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 the cluster Abell 85, about 700 million light-years away in Cetus, where the galaxy's core is strangely empty of stars.
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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 |
| How the mass is measured | modelling the speeds of stars in the galaxy’s centre |
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.
The hollow heart of Holmberg 15A
At the centre of this galaxy lies a wide region about 15,000 light-years across where stars are very scarce. Scientists see it as the mark of two galaxies that merged. When galaxies merge, their central black holes pair up and sink toward the middle. As they do, they fling nearby stars outward like a slingshot, leaving an empty space at the centre.
Larger empty cores tend to go with heavier black holes, so a giant black hole was expected in Holmberg 15A even before it was weighed. The name comes from a list of galaxies lying close together that the Swedish astronomer Erik Holmberg compiled in 1937. In 1941 Holmberg also used light bulbs and light meters to imitate two galaxies pulling on each other, long before computers could do such work.
The black hole in depth
Holmberg 15A is a giant elliptical galaxy at the heart of the cluster Abell 85, about 700 million light-years away in Cetus. In 2019 a team from Germany's Max Planck Institute used the Very Large Telescope in Chile to measure in detail how fast the stars near the galaxy's centre move. The result put the central black hole at about 40 billion solar masses.
The masses of black holes in distant quasars are usually estimated indirectly, from the light of their gas. Holmberg 15A's was measured directly from the motions of stars, so it is considered more reliable. Stars move faster near a heavy mass, so their speeds reveal how much mass sits at the centre. Among black holes weighed this way, Holmberg 15A ranks at the very top.
The galaxy is about 14th magnitude, only a faint smudge even in large amateur telescopes on autumn nights, and the black hole cannot be seen. Its event horizon is about 240 billion km across, more than 25 times the width of Neptune's orbit. In the Sky view it appears within Cetus. It could hold the whole solar system many times over.
Frequently asked
- What is the mass of Holmberg 15A?
- The mass of Holmberg 15A is ~40 billion Suns (measured from stellar motions).
- How far is Holmberg 15A from Earth?
- The distance of Holmberg 15A is ~700 million ly.
- What is notable about Holmberg 15A?
- The note of Holmberg 15A is stars scooped from a 15,000-ly core.
- Where in the sky is Holmberg 15A?
- Holmberg 15A lies in the direction of Cetus.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.