2. In the long run, we must expect the stars to grow colder, at
least as to the surface strata. What the average interior
temperatures are is another question; the highest interior
temperatures are thought to be reached at an intermediate or
quite late stage in the process, in accordance with principles
investigated by Lane and others; but the temperatures existing
in the deep interiors seem to have little direct influence in
defining the spectral characters of the stars, which are
concerned more directly with the surface strata.[1] We should
therefore expect the simpler types of spectra, such as we find
in the helium and hydrogen stars, in the early stages of the
evolutionary process. The complicated spectra of the metals,
and particularly the oxides of the metals, should be in
evidence late in stellar life, when the atmospheres of the
stars have become denser and colder.
