Black hole
M33 X-7
An eclipsing massive X-ray binary in the Triangulum Galaxy
A black hole in the Triangulum Galaxy (M33) orbiting a 70-solar-mass giant star every 3.45 days and regularly eclipsed behind its companion. Because the eclipses pin down the orbital inclination, its mass (15.65 M☉) is known with unusual precision.
| Mass | 15.7 solar masses |
|---|---|
| Event horizon radius | 46 km: the size of a city |
| Time for light to circle the event horizon once | 9.7 × 10⁻⁴ seconds |
| Mass | 15,65 M☉ |
| Companion | O7-8 III, ~70 M☉ |
| Orbital period | 3.45 days (eclipsing) |
| Distance | 840 kpc (M33) |
| Discovery | 2007 (Orosz ve ark., Chandra + Gemini) |
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).