we have already seen that the central regions of these stars have also approximately the same temperatures as the centre of the sun, whence it follows that their physical conditions are all substantially the same. Thus the atoms in the central regions of all these stars must be broken down to the same extent as the atoms in the central regions of the sun. The _K_-rings of electrons survive intact, but the outer rings are transformed into a hail of electrons flying about like independent molecules. With sufficient accuracy for our present purpose, all the stars on the main-sequence, except perhaps those at its extreme lower end, may be supposed to be in the same physical condition. On account of this property, the main-sequence forms an admirable base-line from which to carry out a survey of the Russell diagram in respect of the physical conditions of stellar interiors. Fig. 22 shews that a star to the right of the main-sequence has a greater diameter than a main-sequence star of the same weight. Consequently the energy it would emit in shrinking to its present diameter is less, and hence its molecular energy of motion is less (by Poincaré’s theorem). It follows that its internal temperatures are lower, and its atoms are less completely broken up. Red giants such as Antares are found only to have central temperatures of from one to five million degrees, and their atoms probably retain intact not only their _K_-rings of electrons, but also their _L_-rings and part at least of their _M_-rings. To the left of the main-sequence we come to a region in which stars, if they occurred at all, would have shrunk further, and so would have higher temperatures and more thoroughly broken atoms. Actually no stars are encountered until we come to the white dwarfs. Calculation shews that the central temperatures of these must be many hundreds of millions of degrees at least, and that their atoms must be stripped of electrons right down to the nuclei. Except for a small number of atoms which may have escaped this general fate, the stellar matter must consist of nuclei stripped absolutely bare, and of free electrons, all flying independently through the star. The high densities of these stars provide a convincing proof of the accuracy of this result. The mean density of Sirius _B_ is certainly over 50,000, while that of van Maanen’s star is probably over 300,000. There is no way in which matter can be packed as closely as this, except that of stripping the atoms of electrons right down to their bare nuclei. The clearest general impression we can form of the Russell diagram in terms of physical condition is probably obtained as follows: