What Is a Supermassive Black Hole?

Unlike stellar-mass black holes (3-100 solar masses) formed from collapsing stars, supermassive black holes contain millions to billions of solar masses and their formation remains partly mysterious. They may have grown from massive "seed" black holes in the early universe, accreting gas and merging with other black holes over billions of years. The Milky Way central supermassive black hole, Sagittarius A* (Sgr A*), has about 4 million solar masses and was directly imaged by the Event Horizon Telescope in 2022. The largest known, TON 618, exceeds 66 billion solar masses. When actively accreting matter, supermassive black holes power quasars and active galactic nuclei, outshining entire galaxies. The M-sigma relation shows a tight correlation between a galaxy central black hole mass and the velocity dispersion of stars in its bulge, suggesting co-evolution.