How do black holes form in galaxies?

Most black holes in galaxies arise from two related pathways: the collapse of individual massive stars, which produces stellar-mass black holes, and the formation and growth of much larger seeds that become supermassive black holes at galactic centers. Observations and theory together explain the causes and chain of consequences that link black holes to galaxy structure and evolution.

Stellar-scale black holes

When a star with an initial mass above roughly twenty times the Sun exhausts nuclear fuel, its core can collapse under gravity and either explode as a supernova or collapse directly to a black hole. The LIGO Scientific Collaboration at the California Institute of Technology and the Massachusetts Institute of Technology has detected gravitational waves from merging stellar-mass black holes, confirming that core collapse followed by binary evolution produces compact black hole populations. These stellar black holes typically range from a few to tens of solar masses and are distributed through galactic disks and halos, where their mergers and accretion episodes contribute modestly to a galaxy’s high-energy output.

Seeds and growth of supermassive black holes

Supermassive black holes, with masses of millions to billions of suns, reside at the centers of most large galaxies. How they form remains an active research area with several plausible channels. One route begins with remnants of the first generation of very massive, metal-poor stars known as Population III stars; their collapse could leave intermediate-mass seeds that grow by accreting gas and merging with other black holes. Another possibility is direct collapse of dense gas clouds into a massive seed without fragmenting into stars, creating a heavier starting point for rapid growth. Dense stellar clusters can also produce runaway collisions that form massive seeds. These theoretical channels are constrained by observations such as the imaging of the black hole in the galaxy M87 by the Event Horizon Telescope collaboration led in part by Sheperd Doeleman at the Harvard-Smithsonian Center for Astrophysics, and by precise measurements of stars orbiting the Milky Way’s center made by Andrea Ghez at the University of California Los Angeles and Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics, which together demonstrate the presence of compact, massive objects in galactic nuclei.

Relevance, causes, and consequences

The connection between central black hole mass and properties of the host galaxy’s bulge suggests coevolution. Work by John Kormendy at the University of Texas at Austin and collaborators has highlighted empirical correlations that imply feedback: as black holes accrete, they can power active galactic nuclei and jets that heat or expel gas, regulating star formation across large scales. Consequences include the quenching of star formation in massive galaxies, sculpting of galaxy morphology, and the enrichment and heating of circumgalactic gas. On smaller scales, black hole binaries and gravitational-wave events seed dynamical heating and can eject stars or even displace black holes after mergers.

Human and territorial dimensions shape this science. Radio and optical observatories—such as facilities in the Atacama Desert in Chile and on Mauna Kea in Hawaii—provide the high-resolution data needed, and their construction and use involve local communities, whose cultural and environmental concerns influence site access and future observing projects. Understanding black hole formation therefore depends not only on astrophysical theory and global collaborations but also on ethical, cultural, and environmental stewardship of the places where we observe the universe.