-, and M-stars. The difference amounts to
, and is a little greater than that given by the statistical method.
The two stars
and 36 Orionis give a displacement towards the violet end. It has been suggested that they do not belong to the more limited group of Orion stars, but are only projected into that portion of the celestial sphere. This is supported by the fact that both stars have only very small spherical proper motions, and that the radial velocities observed for them differ considerably from the mean of the radial velocities of the others.
This method has not been successfully applied to other stellar systems inasmuch as the nebulæ of those which are available emit such feeble light that it has not been possible to establish the displacement to any degree of accuracy. Eddington recently pointed out that a very important factor had been neglected in the fundamental equations of the early theories concerning the equilibrium of stellar matter, viz. the pressure due to radiation. According to his theory, the equilibrium in the interior of the star (regarded as a gaseous sphere) is determined by three conditions. These are gaseous pressure, radiational pressure, and gravitational forces.
Calculation shows that for very great masses the gravitational pressure is almost entirely balanced by radiational pressure. This implies that any additional force such as that due to a centrifugal field of rotation would lead to an unstable condition.
It can, furthermore, be deduced from Eddington's theory that only stars whose masses exceed a certain minimum value can in the course of their evolution reach the very high surface-temperatures which have been observed in the case of the
