To take the most conspicuous instance of all, there are no stars at all to the left of the main-sequence in the Russell diagram (fig. 22), until we come to the quite detached group of white dwarfs. Why are there no stars in intermediate conditions? Why, to make the example still more precise, does no star exist of the same colour as Sirius but with only a tenth of its luminosity? Why do we have to go down to the white dwarf ο₂ Eridani _B_, with a luminosity of only a ten-thousandth of that of Sirius, before we can find a star to match Sirius in colour? A hypothesis which occurs naturally to the mind is that the main-sequence stars and the white dwarfs may form distinct groups because they are of entirely different ages—they may represent distinct creations. As stars age they decrease in weight and in luminosity, so that it is natural to interpret the small weights and extremely low luminosity of the white dwarfs as evidence of an age far greater than the age of the normal main-sequence stars. Yet this hypothesis does not appear to be tenable. With the single exception of van Maanen’s star, every star which is either known or suspected to be a white dwarf forms one component of a binary system, and in every case its companion is a main-sequence star or (in the case of ο Ceti) a red giant. We have already seen how rare it is for two stars to approach near to one another in space. It must be an almost inconceivably rare event for two stars, originally moving as independent bodies, so to meet in their random wanderings, that the big one “captures” the little one, and they henceforth journey together through space. For it can be shewn that, for such an event to occur, something more than a close approach is needed; a close approach must take place in the presence of yet a third star, so that no fewer than three stars must chance to come near one another simultaneously in their wanderings through the vast emptinesses of space. It is almost inconceivable that this should happen in a single instance, but it is straining the probabilities too far to suppose that it has happened in the case of every single known white dwarf but one. Thus we have to suppose that the white dwarfs and their more normal companions have been together since birth, and so were born at the same time out of the same nebula. The difference between white dwarfs and main-sequence stars cannot, then, be a mere difference of age, and it would seem as though there must be some physical reason militating against the existence of stars in intermediate conditions. Taking a more general view of the question, we are led to investigate whether the absence of stars built to certain specifications can be attributed to such stars needing physical properties which nature cannot provide. This leads directly into the general question of the structure and mechanism of the stars. THE INTERNAL CONSTITUTION OF THE STARS