Part 24 (2/2)
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.
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