Mass-to-light ratio

In astrophysics and physical cosmology the mass to light ratio, normally designated with the Greek upsilon symbol ${\displaystyle \Upsilon }$, 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 ${\displaystyle \Upsilon _{\odot }}$ = 5193 kg/W equal to a solar mass (M_☉) divided by a solar luminosity (L_☉), (M_☉/L_☉). The mass to light ratios of galaxies and clusters are all much greater than ${\displaystyle \Upsilon _{\odot }}$ 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.

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