STELLAR EVOLUTION We have supposed that the stars were born initially as condensations in the outer fringes of spiral nebulae. These condensations would, from the mode of their formation, necessarily be of all sorts of sizes, and subject only to the single restriction that none of them could be below a certain limit of weight. Thus we should not expect the stars, either at birth or subsequently, to be all of the same size or weight, or all in the same physical condition. The stars would start their existences at different points in the Russell diagram, but we may imagine that their initial positions are limited to those parts of the diagram which can be occupied by stars—either, as the liquid-star hypothesis would suggest, because these are the only stable configurations, or else for some other reason so far undiscovered. Each year a star loses a certain amount of weight, and its rate of generating energy, and so also its luminosity, are correspondingly reduced, with the result that it moves to a new position in the diagram. Turning to Seares’ diagram of stellar weights on p. 282, we may think of the curves of equal weight as a flight of steps—very uneven steps, it is true—each representing a lower weight than the one above. Whatever a star’s evolution may be, it is essential that it should always be _down_ the steps: any upward step is impossible. We can trace out two possible paths of stellar evolution in the Russell diagram which involve no entry into regions unoccupied by stars—two roads along which the stellar army may march as they transform their substance into radiation. The first is of course the “main-sequence,” which a great number of considerations suggest to be the main line of march of the stellar army. The branch which starts from the red giants in the Russell diagram represents a second possible line of march, a certain number of stars travelling along this branch until they reach the main-sequence as blue or white stars, and then travelling down the lower half of the main-sequence to end as faint red stars passing on to ultimate extinction. Progress along each of these roads is accompanied by a continuous shrinkage in the size of the star, its diameter steadily decreasing. This is not the same thing as saying that the star’s density continually increases, for the star is continually diminishing in weight, so that even if the star’s density remained the same, its diameter would decrease. Nevertheless a study of Seares’ determinations of mean densities, as shewn in the diagram on p. 282, suggests that there is a continuous increase of density, although this becomes very slight in the middle reaches of the main-sequence.