Double Stars.—Similarly, among the double stars, a pair with components of large mass would settle down at first like Ao, Bo the brighter star being the redder one—a pair of small mass at with the fainter star the redder. The great majority of the double stars fall into one or other of these two classes. A few pairs have components of very different brightness, but the same colour and in some, like the brighter corn ponent is a very hot star, of spectral class B. All these phenom ena are immediately accounted for by the theory.
Course of Evolution.—The question of the history of an indi vidual star is not yet answered. If the expendable fraction of the star's whole mass is small, it must remain of nearly the same brightness all through its history (fig. 5). A star of great mass will linger as a red giant A,, till it has exhausted one store of energy, losing a little brightness as it consumes its mass, then pass rapidly to. A, where it draws on the other store, finally exhausting it and shrinking beyond our ken. One of moderate mass will spend practically its whole life between B, and B,, as a star of moderate temperature (spectral class F) and moderate brightness. One of small mass, after a long career as a red dwarf at may become a white dwarf at C,. If, on the other hand, most of the mass is expendable, a single star may pass through almost the whole known sequence of stellar types. Beginning at A, (fig. 6), perhaps as a long period variable (the coolest and least dense of all known stars), it may become a yellow giant like Capella (A,) and a white star like Sirius At this point, before all the ma terial of the second sort is exhausted, that of the first kind may begin to be consumed. Till it is gone the star must remain on the
main series, passing through a stage resembling the sun (AO, and one like 61 Cygni (A,). Ultimately, after all the material of the first sort is exhausted, the star may become a white dwarf at A6, fed by the unexpended store of the second kind. Many other life-histories, with a less extensive set of changes, are possible with different initial amounts of the two transformable substances.
The question cannot yet be regarded as settled. Indeed, the whole evolutionary scheme here sketched, which follows the lines laid down by Russell in 1927—is of a highly tentative nature. The two processes of energy liberation which are postulated are frankly invented ad hoc, and the best that can be claimed for them is that they account for most of the facts. The advance of atomic physics may replace them by something far more firmly grounded, and very different.
It may be said in conclusion that a very different evolutionary scheme has recently been put forward by Jeans, who regards the rate of liberation of energy as independent of the temperature, like radioactivity, and believes that the stars are unstable when the simple gas-laws are obeyed, and concentrated in certain regions on the diagram because, under the corresponding physical condi tions of partial ionization, the behaviour of the gas departs widely from the ordinary laws, resembling that of a liquid. That such departures from the gas-laws should occur at the observed den sities he has not yet shown, nor is it easy to see how they can hap pen. But the suggestion illustrates that there are probably many possibilities still unexplored.