- and
-stars.
It therefore seems likely that the
- and
-stars have in the process of evolution passed through a stage of which the radiational pressure has brought about a condition of unstable equilibrium, and one might expect them to be surrounded by cosmic dust which has become dissociated from the nuclei of the system.
In some cases this dissociated matter may be in a very fine state of division, and may extend so far into space that the absorption lines they produce in the spectrum of the star they surround may originate from a gravitational field which differs perceptibly from that at the surface of the star. There are definite signs of the existence of such atmospheres. A high percentage of