What is a supermassive black hole, and where are they typically found?

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An extremely massive black hole containing millions to billions of solar masses, found at the centers of most galaxies. A supermassive black hole is an extremely massive black hole containing millions to billions of solar masses, found at the centers of most, if not all, large galaxies. The supermassive black hole at the center of the Milky Way, called Sagittarius A* (Sgr A*), has a mass of approximately 4 million solar masses. These objects are thought to play a crucial role in galaxy formation and evolution, and when actively accreting matter, they power active galactic nuclei and quasars.

A black hole formed from the collapse of a single massive star is a stellar-mass black hole, typically containing 5 to 50 solar masses - millions of times less massive than supermassive black holes. The claim that supermassive black holes are hypothetical and have never been observed is incorrect; the Event Horizon Telescope directly imaged the supermassive black hole in the galaxy M87 in 2019 and Sgr A* in 2022. A large dark cloud of gas that blocks visible light describes a dark nebula, which is an entirely different astronomical object.

Supermassive black holes are an important and frequently tested topic on the DSST Astronomy exam. Students should understand the evidence for their existence (stellar orbits near galactic centers, active galactic nuclei, direct imaging by the Event Horizon Telescope), the relationship between black hole mass and galaxy properties (the M-sigma relation), and the distinction between supermassive, stellar-mass, and intermediate-mass black holes.

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