| Star | Class | Mean density | Radius | Product | ||||
|---|---|---|---|---|---|---|---|---|
| SX | Cas |  | 0.0004 | 0.0002 | 15.3 | 18.6 | 0.006 | 0.004 |
| RX | Cas |  | 0.0005 | 0.0004 | 14.3 | 14.3 | 0.007 | 0.006 |
| RZ | Oph |  | 0.001 | 0.00003 | 10.1 | 33.5 | 0.010 | 0.001 |
| RT | Lac |  | 0.0013 | 0.010 | 4.6 | 4.6 | 0.059 | 0.046 |
| W | Cru |  | 0.00002 | 0.000025 | 94 | 36 | 0.00019 | 0.0009 |
| U | Peg |  | 0.83 | 0.67 | 1.2 | 1.2 | 1.0 | 0.8 |
| W |  | G | 1.8 | 1.8 | 0.9 | 0.9 | 1.6 | 1.6 |
| Sun |  | 1.0 | 1.0 | 1.0 | ||||
In mean density these stars display a range of
, while the range in surface gravity is
, illustrating the significant fact that the mean density varies far more widely than the surface gravity. The latter quantity is the important one in determining the pressure that may be assumed to exist in the reversing layer. If the masses of the very luminous stars of low mean density, such as W Crucis, exceed the solar mass, as they most probably do, the hypothetical radii are increased, and the range in surface gravity becomes even smaller than before.
The data for stars of known mean density and radius permit the estimation of the range in surface gravity, and hence of the range in pressure, encountered in the reversing layer. In the absence of knowledge of the appropriate optical depth, however, the actual pressure cannot be deduced from such considerations, and recourse must be made to more indirect methods. The present view is based upon a number of considerations, none of which would alone be of great weight. All of the conclusions, taken together, however, indicate that the upper limit of the pressure for the region in which the Fraunhofer lines originate is of the order of
.
Attention was first called to the probability of an extremely low pressure in the reversing layer by R. H. Fowler and Milne,[62] in advancing the form of ionization theory which is to be analyzed in later chapters. The conclusion that the pressure in the reversing layer is exceedingly low was a direct outcome of their discussion, and they mentioned that the results from other methods converged in the same direction.