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Astronomers have identified dozens of candidate stellar-mass black holes in parts of the Andromeda Galaxy, but there is no complete, galaxy-wide count. The often-cited figures come from different surveys and classification methods, so they should not be added together. Andromeda also has a separate supermassive black hole at its center.
How many black holes have been found in Andromeda?
The best-known counts are tallies of candidate black holes in selected regions—not an inventory of every black hole in M31. The figures differ because the studies used different observations, coverage and confidence categories.
| Study | What it reported | How to interpret it |
|---|---|---|
| NASA’s 2013 report on Chandra observations | 26 newly identified candidates; 35 in the surveyed region after including nine previously identified candidates | A regional candidate tally from 152 observations spanning about 13 years, not a galaxy-wide census. NASA, June 12, 2013 |
| Barnard et al., 2014 preprint | 50 candidates: 42 strong and eight plausible | A separate study and classification scheme; its total should not be added to the 2013 tally. Study abstract |
| Moon et al., 2024 NuSTAR survey | 20 sources detected at 2-sigma in the 4–25 keV band; 14 X-ray-binary candidates; nine tentative compact-object classifications, including three black holes and six neutron stars | A survey of part of M31’s disk, covering about 0.45 square degrees with ten moderate-depth observations—not a new total for the galaxy. The Astrophysical Journal study |
These counts describe different samples and confidence levels. None establishes how many black holes Andromeda contains overall.
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A black hole does not emit light that a telescope can photograph directly. Some can nevertheless be detected when one is in a binary system with a companion star. Gas drawn from the star heats up as it falls toward the black hole and emits X-rays.
For the 2013 Chandra analysis, researchers assessed X-ray brightness and color, along with changes in brightness over time, to distinguish likely stellar-mass black-hole systems from background sources and neutron-star alternatives. XMM-Newton spectra provided additional evidence for some candidates. These are indirect clues supporting a classification, not direct images of the black holes. NASA’s account of the Chandra findings
Detection also favors systems that are actively pulling material from a nearby companion. As Robin Barnard, lead author of the Chandra work, put it: “Most black holes won’t have close companions and will be invisible to us.” The known candidates therefore reveal only part of the population.
Are the candidates concentrated near Andromeda’s center?
In the 2013 report, seven of the 35 candidates were within 1,000 light-years of M31’s center. The researchers related this concentration to Andromeda’s larger central stellar bulge. It is a pattern in that study’s candidate sample, not proof that all—or most—of the galaxy’s black holes are in that region. NASA’s Chandra summary
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What is M31*?
M31* is the supermassive black hole at Andromeda’s center. It is distinct from the stellar-mass candidates found in X-ray binaries: the candidates are smaller black holes in systems with companion stars, while M31* is the galaxy’s central dark object. NASA’s Hubble account gives a historical mass estimate of 140 million solar masses, based on the work described there. That figure is an estimate for M31* alone, not a count or combined mass for the stellar-mass candidates. NASA’s Hubble account
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What about black holes in Andromeda’s globular clusters?
Some studies model how black-hole populations might form and evolve inside globular clusters. A 2023 MOCCA paper modeled clusters in Milky Way-like and Andromeda-like galaxies; it provides theoretical context about possible populations and dynamics, not a telescope-based count of confirmed black holes in M31. The 2023 modeling study
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