Part 24 (1/2)

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.