The mass of neutral hydrogen gas in a galaxy as a function of its luminosity. This ratio has a number of repercussions in current astronomy research. A high ratio (10 or more) for the galaxy as a whole is seen as a sign of relative youth and can be used to identify potentially interesting galaxies. The value of the ratio in different parts of a galaxy speaks to its dark matter distribution.

The luminosities of stars can also be used to estimate their masses, but the theoretical stellar mass-to-luminosity relationship has been largely unverified.