160 Liquid Stars [ch. v 
This is of course directly comparable with the diagram already given in 
fig. 5 (p. 61), this latter presenting the results of observation while fig. 12 
presents the predictions of theory. It is also very nearly, but not quite, 
directly comparable with the diagram shewn in fig. 6, in which 2100 stars 
are plotted in respect of absolute magnitude and spectral type. 
146. Theory predicts that only those configurations are stable which lie 
in parts of the diagram (fig. 12) in which the lines are drawn thick. Thus, 
unless theory is at fault, stars ought to occur in the corresponding regions 
in the observational diagrams shewn in figs. 5 and 6, and in no others; the 
regions in which the lines are drawn thin ought to be untenanted by stars 
since the corresponding configurations are unstable, to be quitted with all 
possible speed. 
A comparison of the theoretical and observational diagrams shews that 
the predictions of theory are borne out at least to a sufficient extent to 
suggest that the theory is on the right general lines. This being so, it is 
possible to identify the various areas of stable configurations predicted by 
theory with the areas found observationally, and thus to specify the physical 
structure of the stars which inhabit the various areas in the diagrams. The 
identification is shewn in fig. 13, in which the diagram of 2100 stars already 
shewn in fig. 6 (p. 62) is parcelled out into stable and unstable regions in 
accordance with the requirements of theory. The main sequence is seen to 
consist of stars whose atoms are ionised down to their AT-rings and jammed 
together. On the left-hand edge of the sequence the deviations from the 
gas-laws are so great that the atoms are packed almost as closely as they 
will go. The giant branch has a similar interpretation with the difference 
that the atoms are ionised only down to their Z-rings. In the white dwarfs 
the atoms are mainly ionised down to their nuclei, but a few K -ring atoms 
remain and these, although few in number, probably occupy the major part 
of the available space; it is their jamming, rather than that of the nuclei, 
which results in the departures from the gas-laws which ensure the stability 
of the star. 
The theoretical diagram suggests that the white-dwarf series ought to 
extend upwards right into the earliest spectral types. I have suggested* 
that the so-called “Dwarf Wolf-Rayet” stars (O-type stars of small radius) 
may be found to occupy the upper half of this series; if this conjecture is 
confirmed the 0-type stars ought to be found to be divided sharply into stars 
of small and of fairly large radius. 
147. As we have seen, the general similarity of the observational and 
theoretical diagrams makes it possible to superpose them in such a way that 
the areas of stability coincide fairly well. In fig. 13 the diagrams are shewn