You see a galaxy which lies at distance D from the earth. You measure a flux from this galaxy of f. If you assume a mass-to-light ratio of r (i.e. r is the number of kilograms needed to generate one watt of radiation), derive an equation for the stellar mass m of the galaxy.

Answer 1

4*pi*f*r*D^2
Stellar mass , m= L * r where L is the luminosity.

Luminosity can be derived by rearranging the usual inverse-square law equation