Mass to light ratio
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In physical cosmology the mass to light ratio is a comparison of the total mass of a galaxy or a cluster compared to its luminosity. These are often reported using the sun as a baseline ratio. The mass to light ratio of galaxies and clusters are all much greater than the sun due in part to the fact that most of the matter in these objects is not stars and indeed a large fraction is dark matter.
Masses are often calculated from the dynamics of the virialized system or from gravitational lensing. Luminosities are obtained from photometric observations and correcting the observed brightness of the object for the distance dimming effects.
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