A0620-00
What is A0620-00?
Black holes & quasars· Distance 3,300 ly. A black-hole binary in Monoceros that briefly became the brightest X-ray source in the sky during its 1975 outburst. A small star whips around the black hole every 7.75 hours. Until Gaia BH1 in 2022, it was the nearest black hole known.
📍 Visible from where you are right now — computed in your browser
bearing · altitude · rise time · best time tonight
Where A0620-00 is in your sky right now: https://nowsky.org/solar/sky?sel=a0620 — location and time set automatically, no login.
Its place in the sky
A black hole, in the direction of Monoceros.
Mass, distance, nature
| Mass | ~6.6 Suns |
|---|---|
| Distance | ~3,300 ly |
| Companion | small K-type star, 7.75-hour orbit |
| Outbursts | 1917, 1975 (brightest X-ray source of its day) |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Discovery and name
Erupted as an X-ray nova in 1975; its companion's orbit (1986) made it one of the first confirmed stellar black holes.
Frequently asked
- What is the mass of A0620-00?
- ~6.6 Suns.
- How far is A0620-00 from Earth?
- ~3,300 ly.
- Where in the sky is A0620-00?
- In the direction of Monoceros.
- Where is A0620-00 in the sky right now?
- Open the sky view for your location and time and it shows the position in the sky right now.
The first outburst, recorded on a photographic plate in 1917
A0620-00 was discovered when the X-ray satellite Ariel 5 caught a sudden outburst in 1975, but that was not its first. Searching the old photographic plates of Harvard Observatory turned up a record of a star at the same spot brightening to 12th magnitude and fading in 1917 — classed at the time as a nova and given the variable-star name V616 Monocerotis. Two outbursts 58 years apart mean the system swallows the material in its disk all at once every few decades.
The observers of 1917 could not have known it was a black hole; the concept did not yet exist. Harvard's half-million plates are now digitised and used to trace the sky of a century ago, and A0620-00 is the leading case of those plates revealing the past of a modern object. If the cycle holds, the next outburst falls around the 2030s, and this time it will be recorded not as a star but as a black hole waking up.
The black hole in depth
A0620-00 is a black-hole binary about 3,300 light-years away in Monoceros, with a black hole of roughly 6.6 solar masses. Its companion, a K-type star about half the Sun's mass, completes an orbit in just 7.75 hours. In August 1975 Britain's Ariel 5 satellite caught an X-ray outburst that outshone every other X-ray source in the sky for weeks; the spot matched V616 Monocerotis, a variable that had flared like a nova on photographic plates in 1917.
After the outburst faded, radial-velocity measurements of the companion in 1986 set the unseen object's minimum mass above three Suns, ruling out a neutron star. It was among the first black holes weighed in quiescence rather than from a bright companion, and the technique became the standard way to certify dormant X-ray novae as black holes. If its roughly 60-year outburst cycle holds, it could wake again in the 2030s.
At its normal eighteenth magnitude the companion needs a large telescope, but the next outburst should bring it near magnitude 11. Select A0620-00 in the Sky view to see its position in Monoceros just east of Orion, best observed on winter evenings.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.