Nebula M. 51 Canum Venaticorum—after Ritchey.
Although Lockyer has long been of opinion that the nebulæ are composed of meteorites, the present argument differs from his. The way in which their spectra establish their constitution may be outlined as follows: the white nebulæ are from their structure evidently in process of evolution, and if they are in stable motion, as we suppose them to be, their parts are moving round their common centre of gravity. As the white nebulæ resist resolution as obstinately as the green, these parts must be not only solid but comminuted (composed of small particles). Now this would be the case were they flocks of meteorites such as we have seen composed our own system once upon a time. Though all are travelling round the centre of gravity of the flock, each is pursuing its own orbit slightly different from, and intersecting those of, its neighbors. Collisions between the meteors must therefore constantly occur, and the question is, are these shocks sufficient to cause light. Let us take our own system and consider two meteorites at our distance from the Sun, travelling in the same sense, the one in an ellipse, the other in a circle, with a major axis five per cent greater and meeting the other at aphelion. This would be no improper jostle for such heavenly bodies. If we calculate the speeds of both and deduct the elliptic from the circular, we shall have the relative speed of collision. It proves to be a half a mile a second or 30 times the speed of an express train. As such a train brought up suddenly against a stone wall would certainly elicit sparks, we see that a speed 30 times as great, whose energy is 900 times greater, is quite competent to a shock sufficient to make us see stars en masse. But, indeed, there must be collisions much more violent than this; both because the central mass is often much greater and because the orbits differ much more, and the effect would increase as the square of the speed. The heat thus generated would cause the meteorites to glow, and at the same time raise the temperature of the gases in and about them. Furthermore, the light would come to us through other non-affected portions of gas between us and the scene of the collision. Thus all three peculiarities of the spectra stand explained: we have a continuous background of light due to heated solid meteorites, the bright lines of glowing gases, and dark lines due to other gases not ignited, lying in our line of sight.
In addition we should perceive another result. Collisions would be both more numerous and more pronounced toward the centre of the nebula, for it must speedily grow denser toward its core owing to the falling in of meteorites, in consequence of shock. Being denser in the centre, the particles would there be thicker and be travelling at greater speed. The nebulæ, therefore, should be brightest at their centres, which is accordant with observation.
Thus from having offered themselves exemplars of the way in which our own system came into being, the white nebulæ assert their present constitution to be that from which we know our system sprang.
Another suggestive fact about the present members of our solar system which has something to say about a past collision is the densities of the different planets. The average density of the four inner planets, Mars, the Earth, Venus, and Mercury is nearly four times that of the four outer ones Neptune, Uranus, Saturn, and Jupiter[[see NOTE 2]]. The discrepancy is striking and cannot be explained by size, as the smallest are the most massive, and if all were primally of like constitution, should be the least compressed. Nor can it be explained simply by greater heat tending to expand them, for Neptune and Uranus show no signs of being very hot. The minor differences between members of each group are probably explicable in part by these two factors, mass and heat, but the great gulf between the two groups cannot so be spanned. We are then driven to the supposition that the materials composing the outer ones were originally lighter. Now this is precisely what should happen had all eight been formed by disruption of a previous body. For its cuticle would be its least dense portion, and on disruption would travel farthest away, not because of being lighter, but because of being on the outside. Parts coming from deeper down would remain near, and be denser intrinsically.
What the present densities of the planets enable us to infer of the cataclysm from which they came, a remarkable set of spectrograms taken not long ago by Dr. V. M. Slipher, at Flagstaff, seems to confirm.
The spectrograms in question were made possible by his production of a new kind of plate. His object was to obtain one which should combine sufficient speed with great photographic extension of the spectrum into the red. For it is in the red end that the absorption lines due to the planets’ atmospheres chiefly lie. With the plates heretofore used it was impossible to go much beyond the yellow, the C line marking the Ultima Thule of attent. Not only was it advisable to get more particularity in the parts previously explored, but it was imperative to go beyond into parts as yet unknown. After several attempts he succeeded, the plates when exposed showing the spectra beyond even the A band. Of their wealth of depiction it is only necessary to say that in the spectrum of Neptune 130 lines and bands can easily be counted between the wave-lengths 4600 µµ, 7600 µµ. Of these, 31 belong to the planet, which compares with 6 found by Huggins, 10 by Vogel, and 9 by Keeler in the part of its spectrum they were able to obtain.
THE SPECTRA OF THE MAJOR PLANETS.
Photographed, in 1907, by V. M. Slipher, at THE LOWELL OBSERVATORY
Flagstaff, Arizona.