Spiral nebula

“ The velocity of escape is 280 km/sec.
At the lower limit of average distance of spirals the typical nebulae would be fifteen light-years in diameter, forty-five million times as massive as our sun, and 3 × 1013 as dense as our atmosphere. On the other hand, if the typical spiral nebula is placed at a suitable distance, its dimensions assume the same order of magnitude as those of our own stellar system. ”

“ This is precisely what we see in the spiral nebula. Nor is there any other action that we know of which would thus handle the body. If it were to disintegrate under
increased speed of rotation due to contraction upon itself, parts of its periphery should be shed continually and a pinwheel of matter, not a two-armed spiral, be thrown off. If explosion were the disintegrating cause, disruption would occur unsymmetrically in one or more directions, not symmetrically as here. ”

“ Taking these considerations into account, it becomes obvious that the estimate of Professor Keeler as to the number of spiral nebulæ in the heavens, vast as that estimate seems, may still fall short of the truth. Thus we are led to one of the most remarkable conclusions of modern astronomy, viz. that the spiral nebula, next to a star itself, is the most characteristic object in the sidereal heavens. ”

“ The character of its spectrum may show us that the spiral nebula is not very young, that it has attained a considerable age in its evolution as compared with other nebulæ which do not show the spiral character and which have a gaseous spectrum. ”

“ The Spiral nebulæ are very curious and complicated objects, but they are visible only in the largest telescopes. Prominent above all is the double spiral nebula No. 3,572, in Canes Venatici, which is not far from η Ursa Majoris. In Lord Rosse's telescope, this object presents a wonderful spiral disposition, looking somewhat like one of the fire-works called pin-wheels, and forming long, curved wisps, diverging from two bright centres. The spectrum of this object, however, is not that of a gas. In the constellation Virgo, Rosse has detected another such nebula. ”

“ We must, however, admit that the lessons as to the origin of the solar system, derived from the spiral nebulæ, must be received with due limitation. We may say at once that the great spiral nebulæ do not appear to be evolving into systems like the sun and planets; their work is of a higher order of magnitude altogether. The great spiral nebulæ seem to be more analogous to galaxies, like the Milky Way (Fig. 32) , than to solar systems. The spiral nebula instead of being described as a system, should perhaps be described as a system of systems. ”

“ There are marked advantages in starting the evolution of the solar system from a spiral nebula, aside from the fact that spirals are abundant, and therefore represent a standard product of development. The material is thinly and very irregularly distributed in a plane passing through the Sun, and the motions around the Sun are all in the same direction. ”

“ We must not forget that the only spiral nebulæ which lie within the reach of our powers of observation, whether telescopic or photographic, appear to be objects of enormously greater cosmical magnificence than was that primæval nebula from which so insignificant an object as the solar system has sprung. The great spirals, so far as we can tell at present, appear to be thousands of times, or even millions of times, greater in area than the solar system. ”

“ Even if they do not approach within this fatal limit, which is a little less than two and one-half times the radius of the body, they may come so near as to change their character entirely, and, through their tidal influence on each other, form into a rotating spiral nebula with two arms projecting from opposite sides of the spiral. ”

“ As a nebulous spot it is an object which can be seen with any moderately good telescope, but to detect those details which indicate the spiral structure demands an instrument of first-class power. This object had indeed been studied by many astronomers before Lord Rosse turned his colossal reflector upon it. Then it was that the wonderful whirlpool structure was first discovered, and thus the earliest spiral nebula became known. ”

“ Starting with a spherical nebula, he argues that in cooling it will assume the form illustrated above, and that the smaller part will separate and form a satellite rotating independently but within a distance influenced by the parent mass.
The spiral nebula in Canes Venatici, a revolving mass of gas or meteorites, supplies, according to the nebular hypothesis of Messrs. Chamberlin and Moulton, an excellent example of how the earth and moon were formed. We may reasonably imagine the smaller spiral to represent the moon in the act of being thrown off by the earth. ”

“ The theory which at present seems more nearly than any other to offer a reasonable explanation of most planetary phenomena is based upon the supposition that the nebula from which the sun and planets were evolved was in the shape of a spiral, and not the gaseous mass that the original nebular hypothesis supposed. The fact that among the many thousands of nebulæ that have been discovered and observed a very large proportion of them are in this form, aside from any other consideration, suggests a great probability that the one from which the solar system was evolved was a spiral. ”

“ The nebulous mass formed as a result of the disintegration of the bodies through their furious impact would thus come into being with a spinning movement, and a spiral would ensue. Again, the stars may not actually collide, but merely approach near to each other. If very close, the interaction of gravitation will give rise to intense strains, or tides, which will entirely disintegrate the bodies, and a spiral nebula will similarly result. ”

“ In the case of many of the spiral nebulæ, stars are often strung along the coils of the spiral, while other curved lines of stars are seen just outside the nebula, so that it is impossible to avoid the conclusion that both are really connected with it, the outer lines of stars indicating a former greater extension of the nebula whose material has been used up in the growth of these stars. ”

“ In Nova Persei, then, we actually witnessed a spiral nebula evolved from a disrupted star.
What was this ejectum and what drove it forth? Professor Very regarded it as composed of corpuscles such as give rise to cathode rays discharged from the star under the stress of light pressure or electric repulsion. But I think we may see in it something simpler still; to wit, gaseous molecules driven off by light pressure alone—the smoke, as one may say, of the catastrophe—akin exactly to the constituents of comet's tails. The mere light of the conflagration pushed the hydrogen molecules away. ”

“ The discovery of spiral nebulæ was made by Lord Rosse with his great 6–foot reflector. Two good examples of these objects will be found in Ursa Major, while there is another fine one in Canes Venatici (see Plate XXII., p. 314) , a constellation which lies between Ursa Major and Boötes. But the finest spiral of all, perhaps the most remarkable nebula known to us, is the Great Nebula in the constellation of Andromeda, (see Plate XXIII., p. 316) —a constellation just further from the pole than Cassiopeia. ”

“ Almost any collision between unequal masses of diffused matter would, in the absence of any massive central body round which they would be forced to revolve, lead to spiral motions. It is to be noted that, although the stars formed in the spiral convolutions of the nebulæ follow those curves, and retain them after the nebulous matter has been all absorbed by them, yet, whenever such a nebula is seen by us edgewise, the convolutions with their enclosed stars will appear as straight lines ”

“ The spiral nebulæ we see in the heavens, however, are centrifugal spirals. This is clearly shown in Plates XV., XII., and the frontispiece of Nichol’s “Architecture of the Heavens,” as well as in Plates XIII. and XIV. Plate XV.—the open spiral—is directly contradictory of any phenomena which could occur in accordance with the nebular [279] theory of Laplace. The frontispiece shows the only form which such a nebula could assume at any stage of its career,—that is, a close spiral with nearly circular convolutions. ”

“ The earth is going about twelve miles a second—that's more than a million miles a day—directly toward the center of the nebula. It has taken ten days to go through the spiral that we have encountered, making that about ten million miles thick. It's not likely that the gap between this spiral and the nucleus of the nebula is more than thirty million miles across, at the most; so you see we'll probably be in the nucleus within a month, and possibly much less than a month. ”

“ It was discovered by Messier while comet-hunting on October 13, 1773. Telescopes of moderate power merely show two nebulæ nearly in contact, but Lord Rosse saw it as a wonderful spiral, and his drawing agrees fairly well with a photograph taken by Dr. Roberts in April, 1889. The nebula has also been photographed by Dr. Common. Dr. Roberts says: “The photograph shows both nuclei of the nebula to be stellar, surrounded by dense nebulosity, and the convolutions of the spiral in this as in other spiral nebulæ are broken up into star-like condensations [167] with nebulosity around them. ”

“ Double nebulae have two principal centres of condensation. The planetary nebulae are nearly uniformly illuminated compact patches of light generally circular or elliptical in shape; they were so called because they appeared to possess disks like planets. Elliptical nebulae are usually nebulae of some flat type (such as annular or spiral) seen rather edgeways, so that the structure is not readily recognizable. The typical spiral nebulae are in the form of a double spiral, the two branches of which proceed from diametrically opposite points of a bright nucleus and wind round it in the same sense ”

“ The great majority of the nebulae, including the abundant, small nebulae which shine with a white light (in contrast with the blue-green light of the planetary and irregular nebulae—see below Spectra of nebulae) , are generally classed as spiral nebulae. The spiral structure has been shown to exist in a few of them, but for the remainder it is only inferred. Nebulous stars are true stars surrounded by an atmosphere or aureole of nebulous light. Diffused nebulosities are very faint nebulae of enormous extent, sometimes forming the background of a whole constellation. ”

“ We might further mention the star-clusters, dense, ball-shaped agglomerations of stars, and the so-called planetary nebulæ, which—at least in their visible outer shell—also consist of gas accumulations with a spherical or ellipsoid conformation. The numerous spiral nebulæ on the other hand, those strange stellar bodies to which we45 later shall have occasion to return, are beyond comparison more frequent in the regions surrounding the poles of the Galaxy than in the rest of the heavens.
Many astronomers have considered the Milky Way itself a nebula. ”

“ Look now again at the glowing spirals. We observe that hardly have they left the central mass before they begin to coagulate. In some places they have a “ropy” aspect; or they are like peascods filled with growing seeds, which eventually will become stars. The great focus itself shows a similar tendency, especially around its circumference. The sense that it imparts of a tremendous shattering force at work is overwhelming. There is probably more matter in that whirling and bursting nebula than would suffice to make a hundred solar systems! ”

“ Closer inspection shows that in addition to the principal focus there are various smaller condensations scattered through the mass. These are conspicuous in the spirals. Some of them are stellar points, and but for the significance of their location we might suppose them to be stars which happen to lie in a line between us and the nebula. But when we observe how many of them follow most faithfully the curves of the spirals we cannot but conclude that they form an essential part of the phenomenon; it is not possible to believe that their presence in such situations is merely fortuitous. ”

“ We have even dared to change the course of suns, to prevent collisions between them that would have annihilated their circling worlds. It might seem, indeed, that we, the massed peoples of the Galaxy, have risen to such power that all things in it are subject to our will, obedient to our commands. But we have not. One thing alone in the Galaxy remains beyond our power to change or alter, one thing beside which all our power and our science are as nothing. And that is the nebula.
"A nebula is the vastest thing in all our universe, and the most mysterious. ”

“ We were very near to the pale beam, now, flashing above the huge pit straight toward it. I had a glimpse of the great pit's perpendicular black metal walls, dropping down for miles into depths inconceivable, of something in those dusky depths that burned like a great white star of light, and then Jor Dahat suddenly uttered a choking cry, flinging an arm out toward the livid ray before us.
"That ray!" he cried. "It's not light—it's force! The nebula—stop the ship!" ”

“ Nearer and nearer we flashed toward it, with Sar Than and Jor Dahat beside me gazing forward, their interest as tense as my own. And now we saw that the pit was of gigantic size, its circular mouth all of five miles in diameter, and that from its center there drove up toward the zenith a flickering beam of pale and ghostlike white light, so pale as hardly to be visible, a livid white ray that stabbed straight up toward the fires of the nebula far above. ”