Supermassive Black Holes
A supermassive black hole is a black hole with a mass ranging from roughly a hundred thousand to tens of billions of times the Sun's mass, found at the centre of most, if not all, large galaxies including the Milky Way — despite this enormous mass, their formation mechanism and the reason essentially every large galaxy hosts one remain only partially understood.
Curious
Supermassive black holes are, in a sense, a different phenomenon from the stellar-mass black holes formed by individual collapsing stars (see the Codex's black holes (astrophysical) entry) — not simply larger versions of the same thing, but objects whose formation mechanism, occupying a mass range from roughly a hundred thousand to tens of billions of solar masses, remains an active and only partially resolved area of research.
The Milky Way's own central supermassive black hole, called Sagittarius A* (pronounced "Sagittarius A-star"), has a mass of roughly 4 million solar masses — its presence was inferred for decades from the orbital motion of stars circling tightly around an invisible central mass, before the Event Horizon Telescope produced a direct image of it in 2022, following its earlier 2019 image of the even more massive black hole at the centre of the galaxy M87.
Exploring
| Object | Approximate mass | Galaxy |
|---|---|---|
| Sagittarius A* | ~4 million solar masses | Milky Way |
| M87* | ~6.5 billion solar masses | M87 |
Deep Dive
The M-sigma relation, discovered observationally around 2000, revealed a surprisingly tight correlation between a supermassive black hole's mass and its host galaxy's central stellar velocity dispersion (a measure of how fast stars move in the galaxy's central bulge) — a relationship that strongly suggests some form of co-evolution between a galaxy and its central black hole, even though the black hole's mass is typically only a small fraction of the total galaxy mass, and the precise physical mechanism linking the two remains actively debated.
The formation puzzle: How supermassive black holes first formed, particularly the extremely massive ones already observed at very high redshift (implying they grew to billions of solar masses within less than a billion years after the Big Bang), remains one of astrophysics' more significant unsolved problems. Leading proposed formation channels include growth from stellar-mass "seed" black holes via sustained accretion and mergers, or direct collapse of extremely massive primordial gas clouds without an intermediate stellar phase — current observations, including those from the James Webb Space Telescope, have found some very massive early black hole candidates that are proving challenging for standard growth-timescale models to fully explain, an active area of ongoing research.
Supermassive black holes actively accreting large amounts of surrounding matter power some of the most luminous, energetic phenomena in the universe — active galactic nuclei and quasars (see the Codex's forthcoming Active Galactic Nuclei and Quasars entries), which can, at their most extreme, outshine their entire host galaxy's combined starlight, providing an important, independent observational window into supermassive black hole growth across cosmic history.
Concept Relationship Map
Requires: black holes (astrophysical)
Enables: active galactic nuclei [PENDING FULL ENTRY], galaxy formation
Commonly confused with: assuming a supermassive black hole's event horizon is proportionally enormous (it's actually quite compact relative to its host galaxy)
Sources
- Ghez, A.M., et al. (1998). The Astrophysical Journal, 509(2), 678–686.
- Event Horizon Telescope Collaboration (2022). "First Sagittarius A* Event Horizon Telescope Results." The Astrophysical Journal Letters, 930(2), L12.