-stars, in the spectrum of which these lines occur, themselves have very small radial velocities. As Young remarked, it seems very strange that these calcium clouds should so consistently choose to lie in front of stars of type
or earlier. An objection against this hypothesis is to be found in the fact that in the case of various systems these two calcium lines are not at rest but move, although with somewhat less amplitude than the other proper lines of the double star.
An additional circumstance which lends support to the theory that calcium lines denote the presence of an atmosphere around the star is that a great number of helium-stars are enveloped in a nebulous atmosphere which is actually visible.
Assuming then that the calcium absorption lines are due to such atmospheres, we may apply the same process as in the case of the Orion nebula, i.e. if the shifts of the spectral-lines of the stars be systematically falsified by a superposed gravitational effect, this should be expressed by the lines of the actual spectrum from a double star being displaced towards the red as compared with the fixed calcium lines.
This phenomenon has been clearly observed. The result has not yet been quantitatively fixed, as the numbers taken are not regarded as final.
All stars in the spectra of which the
and