Black hole
Sagittarius A*
The supermassive black hole at the centre of the Milky Way
The dynamical centre of the Milky Way: a supermassive black hole of 4.3 million solar masses. The orbits of the surrounding S-stars (S2, S38, S55) let us measure its mass directly; in 2022 the Event Horizon Telescope imaged its shadow. The event horizon is 12.7 million km across (~18 solar radii), its shadow 52 micro-arcseconds on the sky.
| Mass | 4.3 million solar masses |
|---|---|
| Event horizon radius | 12.7 million km: 18 times the Sun's radius |
| Time for light to circle the event horizon once | 4.4 minutes |
| Mass | 4,297 × 10⁶ M☉ |
| Distance | 8,178 pc (26,700 light-years) |
| Shadow diameter | ~52 µas (EHT 2022) |
| Nobel | 2020 — Genzel & Ghez (S-star orbits) |
| Status | Quiet (low accretion) |
| Discovery | 1974 (Balick & Brown, radyo) |
Through a telescope from Earth
The event horizon is the boundary where the escape velocity equals the speed of light: nothing inside, not even light, can get out. Its radius grows in direct proportion to the mass (the Schwarzschild radius, for a non-rotating black hole).