"The mass of an object is one of the most fundamental properties of the object. As such, reliable ways to measure mass are exceedingly important. One way is to simply measure the optical emission (luminosity of a galaxy) and to (independently) determine the average mass of a star in the galaxy. This will then lead to an estimate for the mass of the galaxy if all of the luminosity is in fact due to the stars and all material radiates in a similar manner. This method has problems because it is sometimes difficult to know whether all mass produces radiation."