Building black holes from gas and from dead stars
In astronomy, the creation of black holes is one of those things we talk about… a lot. We know Big stars form small black holes. We know stars can only get so big. We even suspected that at a certain point stars exploded rather than forming black holes. This meant, from theory, stars could form black holes with just one range of sizes.
We also know that larger and larger black holes can be formed by merging black holes or neutron stars, and this technique allows black holes of unlimited size to ultimately come into being.
But how do we know when a black hole has formed through the collapse of a star, or through the merger of two smaller black holes?
It turns out, when black holes merge, they end up with more angular momentum than when one giant star collapses. We don’t generally have the ability to look at black holes and say “Ah ha, your angular momentum is…” BUT… when we observe two black holes merging using gravitational waves, we can get both their starting sizes and angular momentums.
A team of researchers, led by Fabio Antonini (Cardiff University), recently reviewed gravitational wave data from LIGO–Virgo–KAGRA to see if they could figure out just how big a black hole one star could make. Differences in angular momentum made the break in the population of black holes formed by mergers and by stellar collapse easy to separate. The data suggests that above 44.3 Solar masses, give or take, black holes consistently form via mergers. This result is so clean it even helps place limits on specific nuclear reactions in stars and also shows us that black holes are growing via mergers in dense star clusters.
While it is possible that the largest stars may also form even larger blackholes, somewhere above 130 solar masses, those kinds of mergers are at the limits of our detectors and we don’t have the detection numbers to explore this population … at least not yet. This paper, appearing in Nature Astronomy, left some mysteries for others to explore.
Sources
- Cardiff University Press Release via EurekAlert
- Antonini, F., Romero-Shaw, I.M., Callister, T. et al. Gravitational-wave constraints on the pair-instability mass gap and nuclear burning in massive stars. Nat Astron (2026). DOI: 10.1038/s41550–026–02847–0
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