# Mass-to-light ratio

In astrophysics and physical cosmology the mass to light ratio, normally designated with the symbol $\Upsilon$[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. These ratios are often reported using the value calculated for the Sun as a baseline ratio which is a constant $\Upsilon_\odot$ = 5133 kg/W equal to a solar mass divided by a solar luminosity. The mass to light ratios of galaxies and clusters are all much greater than $\Upsilon_\odot$ 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.
Typical mass to light ratios for galaxies range from 2 to 10 $\Upsilon_\odot$ while on the largest scales, the mass to light ratio of the observable universe is approximately 100 $\Upsilon_\odot$, in concordance with the current best fit cosmological model.