Black hole
M31*
The sleeping giant at the centre of Andromeda (140 million M☉)
The 140-million-solar-mass black hole in the nucleus of the Andromeda Galaxy; 30 times heavier than Sgr A* but likewise almost starved. Around it orbit the eccentric disc of old stars discovered by Hubble (the P1/P2 double nucleus) and the P3 cluster of young blue stars.
| Mass | 140 million solar masses |
|---|---|
| Event horizon radius | 2.76 AU: 0.092 times Neptune's orbit |
| Time for light to circle the event horizon once | 2.4 hours |
| Mass | 1,4 × 10⁸ M☉ |
| Distance | 765 kpc |
| Status | Very low accretion (L ≈ 10⁻¹⁰ L_Edd) |
| Discovery | 1988 (çift çekirdek, Lauer & Dressler), 2005 (kütle, Bender ve ark.) |
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).