Stellar Black Holes
A stellar black hole is a black hole formed by the gravitational collapse of the core of a massive star, typically with a mass between about 3 and 100 solar masses, so far detected through the motion of companion stars in X-ray binaries, through gravitational waves from mergers, and, in a few cases, through gravitational microlensing.
Curious
Most stars end quietly as white dwarfs. The cores of the most massive stars, though, are too heavy for any known pressure to resist, and they collapse. If enough mass remains, the result is a black hole a few to a few dozen times the Sun's mass, only tens of kilometres across. Roughly one in a thousand stars is expected to form one; the Milky Way may hold tens of millions or more.
Because black holes emit no light, they are found by their effects: gas falling from a companion star heats to millions of degrees and shines in X-rays (see Accretion Discs), and when two black holes merge they send out gravitational waves, first detected by LIGO in 2015.
Exploring
| Detection method | Example | Constraint obtained |
|---|---|---|
| X-ray binary | Cygnus X-1 | Companion orbit gives mass (~21 M☉) |
| Gravitational waves | GW150914 | Masses ~36 and ~29 M☉ merging |
| Microlensing | OGLE-2011-BLG-0462 | Isolated candidate; mass estimates differ between analyses |
Deep Dive
Formation. Stars above roughly 20–25 solar masses are expected to leave black holes, either by direct collapse or by fallback after a weak supernova, though the mapping between initial mass and remnant is not well settled and depends on metallicity and rotation. Neutron stars have a maximum mass (the Tolman–Oppenheimer–Volkoff limit, probably 2–2.5 solar masses; its exact value depends on the unknown dense-matter equation of state), so a remnant above it must become a black hole (see Neutron Stars).
Mass gaps (open). X-ray binary data once suggested a “lower mass gap” between about 2.5 and 5 solar masses with no black holes. Gravitational-wave detections of objects in this range, such as GW230529 (reported 2024), have challenged that picture. At the upper end, pair-instability supernovae are predicted to leave no black holes between roughly 60 and 130 solar masses, yet LIGO–Virgo–KAGRA has reported mergers with components in or near this range (for example GW231123, announced 2025), suggesting hierarchical mergers or other formation channels. These interpretations are under active debate.
Tests of general relativity. Merger signals test the strong-field regime of Einstein's theory (see Schwarzschild Metric); the ringdown after merger is consistent with a Kerr black hole to the precision available. The Event Horizon Telescope has imaged supermassive, not stellar, black holes, because stellar ones are too small on the sky.
Concept Relationship Map
Requires: stellar evolution, Schwarzschild metric, supernovae
Enables: gravitational-wave astronomy, accretion physics
Commonly confused with: supermassive black holes (106–1010 M☉)
Sources
- Abbott, B.P., et al. (2016). Physical Review Letters, 116, 061102.
- Miller-Jones, J.C.A., et al. (2021). “Cygnus X-1 contains a 21-solar mass black hole.” Science, 371, 1046–1049.
- LIGO-Virgo-KAGRA Collaboration, Gravitational Wave Transient Catalogues (GWTC).