The M–σ relation, also known as the "mass–sigma relation," describes a correlation between the mass of a supermassive black hole (SMBH) at the center of a galaxy and the velocity dispersion (σ) of stars in that galaxy's bulge.
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The M-sigma relation is an empirical correlation between supermassive black hole mass and the stellar velocity dispersion of its host's spheroidal component. A commonly used rough parametrization has exponent near four, but the normalization, exponent, scatter, and selection effects depend on the observed sample. It is distinct from the black-hole--bulge mass relation.