THERE are several lines of evidence in support of the order of evolution which we have outlined.
1. The close relationship of the bright-line nebular spectrum, the bright-line stellar spectrum and the spectra of the simplest helium stars; the practically continuous sequence of spectra from the helium stars to the red stars.
2. In the long run, we must expect the stars to grow colder, at least as to the surface strata. What the average interior temperatures are is another question; the highest interior temperatures are thought to be reached at an intermediate or quite late stage in the process, in accordance with principles investigated by Lane and others; but the temperatures existing in the deep interiors seem to have little direct influence in defining the spectral characters of the stars, which are concerned more directly with the surface strata.[1] We should therefore expect the simpler types of spectra, such as we find in the helium and hydrogen stars, in the early stages of the evolutionary process. The complicated spectra of the metals, and particularly the oxides of the metals, should be in evidence late in stellar life, when the atmospheres of the stars have become denser and colder.
[1] This important point seems not to have been realized by all theorists.
3. The velocities of the Orion nebula, the Trifid nebula, the Carina nebula, and of several other irregular nebulae, have been measured with the spectroscope. These bodies seem to be nearly at rest with reference to the stellar system. The helium stars have the lowest-known stellar velocities, and the average velocities of the stars are higher and higher as we pass from the helium stars, through the hydrogen and solar stars, up to the red stars. The average velocities of the brighter stars of the different spectral classes, as determined with the D. O. Mills spectrographs at Mount Hamilton and in Chile, are as in the following table:
Spectral No. of Class Stars Average Velocity in Space
B 225 12.9 km. per Sec.
A 177 21.9
F 185 28.7
G 128 29.9
E 382 33.6
M 73 34.3
We can not place the irregular nebulae after the red stars: their velocities are too small, and their spectra have no resemblances to the red-star spectra.
4. Wherever we find large irregular gaseous nebulae we find stars in the early subdivisions of the helium group. They are closely related in position. This is true of the Orion and other similar regions. The irregular, gaseous nebulae are in general found in and near the Milky Way, and so are the helium stars. The yellow and red stars, at least the brighter ones, do not cluster in nebulous regions.
5. The stars are more and more uniformly distributed over the sphere as one goes from the helium stars through the hydrogen and solar stars, to the red stars. The Class M stars show little or no preference for the Milky Way. Of course, I am speaking here of the brighter and nearer stars which we have been able to study by means of the spectroscope, and not at all of the faint stars which form the unstudied distant parts of the Milky Way structure. The helium stars are young, their motions are slow, and they have not wandered far from the place of their birth. Not so with the older stars.
6. The visual double stars afford strong evidence that the order of evolution described is correct. The 36-inch refractor has shown that one star in 18, on the average, brighter than the ninth visual magnitude, consists of two or more suns which we can not doubt are in slow revolution around each other. The number of double stars observable would be very much greater than this if they were not so far away. Of the 20 stars which we say are our nearest neighbors, 8 are well known double stars; one double in each two and one half, on the average. Aitken has made a specialty of observing the double stars whose components in each case are very close together and are in comparatively rapid revolution. His program includes 164 such systems whose types of spectra are known, as in the following table: