper motions
is a characteristic feature of all nebulæ. Indeed, there is hardly a
known case of sensible proper motion of one of these bodies, although a
dozen or more of them show velocities in the line of sight ranging in
amount from +30 to -40 miles per second, the plus sign indicating an
increasing distance. While a part of these velocities may be only
apparent and due to the motion of earth and sun through space, a part at
least is real motion of the nebulæ themselves. These seem to move
through the celestial spaces in much the same way and with the same
velocities as do the stars, and their smaller proper motions across the
line of sight (angular motions) are an index of their great distance
from us. No one has ever succeeded in measuring the parallax of a nebula
or star cluster.
[Illustration: FIG. 148.--A part of the Milky Way.]
The law of gravitation presumably holds sway within these bodies, and
the fact that their several parts and the stars which are involved
within them, although attracted by each other, have shown little or no
change of position during the past century, is further evidence of
their low density and feeble attraction. In a few cases, however, there
seem to be in progress within a nebula changes of brightness, so that
what was formerly a faint part has become a brighter one, or _vice
versa_; but, on the whole, even these changes are very small.
[Illustration: FIG. 149.--The Milky Way near [th] Ophiuchi.--BARNARD.]
219. THE MILKY WAY.--Closely related to nebulæ and star clusters is
another feature of the sky, the _galaxy_ or _Milky Way_, with whose
appearance to the unaided eye the student should become familiar by
direct study of the thing itself. Figs. 148 and 149 are from photographs
of two small parts of it, and serve to bring out the small stars of
which it is composed. Every star shown in these pictures is invisible to
the naked eye, although their combined light is easily seen. The general
course of the galaxy across the heavens is shown in the star maps, but
these contain no indication of the wealth of detail which even the naked
eye may detect in it. Bright and faint parts, dark rifts which cut it
into segments, here and there a hole as if the ribbon of light had been
shot away--such are some of the features to be found by attentive
examination.
[Illustration: FIG. 150.--The Milky Way near [b] Cygni.--BARNARD.]
Speaking generally, the course of the Milky Way is a great circle
completely girdling the sky and having its north pole in the
constellation Coma Berenices. The width of this stream of light is very
different in different parts of the heavens, amounting where it is
widest, in Lyra and Cygnus, to something more than 30°, although its
boundaries are too vague and ill defined to permit much accuracy of
measurement. Observe the very bright part between [b] and [g] Cygni,
nearly opposite Vega, and note how even an opera glass will partially
resolve the nebulous light into a great number of stars, which are here
rather brighter than in other parts of its course. But the resolution
into stars is only partial, and there still remains a background of
unresolved shimmer. Fig. 150 is a photograph of a small part of this
region in which, although each fleck of light represents a separate
star, the galaxy is not completely resolved. Compare with this region,
rich in stars, the nearly empty space between the branches of the galaxy
a little west of Altair. Another hole in the Milky Way may be found a
little north and east of [a] Cygni, and between the extremes of
abundance and poverty here noted there may be found every gradation of
nebulous light.
The Milky Way is not so simple in its structure as might at first be
thought, but a clear and moonless night is required to bring out its
details. The nature of these details, the structure of the galaxy, its
shape and extent, the arrangement of its parts, and their relation to
stars and nebulæ in general, have been subjects of much speculation by
astronomers and others who have sought to trace out in this way what is
called the _construction of the heavens_.
220. DISTRIBUTION OF THE STARS.--How far out into space do the stars
extend? Are they limited or infinite in number? Do they form a system of
mutually related parts, or are they bunched promiscuously, each for
itself, without reference to the others? Here is what has been well
called "the most important problem of stellar astronomy, the acquisition
of well-founded ideas about the distribution of the stars." While many
of the ideas upon this subject which have been advanced by eminent
astronomers and which are still current in the books are certainly
wrong, and few of their speculations along this line are demonstrably
true, the theme itself is of such grandeur and permanent interest as to
demand at least a brief consideration. But before proceeding to its
speculative side we need to collect facts upon which to build, and
these, however inadequate, are in the main simple and not far to seek.
Parallaxes, proper motions, motions in the line of sight, while
pertinent to the problem of stellar distribution, are of small avail,
since they are far too scanty in number and relate only to limited
classes of stars, usually the very bright ones or those nearest to the
sun. Almost the sole available data are contained in the brightness of
the stars and the way in which they seem scattered in the sky. The most
casual survey of the heavens is enough to show that the stars are not
evenly sprinkled upon it. The lucid stars are abundant in some regions,
few in others, and the laborious star gauges, actual counting of the
stars in sample regions of the sky, which have been made by the
Herschels, Celoria, and others, suffice to show that this lack of
uniformity in distribution is even more markedly true of the telescopic
stars.
The rate of increase in the number of stars from one magnitude to the
next, as shown in § 187, is proof of another kind of irregularity in
their distribution. It is not difficult to show, mathematically, that if
in distant regions of space the stars were on the average as numerous
and as bright as they are in the regions nearer to the sun, then the
stars of any particular magnitude ought to be four times as numerous as
those of the next brighter magnitude--e. g., four times as many
sixth-magnitude stars as there are fifth-magnitude ones. But, as we have
already seen in § 187, by actual count there are only three times as
many, and from the discrepancy between these numbers, an actual
threefold increase instead of a fourfold one, we must conclude that on
the whole the stars near the sun are either bigger or brighter or more
numerous than in the remoter depths of space.
221. THE STELLAR SYSTEM.--But the arrangement of the stars is not
altogether lawless and chaotic; there are traces of order and system,
and among these the Milky Way is the dominant feature. Telescope and
photographic plate alike show that it is made up of stars which,
although quite irregularly scattered along its course, are on the
average some twenty times as numerous in the galaxy as at its poles,
and which thin out as we recede from it on either side, at first rapidly
and then more slowly. This tendency to cluster along the Milky Way is
much more pronounced among the very faint telescopic stars than among
the brighter ones, for the lucid stars and the telescopic ones down to
the tenth or eleventh magnitude, while very plainly showing the
clustering tendency, are not more than three times as numerous in the
galaxy as in the constellations most remote from it. It is remarkable as
showing the condensation of the brightest stars that one half of all the
stars in the sky which are brighter than the second magnitude are
included within a belt extending 12° on either side of the center line
of the galaxy.
In addition to this general condensation of stars toward the Milky Way,
there are peculiarities in the distribution of certain classes of stars
which are worth attention. Planetary nebulæ and new stars are seldom, if
ever, found far from the Milky Way, and stars with bright lines in their
spectra especially affect this region of the sky. Stars with spectra of
the first type--Sirian stars--are much more strongly condensed toward
the Milky Way than are stars of the solar type, and in consequence of
this the Milky Way is peculiarly rich in light of short wave lengths.
Resolvable star clusters are so much more numerous in the galaxy than
elsewhere, that its course across the sky would be plainly indicated by
their grouping upon a map showing nothing but clusters of this kind.
On the other hand, nebulæ as a class show a distinct aversion for the
galaxy, and are found most abundantly in those parts of the sky farthest
from it, much as if they represented raw material which was lacking
along the Milky Way, because already worked up to make the stars which
are there so numerous.
222. RELATION OF THE SUN TO THE MILKY WAY.--The fact that the galaxy is
a _great circle_ of the sky, but only of moderate width, shows that it
is a widely extended and comparatively thin stratum of stars within
which the solar system lies, a member of the galactic system, and
probably not very far from its center. This position, however, is not to
be looked upon as a permanent one, since the sun's motion, which lies
nearly in the plane of the Milky Way, is ceaselessly altering its
relation to the center of that system, and may ultimately carry us
outside its limits.
The Milky Way itself is commonly thought to be a ring, or series of
rings, like the coils of the great spiral nebula in Andromeda, and
separated from us by a space far greater than the thickness of the ring
itself. Note in Figs. 149 and 150 how the background is made up of
bright and dark parts curiously interlaced, and presenting much the
appearance of a thin sheet of cloud through which we look to barren
space beyond. While, mathematically, this appearance can not be
considered as proof that the galaxy is in fact a distant ring, rather
than a sheet of starry matter stretching continuously from the nearer
stellar neighbors of the sun into the remotest depths of space,
nevertheless, most students of the question hold it to be such a ring of
stars, which are relatively close together while its center is
comparatively vacant, although even here are some hundreds of thousands
of stars which on the whole have a tendency to cluster near its plane
and to crowd together a little more densely than elsewhere in the region
where the sun is placed.
223. DIMENSIONS OF THE GALAXY.--The dimensions of this stellar system
are wholly unknown, but there can be no doubt that it extends farther in
the plane of the Milky Way than at right angles to that plane, for stars
of the fifteenth and sixteenth magnitudes are common in the galaxy, and
testify by their feeble light to their great distance from the earth,
while near the poles of the Milky Way there seem to be few stars fainter
than the twelfth magnitude. Herschel, with his telescope of 18 inches
aperture, could count in the Milky Way more than a dozen times as many
stars per square degree as could Celoria with a telescope of 4 inches
aperture; but around the poles of the galaxy the two telescopes showed
practically the same number of stars, indicating that here even the
smaller telescope reached to the limits of the stellar system. Very
recently, indeed, the telescope with which Fig. 140 was photographed
seems to have reached the farthest limit of the Milky Way, for on a
photographic plate of one of its richest regions Roberts finds it
completely resolved into stars which stand out upon a black background
with no trace of nebulous light between them.
224. BEYOND THE MILKY WAY.--Each additional step into the depths of
space brings us into a region of which less is known, and what lies
beyond the Milky Way is largely a matter of conjecture. We shrink from
thinking it an infinite void, endless emptiness, and our intellectual
sympathies go out to Lambert's speculation of a universe filled with
stellar systems, of which ours, bounded by the galaxy, is only one.
There is, indeed, little direct evidence that other such systems exist,
but the Andromeda nebula is not altogether unlike a galaxy with a
central cloud of stars, and in the southern hemisphere, invisible in our
latitudes, are two remarkable stellar bodies like the Milky Way in
appearance, but cut off from all apparent connection with it, much as we
might expect to find independent stellar systems, if such there be.
These two bodies are known as the Magellanic clouds, and individually
bear the names of Major and Minor Nubecula. According to Sir John
Herschel, "the Nubecula Major, like the Minor, consists partly of large
tracts and ill-defined patches of irresolvable nebula, and of nebulosity
in every stage of resolution up to perfectly resolved stars like the
Milky Way, as also of regular and irregular nebulæ ... of globular
clusters in every stage of resolvability, and of clustering groups
sufficiently insulated and condensed to come under the designation of
clusters of stars." Its outlines are vague and somewhat uncertain, but
surely include an area of more than 40 square degrees--i. e., as much as
the bowl of the Big Dipper--and within this area Herschel counted
several hundred nebulæ and clusters "which far exceeds anything that is
to be met with in any other region of the heavens." Although its
excessive complexity of detail baffled Herschel's attempts at artistic
delineation, it has yielded to the modern photographic processes, which
show the Nubecula Major to be an enormous spiral nebula made up of
subordinate stars, nebulæ, and clusters, as is the Milky Way.
Compared with the Andromeda nebula, its greater angular extent suggests
a smaller distance, although for the present all efforts at determining
the parallax of either seem hopeless. But the spiral form which is
common to both suggests that the Milky Way itself may be a gigantic
spiral nebula near whose center lies the sun, a humble member of a great
cluster of stars which is roughly globular in shape, but flattened at
the poles of the galaxy and completely encircled by its coils. However
plausible such a view may appear, it is for the present, at least, pure
hypothesis, although vigorously advocated by Easton, who bases his
argument upon the appearance of the galaxy itself.
225. ABSORPTION OF STARLIGHT.--We have had abundant occasion to learn
that at least within the confines of the solar system meteoric matter,
cosmic dust, is profusely scattered, and it appears not improbable that
the same is true, although in smaller degree, in even the remoter parts
of space. In this case the light which comes from the farther stars over
a path requiring many centuries to travel, must be in some measure
absorbed and enfeebled by the obstacles which it encounters on the way.
Unless celestial space is transparent to an improbable degree the
remoter stars do not show their true brightness; there is a certain
limit beyond which no star is able to send its light, and beyond which
the universe must be to us a blank. A lighthouse throws into the fog its
beams only to have them extinguished before a single mile is passed, and
though the celestial lights shine farther, a limit to their reach is
none the less certain if meteoric dust exists outside the solar system.
If there is such an absorption of light in space, as seems plausible,
the universe may well be limitless and the number of stellar systems
infinite, although the most attenuated of dust clouds suffices to
conceal from us and to shut off from our investigation all save a minor
fraction of it and them.