CHAPTER VI 
THE EVOLUTION OF THE STARS 
General Principles. 
153. The early spectroscopists believed that the spectrum of a star 
provided a sure indication of the star’s age. Huggins and Lockyer had found, 
for instance, that the spectrum of Sirius exhibited hydrogen lines very strongly 
and calcium lines rather weakly; in the solar spectrum the relative strength 
of these two sets of lines was reversed, calcium being strong and hydrogen 
weak. They concluded that hydrogen was specially prominent in the con 
stitution of Sirius and calcium in that of the sun. Believing that Sirius must 
one day develop into a star similar to our sun, they conjectured that its 
substance must gradually change from hydrogen into calcium and other more 
complex elements, thus finding support for the long-established hypothesis 
that the more complex elements were formed by gradual evolution out of the 
simplest. In this way they were led to regard a star’s spectrum as an index 
to its age. 
As we have seen, the true interpretation of these observations is merely 
that the surface of Sirius is at a temperature at which hydrogen is specially 
active in emitting and absorbing radiation, while the sun’s surface is at a 
lower temperature at which hydrogen is comparatively inert, while calcium, 
iron, etc., have become active in its place. Just as the laboratory physicist 
can produce different spectra from the same vacuum tube by varying the 
mode and conditions of excitation, so Nature produces different spectra from 
the same stellar material by varying its temperature. The linear sequence 
into which the spectra of stars fall is merely one of varying surface temperature. 
Clearly this circumstance robs stellar spectra of all direct evolutionary 
significance. The spectra of the stars merely inform us as to their present 
surface temperatures, so that even if we could arrange the stars in order of 
age, a comparison of their spectra would only shew whether their surfaces were 
becoming hotter or cooler; it would give no information as to any evolutionary 
or chemical changes occurring in their substance. 
To obtain evidence as to evolutionary changes in a star, we must probe 
deeper into the star than we can by a study of the star’s surface spectrum 
and try to obtain information as to changes in progress in the star’s interior. 
We have already been driven to look to the annihilation of matter for the 
source of stellar energy. It necessarily follows that the primary evolutionary 
change in a star is decrease of mass—the older a star gets, the less massive 
it becomes. Other evolutionary changes follow as an inevitable corollary. The