Spiral galaxy

“ A tentative cosmogonic hypothesis has been formulated to account for the motions, distribution, and observed structure of clusters and spiral nebulæ.
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 clusters as at present. ”

“ The sidereal system is altogether more complicated and more varied in structure than has hitherto been supposed; in the same region of the stellar depths coexist stars of many orders of real magnitude; all orders of nebulæ, gaseous or stellar, planetary, ring-formed, elliptical, and spiral, exist within the limits of the galaxy; and, lastly, the whole system is alive with movements, the laws of which may one day be recognized, though at present they appear too complex to be understood. ”

“ We now know for a certainty that our glorious Galaxy of stars is itself but an item in space, for since the recent investigations with the 60- and the 100-inch telescopes at Mount Wilson observatory with which the outer portions of the spiral nebulæ were resolved into swarms of stars, we know that there are hundreds of thousands of Galaxies separated from ours by a million light years, and from one another equally far. ”

“ It is very probable that our own Galaxy is a similar disk-like aggregation of stars, involving spiral starstreams, and surrounded or interpenetrated by an absorbing medium which is most extensive in the plane of the disk. ”

“ This beautiful star-swarm, visible to the naked eye as a glowing patch in the Milky Way, is indicated in Chart X by a double cluster of dots above the star Eta (η) . Seen with a powerful opera-glass, or better with a small telescope, it is an object that one can never cease to admire and wonder at. It is so bright that the unassisted eye sees it as soon as it is directed toward that part of the sky. It seems to throw a halo over the surrounding sky, as if at that point the galaxy had been tied into a gleaming knot. It is popularly called the “Sword Hand of Perseus.” ”

“ Accordingly, we are led to believe that the spirals studied thus far have diameters 20 times or 100 times, or in some cases several thousand times, the diameter of our solar system. It is difficult to avoid the conclusion that in general they are immensely more massive than is our solar system. The spiral which has been assumed as the forerunner of our system must have been of diminutive size as compared with the larger and brighter spirals which we see to-day. ”

“ Now, I do not say that it is inconceivable that an outlying star-system, so far off as to be irresolvable by any but the most powerful telescopes, should cover so large a space on the heavens. On the contrary, I do not believe that a galaxy resembling our own would be resolvable at all, unless it were so near as to appear much larger than the Orion nebula. I believe astronomers have been wholly mistaken in considering any of the nebulas to be such systems as our own. ”

“ There can be little doubt that these stars belong to the Milky Way cluster, so that their presence is a property of the cluster rather than of the galactic plane in general. Spiral nebulae have the remarkable characteristic of avoiding the galactic plane, and it has been suggested that the space outside the limits of the stellar universe is filled with them. ”

“ Now, a field-glass, if of sufficient power, will divide the larger of these stars again into two—a fine test.
Sweep around α and γ for the splendid star-fields that abound in this neighborhood; also around the upper part of the figure of the cross. We are here in one of the richest parts of the Milky-Way. Between the stars α, γ, ε, is the strange dark gap in the galaxy called the Coal-Sack, a sort of hole in the starry heavens. ”

“ And astonishing as this statement may be, we have still to add that, in face of the actual facts, it may be regarded as even a moderate estimate of the abundance of spirals in the universe. We must remember that a spiral nebula is a flat object with long arms extending from it which lie nearly in the same plane. If we are actually to see that such an object is spiral, it is necessary for it to be turned squarely towards the earth. If the object be too much foreshortened, it is quite plain that we can hardly expect to detect its spiral character. ”

“ Suns were flashing by us on either side now, at a rate that was an index to our appalling speed, but still we flashed on with greater and greater speed, racing out between the thronging suns of the galaxy toward its edge, the great ball of suns of the Cancer cluster expanding before us as we raced on in its direction. On—on—until the mighty cluster lay full to our right, until we were flashing past it, the blackness of outer space stretching ahead, and in that far-flung blackness the dim little patch of light that was the Andromeda universe. ”

“ When they reached a distance of three billion miles from the Galaxy's edge they planned to turn off the great condenser, and their dark star would then shoot past the Galaxy's edge, jerking out our sun with it, from that edge, by its own terrific gravitational grip. If the condenser were turned off before they came that close, however, they would pass the sun at a distance too far to pull it out with them, and would then speed on out into space alone, toward the freezing of their world and their own extinction. ”

“ These systems are not, however, scattered at random, but are divided up into greater systems which appear in our telescopes as clusters of stars. An immense number of these clusters make up our galaxy, and form the visible universe as seen in our telescopes. There may be yet greater systems, each made up of galaxies, and so on indefinitely, only their distance is so immense as to elude our observation. Each of the smaller systems visible to us has its central body, the mass of which is much greater than that of those which revolve around it. ”

“ At the rectangle's lower edge there gleamed on the smooth metal a score or more of little circles of glowing light, of varying sizes, representing the suns at the edge of the Galaxy behind us. Outermost of these glowed the light-disk that was our own sun, and around this Hurus Hol had drawn a shining line or circle lying more than four billion miles from our sun, on the chart. He had computed that if the approaching dark star came closer than that to our sun its mighty gravitational attraction would inevitably draw the latter out with it into space ”

“ This vast system of stars resembles a gigantic sidereal billow flowing towards the Milky Way as towards some mighty shore-line. Nor is this description altogether fanciful; since one of the most marked instances of star-drift presently to be adduced refers to this very region. These associated stars are urging their way towards the galaxy, and that at a rate which, though seemingly slow when viewed from beyond so enormous a gap as separates us from this system, must in reality be estimated by millions of miles in every year. ”

“ Full in the heavens before us it lay, a mighty galaxy fully as large as our own, as the Andromeda universe, but infinitely different, a galaxy not of life but of death. In all its mighty mass were no flaming white or blue or yellow suns like those of our own galaxy, no brilliant young stars surrounded by circling, sun-warmed worlds. ”

“ Kinnison asserted. "True, they are closer together in space, relative to their actual linear dimensions, than are stars; but on the other hand, their relative motions are slower—that is, a star will traverse the average interstellar distance much quicker than a Galaxy will the intergalactic one—so that the whole thing evens up. As nearly as Wacky and I could figure it, two Galaxies will collide deeply enough to produce a significant number of planetary solar systems on an average of once in just about one point eight times ten to the tenth years. ”

“ Russell writing of this region says: "Here the Milky Way is crossed by a dark streak which immediately suggests a passing cloud. But, year in and year out, on the clearest nights, the dark region is there. Its origin must be interstellar space—perhaps in an actual thinning of the stars of the Galaxy, perhaps in the interposition of some cosmic cloud of overwhelming vast dimensions."
Many think the Galaxy a universe by itself and our sun one of its myriad stars. ”

“ If it be accepted it greatly increases our estimate of the size of the material universe. For our central, lens-shaped system is calculated to extend towards the Milky Way for more than twenty thousand times a million million miles, and about a third of this distance towards what we have called the poles. If, as we suppose, each spiral nebula is an independent stellar universe comparable in size with our own, then, since there are hundreds of thousands of spiral nebulae, we see that the size of the whole material universe is indeed beyond our comprehension. ”

“ In a low pit at the heart of one of their cities—this city, in fact—they placed the condenser, a mass of brilliant hemispherical ray-attracters which caused more of the Galaxy's outward-shooting attractive force to fall upon the dark star, which condensed and concentrated that radiating force upon the dark star, thereby pulling the dark star inward toward the Galaxy's edge in a great curve. ”

“ From this point the density gradually diminishes out to a distance of 2500 “light years,” at which distance it is reduced to about one-fifth of the density in the sun’s vicinity. [457]
In a letter to the late Mr. Proctor (Knowledge, November, 1885, p. 21) , Sir John Herschel suggested that our Galaxy (or stellar system) “contained within itself miniatures of itself.” This beautiful idea is probably true. In his account of the greater “Magellanic cloud,” Sir John Herschel describes one of the numerous objects it contains as follows:—
“Very bright, very large ”

“ You know that it is gravitational force alone which keeps the suns and planets to their courses, and you know that the gravitational force of any body, sun or planet, is radiated out from it in all directions, tending to pull all things toward that body. In the same way there is radiated outward perpetually from the Galaxy the combined attractive gravitational force of all its swarming suns, and a tiny fraction of this outward-radiating force, of course, struck the dark star, pulling it weakly toward the Galaxy. ”

“ The system to which our sun belongs he described as "a very extensive branching congeries of many millions of stars, which probably owes its origin to many remarkably large as well as pretty closely scattered small stars, that may have drawn together the rest." [48] But the continued action of this same "clustering power" would, he supposed, eventually lead to the breaking-up of the original majestic Galaxy into two or three hundred separate groups, already visibly gathering. ”

“ This will be the richness of the region in stars. Then we take regions nearer the horizontal Milky Way—say that contained between 10 degrees and 20 degrees from the zenith—and, by a similar count, find its richness in stars. We do the same for other regions, nearer and nearer to the horizon, till we reach the galaxy itself. The result of all the counts will be that the richness of the sky in stars is least around the galactic pole, and increases in every direction towards the Milky Way. ”

“ These systems are, however, united together into one vast aggregation of worlds, having one common controlling centre of their own, and by their unity form a constellation, a larger and grander mechanism. Throughout the whole constellation there is the same order, and harmonious working of part with part, that characterise the solar system. Then these constellations increasing in their aggregations form a still larger complexity of systems, called a Galaxy ”

“ To make apparent any general absorption of radiation by the interstellar medium, it becomes necessary to investigate the properties of space far beyond the limits of the Galaxy and its outlying shells of sparsely distributed stars, and, crossing the immense voids of surrounding ether, to inquire whether they contain other galaxies of dimensions comparable with our own, and whether these afford any evidence of a gradual absorption of luminous energy by the intervening medium. ”

“ But if this annular form with some kind of central nucleus, often very large, is produced under certain conditions by the action of the ordinary laws of motion upon more or less extensive masses of discrete matter, why may not the same laws acting upon similar matter once dispersed over the whole extent of the existing stellar universe, or even beyond what are now its farthest limits, have led to the aggregation of the vast annular formation of the Milky Way, with all the subordinate centres of concentration or dispersal to be found within or around it? ”

“ But this mass was probably condensed on meteors which immigrated from outside, and when these meteors coalesced dark planets were produced which move about the central body, and which have gathered about them most of the gases. If that central body were as heavy as our sun, the matter in the ring should revolve about it in five thousand years. That rotation would suffice to wipe out the original spiral shape, enough of which has yet been left to permit of our [Pg 177] distinctly discerning the two wings of the spiral. ”