As possibly I may have assumed too great a thickness for the nebula, let us take a thickness of one-tenth of that used above, or one thousandth of the length of the nebula. This gives a mass of 8 million times the sun’s mass. This seems a more probable mass if the nebula is—as Bohlin’s parallax implies—a member of our sidereal system.
If we assume a parallax of say 0″·01—or one-hundredth of a second of arc—which would still keep the nebula within the bounds of our sidereal system—we have the dimensions of the nebula increased 17 times, and hence its mass nearly 5000 times greater (173) than that found above. The mass would then be 40,000 million times the sun’s mass! This result seems highly improbable, for even this small parallax would imply a light journey of only 326 years, whereas the distance of the Milky Way has been estimated by Prof. Newcomb at about 3000 years’ journey for light.
In Dr. Roberts’ photograph many small stars are seen scattered over the surface of the nebula; but these do not seem to be quite so numerous as in the surrounding sky. If the nebula lies nearer to us than the fainter stars visible on the photograph, some of them may be obscured by the denser portions of the nebula; some may be visible through the openings between the spiral branches; while others may be nearer to us and simply projected on the nebula.
To add to the difficulty of solving this celestial problem, the spectroscope shows that the Andromeda nebula is not gaseous. The spectrum is, according to Scheiner, very similar to that of the sun, and “there is a surprising agreement of the two, even in respect to the relative intensities of the separate spectral regions.”[366] He thinks that “the greater part of the stars comprising the nucleus of the nebula belong to the second spectral class” (solar), and that the nebula “is now in an advanced stage of development. No trace of bright nebular lines are present, so that the interstellar space in the Andromeda nebula, just as in our stellar system, is not appreciably occupied by gaseous matter.”[366] He suggests that the inner part of the nebula [the “nucleus”] “corresponds to the complex of those stars which do not belong to the Milky Way, while the latter corresponds to the spirals of the Andromeda nebula.”[366] On this view of the matter we may suppose that the component particles are small bodies widely separated, and in this way the mean density of the Andromeda nebula may be very small indeed. They cannot be large bodies, as the largest telescopes have failed to resolve the nebula into stars, and photographs show no sign of resolution.
It has often been suggested, and sometimes definitely stated, that the Andromeda nebula may possibly be an “external” universe, that is an universe entirely outside our sidereal system, and comparable with it in size. Let us examine the probability of such hypothesis. Assuming that the nebula has the same diameter as the Milky Way, or about 6000 “light years,” as estimated by Prof. Newcomb, I find that its distance from the earth would be about 150,000 “light years.” As this is about 8000 times the distance indicated by Bohlin’s parallax, its dimensions would be 8000 times as great, and hence its volume and mass would be 8000 cubed, or 512,000,000,000 times greater than that found above. That is, about 4 trillion (4 × 1018) times the sun’s mass! As this appears an incredibly large mass to be compressed into a volume even so large as that of our sidereal system, we seem compelled to reject the hypothesis that the nebula represents an external universe. The sun placed at the distance corresponding to 150,000 light years would, I find, shine as a star of less than the 23rd magnitude, a magnitude which would be invisible in the largest telescope that man could ever construct. But the combined light of 4 trillion of stars of even the 23rd magnitude would be equal to one of minus 23·5 magnitude, that is, 23½ magnitude brighter than the zero magnitude, or not very much inferior to the sun in brightness. As the Andromeda nebula shines only as a star of about the 5th magnitude the hypothesis of an external universe seems to be untenable.
It is evident, however, that the mass of the Andromeda nebula must be enormous; and if it belongs to our sidereal system, and if the other great nebulæ have similar masses, it seems quite possible that the mass of the visible universe may much exceed that of the visible stars, and may be equal to 1000 million times the sun’s mass—as supposed by the late Lord Kelvin—or even much more.
With reference to the small star which suddenly blazed out near the nucleus of the Andromeda nebula in August, 1885, Prof. Seeliger has investigated the decrease in the light of the star on the hypothesis that it was a cooling body which had suddenly been raised to an intense heat by the shock of a collision, and finds a fair agreement between theory and observation. Prof. Auwers points out the similarity between this outburst and that of the “temporary star” of 1860, which appeared in the cluster 80 Messier, and he thinks it very probable that both phenomena were due to physical changes in the nebulæ in which they appeared.
The appearance of this temporary star in the Andromeda nebula seems to afford further evidence against the hypothesis of the nebula being an external universe. For, as I have shown above, our sun, if placed at a distance of 150,000 light years, would shine only as a star of the 23rd magnitude, or over 15 magnitudes fainter than the temporary star. This would imply that the star shone with a brightness of over a million times that of the sun, and would therefore indicate a body of enormous size. But the rapid fading of its light would, on the contrary, imply a body of comparatively small dimensions. We must, therefore, conclude that the nebula, whatever it may be, is not an external universe, but forms a member of our own sidereal system.
In Sir John Herschel’s catalogue of Nebulæ and Clusters of Stars, published in 1833, in the Philosophical Transactions of the Royal Society, there are many curious objects mentioned. Of these I have selected the following:—
No. 496 is described as “a superb cluster which fills the whole field; stars 9, 10 ... 13 magnitude and none below, but the whole ground of the sky on which it stands is singularly dotted over with infinitely minute points.” This is No. 22 of Sir William Herschel’s 6th class, and will be found about 3 degrees south and a little east of the triple star 29 Monocerotis.