-stars are found to be spectroscopic double stars, i.e. their spectral lines fluctuate periodically about some mean position. Hartmann was the first to notice that in the spectrum of the
-star
. Orionis the absorption lines
and
of calcium, viz. 3933.82 and 3968.63 Ångstroms, occur, but that they do not share in the periodic movements of the other lines. A number of other stars belonging to early spectral types contain calcium absorption lines in their spectra, which exhibit a similar anomaly, inasmuch as they either remain immovable or execute periodic motions which are of feeble amplitude compared with the proper stellar lines. In view of the important rôle that calcium plays in the outermost layers of the gaseous atmosphere encircling the sun, and in view of the discussion above, the suggestion forces itself upon one that these calcium lines indicate the presence of an extensive atmosphere surrounding the star.
It has often been put forward that these lines are due to the light from these stars being absorbed by vast interstellar clouds of calcium. Evershed considers that this is supported by the fact that when the motion of the solar system is subtracted from that calculated from the fixed calcium lines (owing to the ordinary Doppler effect), the remaining motion is very small. But this argument does not carry weight inasmuch as it is known that the