Holmberg 15A
What is Holmberg 15A?
Black holes & quasars· Distance 700 million ly. One of the heaviest black holes whose mass has been measured directly from the motions of stars. It sits in the central galaxy of cluster Abell 85, 700 million light-years away in Cetus, and has carved a vast starless core in its host.
📍 Visible from where you are right now — computed in your browser
bearing · altitude · rise time · best time tonight
Where Holmberg 15A is in your sky right now: https://nowsky.org/solar/sky?sel=holmberg 15a — location and time set automatically, no login.
Its place in the sky
A black hole, in the direction of Cetus.
Mass, distance, nature
| Mass | ~40 billion Suns (measured from stellar motions) |
|---|---|
| Distance | ~700 million ly |
| Host | central galaxy of cluster Abell 85 |
| Note | stars scooped from a 15,000-ly core |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Discovery and name
The central galaxy of Abell 85; in 2019 stellar motions directly weighed its 40-billion-Sun black hole.
Frequently asked
- What is the mass of Holmberg 15A?
- ~40 billion Suns (measured from stellar motions).
- How far is Holmberg 15A from Earth?
- ~700 million ly.
- What is notable about Holmberg 15A?
- stars scooped from a 15,000-ly core.
- Where in the sky is Holmberg 15A?
- In the direction of Cetus.
- Where is Holmberg 15A in the sky right now?
- Open the sky view for your location and time and it shows the position in the sky right now.
Holmberg — the Swedish catalogue of galaxy pairs, 1937
The name Holmberg 15A comes from the fifteenth entry in the "catalogue of double galaxies" compiled in 1937 by the Swedish astronomer Erik Holmberg. He picked some 800 pairs of galaxies lying close together from photographic plates and was the first to examine systematically the idea that galaxies interact through gravity. In 1941 he ran an "analogue simulation" of a galaxy collision with 74 light bulbs and photocells, showing without a computer that merging galaxies grow tidal tails.
The giant empty core of Holmberg 15A is a product that proves him right. When two galaxies merged, their central black holes paired up and sank inward, flinging the central stars outward like a slingshot and leaving a hollow core 15,000 light-years across. Because the size of such a core scales with the total mass of stars ejected, the core's size is independent evidence for the black hole's 40-billion-Sun mass.
The black hole in depth
Holmberg 15A is a giant elliptical galaxy at the centre of the cluster Abell 85, about 700 million light-years away in Cetus. In 2019 a Max Planck team used the Very Large Telescope to map the velocities of stars in the galaxy's core and pinned the central black hole at roughly 40 billion solar masses. Among black holes weighed directly from stellar motions rather than by indirect quasar methods, it ranks near the very top.
The galaxy's centre is a depleted core about 15,000 light-years across, nearly empty of stars. Such cores are thought to form when two galaxies merge: their black holes pair up and sink toward the centre, slingshotting nearby stars outward. Larger cores tend to host heavier black holes, and since Holmberg 15A has the largest core known, a monster was expected even before the measurement.
The galaxy shines around fourteenth magnitude and appears as a faint smudge in large amateur telescopes on autumn nights in Cetus, though the black hole itself remains unseen. Select Holmberg 15A in the Sky view to find its place within Cetus. Its event horizon spans about 240 billion kilometres, enough to engulf the entire solar system.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.