In astrophysics and physical cosmology the mass-to-light ratio, normally designated with the Greek letter upsilon, ϒ,[1] is the quotient between the total mass of a spatial volume (typically on the scales of a galaxy or a cluster) and its luminosity.
The mass-to-light ratios of galaxies and clusters are all much greater than ϒ☉ due in part to the fact that most of the matter in these objects does not reside within stars and observations suggest that a large fraction is present in the form of dark matter.
In general, unless a complete spectrum of the radiation emitted by the object is obtained, a model must be extrapolated through either power law or blackbody fits.
Masses are often calculated from the dynamics of the virialized system or from gravitational lensing.
Typical mass-to-light ratios for galaxies range from 2 to 10 ϒ☉ while on the largest scales, the mass to light ratio of the observable universe is approximately 100 ϒ☉, in concordance with the current best fit cosmological model.