-stars, called Helium stars on account of the predominance of helium lines in their spectra. Other observations have led astronomers to infer that the
-stars have unusually great masses but small densities. The result has been distinctly in favour of the Einstein shift on the basis of the foregoing discussion. The same was found to hold for the bright
- and
-stars, which are considered to be at a lower temperature and possessed of enormous surface extent, which accounts for their brilliance.
If we indicate the mean shift of the lines towards the red by
, then for