The unstable areas are so marked; the remaining areas are stable. The dots which form a sort of background to the diagram represent 2100 stars whose absolute magnitudes are known through their parallaxes having been determined spectroscopically at Mount Wilson. The observational material is not perfect, for considerable uncertainty attaches to all spectroscopic parallaxes of _B_-type stars, and _A_-type stars are almost unrepresented because it is practically impossible to obtain their parallaxes by the spectroscopic method. The theoretical curves are probably still more imperfect, yet, such as they are, they seem to suggest very forcibly that the occupied and unoccupied regions coincide with those representing stable and unstable configurations; after making all possible allowances for the imperfections both of theory and of observation, too much agreement remains to be explained away as mere coincidence. Thus the conclusion to which mathematical discussion seems to lead is that the regions in the Russell diagram which are occupied, represent stars whose central regions are in a liquid, or nearly liquid, state. All other stars are unstable, so that the corresponding regions in the Russell diagram are necessarily vacant. To put it in less technical language, all the stars in the sky must have liquid, or nearly liquid, centres. Here we have a piece of the puzzle which seems to fit on to the piece we unearthed in Chapter IV, where we found that a star could only break up by fission if it had a liquid, or nearly liquid centre. Evidence accumulates that the stars have liquid rather than gaseous centres. Criticism of the foregoing hypothesis—generally described as the “liquid-star” hypothesis—has mainly taken the form that the diameters of the _K_-rings of atoms are so small that the _K_-ring atoms in the sun’s central regions cannot possibly be packed closely enough to involve any substantial departure from the gaseous state. It is difficult to discuss, and still more to meet, this criticism without knowing the precise diameters of these _K_-ring atoms. We of course know the diameters assigned to the _K_-ring by Bohr’s theory (p. 129), but no one any longer contends that this theory gives a true picture of the atom. It provides a good working model within limits, but we do not know where the limits end. The only practical experience we have of _K_-ring atoms is with helium atoms; Bohr’s theory assigns to these a diameter of 0·54 × 10⁻⁸ cms. Yet solid and liquid helium provide a practical illustration of the closeness with which helium atoms can be packed; in these each atom occupies a sphere of diameter 4 × 10⁻⁸ cms., or over 400 times the space allotted to it by Bohr’s theory. It looks as though we are still far from definite knowledge of the dimensions of _K_-rings of electrons.