_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 Aurigae (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 a.s.sociated 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 surpa.s.sing 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 surpa.s.s 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 cla.s.s, 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.

[Ill.u.s.tration: 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. CLa.s.sES 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 cla.s.sification. 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 cla.s.sification 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 Cl.u.s.tERS.--In previous chapters we have noted the Pleiades and Praesepe as star cl.u.s.ters 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 cl.u.s.ter in Hercules, whose appearance in a telescope of moderate size is shown in Fig. 135, while Fig. 136 is a photograph of the same cl.u.s.ter taken with a very large reflecting telescope. This is only a type of many telescopic cl.u.s.ters 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 cl.u.s.ter. Within an area which could be covered by a third of the full moon's face are crowded in this cl.u.s.ter 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 cl.u.s.ter, 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.

[Ill.u.s.tration: FIG 135.--Star cl.u.s.ter in Hercules.]

An interesting discovery of recent date, made by Professor Bailey in photographing star cl.u.s.ters, 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 cl.u.s.ters, 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 cl.u.s.ter, 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."

[Ill.u.s.tration: FIG. 136.--Star cl.u.s.ter in Hercules.--KEELER.]

[Ill.u.s.tration: FIG. 137.--The Andromeda nebula as seen in a very small telescope.]

[Ill.u.s.tration: FIG. 138.--The Andromeda nebula and Holmes's comet.

Photographed by BARNARD.]

[Ill.u.s.tration: FIG. 139.--A drawing of the Andromeda nebula.]

[Ill.u.s.tration: FIG. 140.--A photograph of the Andromeda nebula.--ROBERTS.]

214. NEBULae.--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 cl.u.s.ter 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 cl.u.s.ter. In many cases, however, this is not possible, and we have an _irresolvable cl.u.s.ter_ 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 gla.s.s 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 nebulae 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 pa.s.sing 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.

[Ill.u.s.tration: FIG. 141.--Types of nebulae.]

A comparison of these several representations of the same thing will serve to ill.u.s.trate the vagueness of its outlines, and how much the impressions to be derived from nebulae 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).

[Ill.u.s.tration: FIG. 142.--The Trifid nebula.--KEELER.]

215. TYPICAL NEBULae.--Some of the fantastic forms which nebulae 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 nebulae, 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 a.s.sociated with the star ? Ophiuchi.

[Ill.u.s.tration: FIG. 145.--Great nebula about the star ?

Ophiuchi.--BARNARD.]

Probably the most impressive of all nebulae is the great one in Orion (Fig. 146), whose position is shown on the star map between Rigel and ? Orionis. Look for it with an opera gla.s.s 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.

[Ill.u.s.tration: FIG. 146.--The Orion nebula.]

The Pleiades (Plate IV) present a case of a resolvable star cl.u.s.ter 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 cl.u.s.ter and not merely a background for it.

[Ill.u.s.tration: THE PLEIADES (AFTER A PHOTOGRAPH)]

Fig. 147 shows a series of so-called double nebulae perhaps comparable with double stars, although the most recent photographic work seems to indicate that they are really faint spiral nebulae in which only the brightest parts are shown by the telescope.

According to Keeler, the spiral is the prevailing type of nebulae, 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.

[Ill.u.s.tration: FIG. 147.--Double nebulae. HERSCHEL.]

Another type of nebula of some consequence presents in the telescope round disks like those of Ura.n.u.s or Neptune, and this appearance has given them the name _planetary nebulae_. The comet in Fig. 138, if smaller, would represent fairly well the nebulae 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 NEBULae.--A star cl.u.s.ter, 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 nebulae 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 nebulae all have spectra of this kind and make up about half of all the known gaseous nebulae. It is worthy of note that a century ago Sir William Herschel had observed a green shimmer in the light of certain nebulae 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 nebulae.

In faint nebulae this line const.i.tutes the whole of their visible spectrum, but in brighter ones two or three other and fainter lines are usually a.s.sociated 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 nebulae, and there are indications of one or two other known substances.

217. DENSITY OF NEBULae.--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 a.s.sume that this shining gas is all that exists in the nebula; so far as telescope or camera are concerned, there may be a.s.sociated 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 nebulae.

But, whatever may be a.s.sociated 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 s.p.a.ce 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 ma.s.s by its attraction would exert a sensible effect upon the motion of the sun through s.p.a.ce. 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 ma.s.s 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 NEBULae.--The extreme minuteness of their proper motions is a characteristic feature of all nebulae. 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 s.p.a.ce, a part at least is real motion of the nebulae themselves. These seem to move through the celestial s.p.a.ces 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 cl.u.s.ter.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: FIG. 149.--The Milky Way near ? Ophiuchi.--BARNARD.]

219. THE MILKY WAY.--Closely related to nebulae and star cl.u.s.ters 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.

[Ill.u.s.tration: FIG. 150.--The Milky Way near 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 and ? Cygni, nearly opposite Vega, and note how even an opera gla.s.s 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 s.p.a.ce 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.