Astronomy: The Science of the Heavenly Bodies Part 24

Dealing with the very important question whether the two streams are actually intermingled in s.p.a.ce, Eddington finds them nearly at the same mean distance and thoroughly intermingled, and there is no possible hypothesis of Drifts I and II pa.s.sing one behind the other in the same line of sight. A third drift, to which all the Orion stars belong, is under investigation, together with comprehensive a.n.a.lysis of the drifts according to the spectral type of all the stars included.

The farther research on star-streaming is pushed, the more it becomes evident that a third stream, called Drift O, is necessary, especially to include B-type stars. The farther we recede from the sun, the more this drift is in evidence. At the average distances of B-type stars, the observed motions are almost completely represented by Drift O alone.

Halm of Cape Town concludes from recent investigations that the double-drift phenomena (Drifts I and II) is of a distinctly _local_ character, and concerns chiefly the stars in the vicinity of the solar system; while stars at the greatest distances from the sun belong preeminently to Drift O.

The 60-inch reflector on Mount Wilson gathers sufficient light so that the spectra of very faint stars can be photographed, and a discussion of velocities derived in this manner has shown that Kapteyn's two star streams extend into s.p.a.ce much farther than it was possible to trace them with the nearer stars. Star-streaming, then, may be a phenomenon of the widest significance in reference to the entire universe.

As to the fundamental causes for the two opposite and nearly equal star streams, it is early perhaps to even theorize upon the subject.

Eddington, however, finds a possible explanation in the spiral nebulae, which are so numerous as to indicate the certainty of an almost universal law compelling matter to flow in these forms. Why it does so, we cannot be said to know; but obviously matter is either flowing into the nucleus from the branches of the spiral, or it is flowing out from the nucleus into the branches. Which of the two directions does not matter, because in either case there would be currents of matter in opposite directions at the points where the arms merge in the central aggregation. The currents continue through the center, because the stars do not interfere with one another's paths. As Eddington concludes: "There then we have an explanation of the prevalence of motions to and fro in a particular straight line; it is the line from which the spiral branches start out. The two star streams and the double-branched spirals arise from the same cause."

CHAPTER LVII

THE GALAXY OR MILKY WAY

Grandest of all the problems that have occupied the mind of man is the distribution of the stars throughout s.p.a.ce. To the earliest astronomers who knew nothing about the distances of the stars, it was not much of a problem because they thought all the fixed stars were attached to a revolving sphere, and therefore all at essentially the same distance; a very moderate distance, too. Even Kepler held the idea that the distances of individual stars from each other are much less than their distances from our sun.

Thomas Wright, of Durham, England, seems to have been the first to suggest the modern theory of the structure of the stellar universe, about the middle of the eighteenth century. His idea was taken up by Kant who elaborated it more fully. It is founded on the Galaxy, the basal plane of stellar distribution, just as the ecliptic is the fundamental circle of reference in the solar system.

What is the Galaxy or Milky Way?

Here is a great poet's view of the most poetic object in all nature:

A broad and ample road, whose dust is gold, And pavement stars, as stars to thee appear Seen in the Galaxy, that Milky Way Which nightly as a circling zone thou seest Powder'd with stars.

_Milton, P. L._ vii, 580.

Were the earth transparent as crystal, so that we could see downward through it and outward in all directions to the celestial sphere, the Galaxy or Milky Way would appear as a belt or zone of cloudlike luminosity extending all the way round the heavens. As the horizon cuts the celestial sphere in two, we see at anyone time only one-half of the Milky Way, spanning the dome of the sky as a cloudlike arch.

As the general plane of the Galaxy makes a large angle with our equator, the Milky Way is continually changing its angle with the horizon, so that it rises at different elevations. One-half of the Milky Way will always be below our horizon, and a small region of it lies so near the south pole of the heavens that it can never be seen from medium northern lat.i.tudes.

Galileo was the first to explain the fundamental mystery of this belt, when he turned his telescope upon it and found that it was not a continuous sheet of faint light, as it seemed to be, but was made up of countless numbers of stars, individually too faint to be visible to the naked eye, but whose vast number, taken in the aggregate, gave the well-known effect which we see in the sky. In some regions, as Perseus, the stars are more numerous than in others, and they are gathered in close cl.u.s.ters. The larger the telescope we employ, the greater the number of stars that are seen as we approach the Galaxy on either side; and the farther we recede from the Galaxy and approach either of its poles fewer and fewer stars are found. Indeed, if all the stars visible in a 12-inch telescope could be conceived as blotted out, nearly all the stars that are left would be found in the Galaxy itself.

The naked eye readily notes the variations in breadth and brightness of the galactic zone. Nearly a third of it, from Scorpio to Cygnus, is split into two divisions nearly parallel. In many regions its light is interrupted, especially in Centaurus, where a dark starless region exists, known as the "coal sack." Sir John Herschel, who followed up the stellar researches of his father, Sir William, in great detail, places the north pole of the Galactic plane in declination 37 degrees N., and right ascension 12 h. 47 m. This makes the plane of the Milky Way lie at an angle of about 60 degrees with the ecliptic, which it intersects not far from the solstices.

Now Kant, in view of the two great facts about the Galaxy known in his time, (1) that it wholly encircles the heavens, and (2) that it is composed of countless stars too faint to be individually visible to the naked eye, drew the safe conclusions that the system of the stars must extend much farther in the direction of the Milky Way than in other directions.

This theory of Kant was next investigated from an observational standpoint by Sir William Herschel, the ultimate goal of whose researches was always a knowledge of the construction of the heavens.

The present conclusion is that we may regard the stellar bodies of the sidereal universe as scattered, without much regard to uniformity, throughout a vast s.p.a.ce having in general the shape of a thick watch, its thickness being perhaps one-tenth its diameter. On both sides of this disk of stars, and cl.u.s.tered about the poles of the sidereal system are the regions occupied by vast numbers of nebulae. The entire visible universe, then, would be spheroidal in general shape. The plane of the Milky Way pa.s.ses through the middle of this aggregation of stars and nebulae, and the solar system is near the center of the Milky Way.

Throughout the watch-form s.p.a.ce the stars are cl.u.s.tered irregularly, in varied and sometimes fantastic forms, but without approach to order or system. If we except some of the star groups and star cl.u.s.ters and consider only the naked-eye stars, we find them scattered with fair approach to uniformity.

[Ill.u.s.tration: STAR CLOUDS AND BLACK HOLES IN SAGITTARIUS. The dark rifts and lanes resemble those in the nearby Milky Way. (_Photo, Yerkes Observatory._)]

[Ill.u.s.tration: THE GREAT NEBULA OF ANDROMEDA, LARGEST (APPARENTLY) OF ALL THE SPIRAL NEBULae. This nebula can be seen very faintly with the naked eye, but no telescope has yet resolved it into separate stars. (_Photo, Yerkes Observatory._)]

The watch-shaped disk is not to be understood as representing the actual form of the stellar system, but only in general the limits within which it is for the most part contained.

A vigorous attack on the problem of the evolution and structure of the stellar universe as a whole is now being conducted by cooperation of many observatories in both hemispheres. It is known as the Kapteyn "Plan of Selected Areas," embracing 206 regions which are distributed regularly over the entire sky. Besides this a special plan includes forty-six additional regions, either very rich or extremely poor in stars, or to which other interest attaches.

Of all investigators Kapteyn has gone into the question of our precise location in the Milky Way most thoroughly, concluding that the solar system lies, not at the center in the exact plane, but somewhat to the north of the Galaxy. Discussing the Sirian stars he finds that if stars of equal brightness are compared, the Sirians average nearly three times more distance from the sun than those of the solar type. So, probably, the Sirians far exceed the Solars in intrinsic brightness. Farther, Kapteyn concludes that the Galaxy has no connection with our solar system, and is composed of a vast encircling annulus or ring of stars, far exceeding in number the stars of the great central solar cl.u.s.ter, and everywhere exceedingly remote from these stars, as well as differing from them in physical type and const.i.tution. So it would be mainly the mere element of distance that makes them appear so faint and crowded thickly together into that gauzy girdle which we call the Galaxy.

The Milky Way reveals irregularities of stellar density and star cl.u.s.tering on a large scale, with deep rifts between great clouds of stars. Modern photographs, particularly those of Barnard in Sagittarius, make this very apparent. Within the Milky Way, nearly in its plane and almost central, is what Eddington terms the inner stellar system, near the center of which is the sun. Surrounding it and near its plane are the ma.s.ses of star clouds which make up the Milky Way. Whether these star clouds are isolated from the inner system or continuous with it, is not yet ascertained.

The vast ma.s.ses of the Milky Way stars are very faint, and we know nothing yet as to their proper motions, their radial motions, or their spectra. Probably a few stars as bright as the sixth magnitude are actually located in the midst of the Milky Way cl.u.s.ters, the fainter ninth magnitude stars certainly begin the Milky Way proper, while the stars of the twelfth or thirteenth magnitude carry us into the very depths of the Galaxy.

It is now pretty generally believed that many of the dark regions of the Milky Way are due not to actual absence of stars so much as to the absorption of light by intervening tracts of nebulous matter on the hither side of the Galactic aggregations and, probably in fact, within the oblate inner stellar system itself. Easton has made many hundred counts of stars in galactic regions of Cygnus and Aquila where the range of intensity of the light is very marked; in fact, the star density of the bright patches of the Galaxy is so far in excess of the density adjacent and just outside the Milky Way, that the conclusion is inevitable that this excess is due to the star clouds.

Of the distance of the Milky Way we have very little knowledge. It is certainly not less than 1,000 pa.r.s.ecs, and more likely 5,000 pa.r.s.ecs, a distance over which light would travel in about 16,000 years. Quite certainly all parts of the Galaxy are not at the same distance, and probably there are branches in some regions that lie behind one another.

While the general regions of the nebulae are remote from the Galactic plane, the large irregular nebulae, as the Trifid, the Keyhole, and the Omega nebulae, are found chiefly in the Milky Way.

In addition to the irregular nebulae many types of stellar objects appear to be strongly condensed toward the Milky Way, but this may be due to the inner stellar system, rather than a real relation to Galactic formations. Quite different are the Magellanic clouds, which contain many gaseous nebulae and are unique objects of the sky, having no resemblance to the true spiral nebulae which, as a rule, avoid the Galactic regions. Worthy of note also is the theory of Easton that the Milky Way has itself the form of a double-branched spiral, which explains the visible features quite well, but is incapable of either disproof or verification. The central nucleus he locates in the rich Galactic region of Cygnus, with the sun well outside the nucleus itself.

By combining the available photographs of the Galaxy, he has produced a chart which indicates in a general way how the stellar aggregations might all be arrayed so as to give the effect of the Galaxy as we see it.

Shapley, at Mount Wilson, has studied the structure of the Galactic system, in which he has been aided by Mrs. Shapley. An interesting part of this work relates to the distribution of the spiral nebulae, and to certain properties of their systematic recessional motion, suggesting that the entire Galactic system may be rapidly moving through s.p.a.ce.

Apparently the spiral nebulae are not distant stellar organizations or "island universes," but truly nebular structures of vast volume which in general are actively repelled from stellar systems. A tentative cosmogonic hypothesis has been formulated to account for the motions, distribution, and observed structure of cl.u.s.ters and spiral nebulae.

An additional great problem of the Galaxy is a purely dynamical one.

Doubtless it is in some sort of equilibrium, according to Eddington, that is to say, the individual stars do not oscillate to and fro across the stellar system in a period of 300 million years, but remain concentrated in cl.u.s.ters as at present. Poincare has considered the entire Milky Way as in stately rotation, and on the a.s.sumption that the total ma.s.s of the inner stellar system is 1,000,000,000 times the sun's ma.s.s, and that the distance of the Milky Way is 2,000 pa.r.s.ecs, the angular velocity for equilibrium comes out 0".5 per century. That is to say, a complete revolution would take place in about 250 million years.

CHAPTER LVIII

STAR CLOUDS AND NEBULae

From star cl.u.s.ters to nebulae, only a century ago, the transition was thought to be easy and immediate. Accuracy in determining the distances of stars was just beginning to be reached, the cl.u.s.ters were obviously of all degrees of closeness following to the verge of irresolvability, and it was but natural to jump to the conclusion that the mystery of the nebulae consisted in nothing but their vaster distance than that of cl.u.s.ters, and it was believed that all nebulae would prove resolvable into stars whenever telescopes of sufficiently great power could be constructed.

But the development of the spectroscope soon showed the error of this hypothesis, by revealing bright lines in the nebular spectra showing that many nebulae emit light that comes from glowing incandescent gas, not from an infinitude of small stars.

In pre-telescope days nothing was known about the nebulae. The great nebula in Andromeda, and possibly the great nebula in Orion, are alone visible to the naked eye, but as thus seen they are the merest wisps of light, the same as the larger cl.u.s.ters are. Galileo, Huygens and other early users of the telescope made observations of nebulae, but long-focus telescopes were not well adapted to this work. Simon Mayer has left us the first drawing of a nebula, the Orion nebula as he saw it in 1612.

The vast light-gathering power of the reflectors built by Sir William Herschel first afforded glimpses of the structure of the nebulae, and if his drawings are critically compared with modern ones, no case of motion with reference to the stars or of change in the filaments of the nebulae themselves has been satisfactorily made out.

Only very recently has the distance of a nebula been determined, and the few that have been measured seem to indicate that the nebulae are at distances comparable with the stars. Of all celestial objects the nebulae fill the greatest angles, so that we are forced to conclude, with regard to the actual size of the greater nebulae as they exist in s.p.a.ce, that they far surpa.s.s all other objects in bulk.

Photography invaded the realm of the nebulae in 1880, when Dr. Henry Draper secured the first photograph of the nebula of Orion.

Theoretically photography ought to help greatly in the study of the nebulae, and enable us in the lapse of centuries to ascertain the exact nature of the changes which must be going on. The differences of photographic processes, of plates, of exposure and development produce in the finished photograph vastly greater differences than any actual changes that might be going on, so that we must rely rather on optical drawings made with the telescope, or on drawings made by expert artists from photographs with many lengths of exposure on the same object.

The great work on nebulae and star cl.u.s.ters recently concluded by Bigourdan of the Paris Observatory and published in five volumes received the award of the gold medal of the Royal Astronomical Society.

While D'Arrest measured about 2,000 nebulae, and Sir John Herschel about double that number in both hemispheres, Bigourdan has measured about 7,000. His work forms an invaluable lexicon of information concerning the nebulae.

Cla.s.sification of the nebulae is not very satisfactory, if made by their shapes alone. There are perhaps fifteen thousand nebulae in all that have been catalogued, described, and photographed. Dreyer's new general catalogue (N.G.C.) is the best and most useful. Many of the nebulae, especially the large ones, can only be cla.s.sified as irregular nebulae.

The Orion nebula is the princ.i.p.al one of this cla.s.s, revealing an enormous amount of complicated detail, with exceptional brilliancy of many regions and filaments. An extraordinary multiple star, Theta Orionis, occupies a very prominent position in the nebula, and photographs by Pickering have brought to light curved filaments, very faint and optically invisible, in the outlying regions which give the Orion nebula in part a spiral character. But the delicate optical wisps of this nebula are well seen, even in very small telescopes. Its spectrum yields hydrogen, helium and nitrogen. The Orion nebula is receding from the earth about eleven miles in every second. Keeler and Campbell have shown that nearly every line of the nebular spectrum is a counterpart of a prominent dark line in the spectrum of the brighter stars of the constellation of Orion. A recent investigator of the distribution of luminosity in the great nebula of Orion finds that radiations from nebulium are confined chiefly to the Huygenian region of the nebula and its immediate neighborhood.

Photography has revealed another extraordinary nebula or group of nebulae surrounding the stars in the Pleiades, which the deft manipulation of Barnard has brought to light. All the stars and the nebula are so interrelated that they are obviously bound together physically, as the common proper motion of the stars also appears to show. Also in the constellation Cygnus, Barnard has discovered very extensive nebulosities of a delicate filmy cloudlike nature which are wholly invisible with telescopes, but very obvious on highly sensitive plates with long exposures.

Another cla.s.s of these objects are the annular and elliptic nebulae which are not very abundant. The southern constellation Grus, the crane, contains a fine one, but by far the best example is in the constellation Lyra. It is a nearly perfect ring, elliptic in figure, exceedingly faint in small telescopes; but large instruments reveal many stars within the annulus, one near the center which, although very faint to the eye, is always an easy object on the photographic plate, because it is rich in blue and violet rays. The parallax of the ring nebula in Lyra comes out only one-sixth of that of the planetary nebulae, and the least greatest diameters of this huge continuous ring are 250 and 330 times the orbit of Neptune.