A THEORY of the physical conditions in the interior of a star should, if sufficiently complete, lead to formulas determining the total radiation emitted, and hence the absolute brightness. In the simplest case-that of a star in the condition of a perfect gas-considerable progress appears to have been made towards the solution of this problem. It appears that in the main the total radiation is a function of the mass. Stars of equal mass but different density, and consequently different spectral type or effective temperature, will show minor differences of brightness which will be ignored in the present brief account of the results; they have, however, been duly allowed for in the comparisons of theory and observation referred to below and illustrated in the diagram. With this understanding we shall regard the absolute brightness as a function of the mass only-always provided that the star is not too dense to be treated as a perfect gas.