How big is the black hole at the Galactic Center?

Sagittarius A* has a mass of about four million times the Sun's, compact enough that its gravitational radius is comparable to the orbit of Mercury, far smaller than the stellar orbits used to measure it.

Why doesn't our central black hole shine like a quasar?

Sagittarius A* is currently accreting very little gas, so it radiates feebly. Quasars and other active galactic nuclei shine because much larger inflows of matter heat their accretion disks to enormous luminosities.

The Galactic Center and Sagittarius A*

At the dynamical heart of the Milky Way lies Sagittarius A*, a compact radio source identified as a supermassive black hole of about four million solar masses.

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Definition

The Galactic Center is the rotational and gravitational center of the Milky Way, hosting a nuclear star cluster and the compact source Sagittarius A*, which the orbits of nearby stars and direct imaging identify as a supermassive black hole of roughly four million solar masses.

Scope

This topic covers the dense nuclear star cluster and interstellar environment of the Galactic Center, the stellar orbits that reveal the central mass, the identification of Sagittarius A* as a supermassive black hole, its low-luminosity accretion and flaring, and the imaging of its shadow at the event horizon scale.

Core questions

What evidence establishes that Sagittarius A* is a supermassive black hole?
How are the orbits of stars near the center used to weigh the central mass?
Why is Sagittarius A* so faint compared with active galactic nuclei?
What did event-horizon-scale imaging reveal about the black hole?

Key theories

Stellar orbits as a black hole scale: Decades of tracking individual stars on Keplerian orbits around an unseen central point mass yield a mass of about four million solar masses confined within a tiny region, requiring a supermassive black hole.
General-relativistic tests near Sgr A*: The orbit of the star S2 shows gravitational redshift and relativistic precession, confirming general relativity in the strong field around the central black hole.
The black hole shadow: Very-long-baseline interferometry resolved a ring of emission around a central brightness depression at Sgr A*, consistent with the shadow predicted for a black hole of the inferred mass.

Clinical relevance

Sagittarius A* is the nearest supermassive black hole and the best laboratory for testing strong-field gravity and black hole accretion; its quiescence also informs how galactic nuclei alternate between active and dormant phases.

History

The radio source Sagittarius A* was discovered in 1974. Infrared monitoring of stellar orbits through the 1990s and 2000s, led by independent teams, established its mass, work recognized with a Nobel Prize. Relativistic effects in the orbit of S2 were detected in 2018, and the Event Horizon Telescope imaged its shadow in 2022.

Key figures

Reinhard Genzel
Andrea Ghez
Donald Lynden-Bell

Seminal works

ghez2008
genzel2010
eht2022

Frequently asked questions

How big is the black hole at the Galactic Center?: Sagittarius A* has a mass of about four million times the Sun's, compact enough that its gravitational radius is comparable to the orbit of Mercury, far smaller than the stellar orbits used to measure it.
Why doesn't our central black hole shine like a quasar?: Sagittarius A* is currently accreting very little gas, so it radiates feebly. Quasars and other active galactic nuclei shine because much larger inflows of matter heat their accretion disks to enormous luminosities.