stellar-mass black hole
When a truly massive star — many times heavier than the Sun — runs out of fuel, no force in nature can hold its core up. Not gas pressure, not electron degeneracy, not even the neutron degeneracy that supports a neutron star. The core simply collapses without limit, packing its mass into a region from which not even light can escape. The result is a stellar-mass black hole: the densest, most extreme grave a star can dig, weighing from about 3 to a few tens of times the mass of the Sun.
Despite their fearsome reputation, these black holes are physically small. A 10-solar-mass black hole has an event horizon — the point of no return — only about 30 kilometres across, smaller than a city. From far away its gravity is unremarkable; if the Sun were magically replaced by a black hole of the same mass, Earth would keep orbiting exactly as before, because a black hole pulls no harder than any object of equal mass. The danger lies only in getting close, where the gravity becomes overwhelming.
Stellar-mass black holes are distinct from the supermassive black holes (millions to billions of solar masses) that lurk in galactic centres; those form by other means and belong to a separate topic. We find stellar-mass black holes by their effect on companions in X-ray binaries, and increasingly by the gravitational waves released when two of them spiral together and merge. A vital correction: a black hole is not a cosmic vacuum cleaner. It does not roam the galaxy 'sucking up' stars — matter only falls in if it strays close, just as it would toward any compact mass.
If the Sun were instantly swapped for a black hole of exactly one solar mass, the sky would go dark but Earth's orbit would not change at all. The black hole's event horizon would be just 3 km across — yet the planets would circle it for billions of years, undisturbed.
Same mass, same orbit: a black hole pulls no harder than the star it replaced.
A black hole is not a vacuum cleaner. Its gravity at a distance equals that of any object of the same mass; things fall in only if they get close enough, the same as toward any star.