supermassive

Supermassive is a term used in astronomy to describe objects that have masses vastly exceeding those of typical stars. The phrase is most often applied to black holes, particularly those at the centers of galaxies, and to hypothetical extremely massive stars. In practice, supermassive black holes contain millions to billions of solar masses.

Most large galaxies, including the Milky Way, host a supermassive black hole at their center. These objects power active galactic nuclei and quasars, emitting across the electromagnetic spectrum. The mass of a central supermassive black hole correlates with properties of its host galaxy, such as bulge mass and stellar velocity dispersion, a relationship commonly called the M-sigma relation.

Formation and growth theories for supermassive black holes include direct collapse of massive gas clouds in the early universe, remnants of massive stars, and hierarchical growth through accretion and mergers with other black holes. Once formed, they grow by accreting gas and by merging during galaxy collisions, linking their evolution to that of their host galaxies.

Observation and measurement rely on several techniques. Tracking the motions of stars and gas near the galactic center reveals the presence and mass of the black hole. Reverberation mapping measures time delays in emission from active nuclei to estimate size and mass. Very long baseline interferometry, exemplified by the Event Horizon Telescope, has imaged the shadows of M87* and provided constraints on Sagittarius A*. Gravitational waves from supermassive black hole mergers are a target for future space-based observatories.

Related topics include supermassive stars, the M-sigma relation, active galactic nuclei, and intermediate-mass black holes.