Note 103 — Chapter XXXI · THE SEPARATION OF SPACE-TIME INTO SPACE AND TIME IN A GRAVITATIONAL FIELD

The Einstein shift in the spectral lines as seen by a definite observer will increase in importance as the atom nears the star or as the star increases in mass. For a star of given mass, the effect will therefore increase as the volume of the star decreases, and hence as its density increases. We understand, therefore, why it is that for two stars of the same mass, the best conditions of observation will be afforded by the star which has the greater density; while for two stars having the same density, the best conditions will be afforded by the star having the greater mass.