176 
THE MASS-LUMINOSITY RELATION 
mass—and changing it considerably. Evolution along PQ involves at the 
most minor changes of mass. 
The dwarf stars enormously outnumber the giants, and if Seares’s 
diagram had shown the true proportions of the stars in space the line QR 
would have been the prominent feature to the exclusion of almost every 
thing else. Indeed, the next most conspicuous feature would have been 
the white dwarfs, and the giants along PQ would scarcely be noticed. We 
call QR the Main Series. We can trace a definite prolongation of it, as 
shown by the broken line, through the hotter types B and 0 ; the latter 
stars are not very numerous in space. 121 
Fig. 4. Schematic Type-Magnitude Diagram. 
121. If evolution is to continue to play the dominant part in our theories 
of the stars that it has done for the last 50 years we must suppose that 
there is an evolution along the main series; and this necessarily involves 
a change of mass. The greater part of the star’s life will be occupied by 
evolution along this series, the stars presumably joining and ultimately 
leaving the line at different points according to their initial masses or 
extraneous influences to which they may have been subjected. We should 
thus regard the giants as stars on the way to join the main series, and the 
white dwarfs as stars which have finally left it. 
If there is no evolution along the main series then each star ultimately 
leaves QR at the point where it reached it, after taking a long rest on the 
line. So far as can be ascertained the stars approaching the line (giants) 
are mostly of considerably larger mass than the stars leaving the line 
(white dwarfs), so that evolution down the line is strongly suggested. 
It will be seen that any modern theory of evolution is bound up with 
the question of the possibility of change of mass of a star.