Black Hole Formation & Mass Classes: Observation, Measurement & Evidence

Stellar black holes contain a few to many tens of solar masses in ordinary examples, intermediate-mass candidates span roughly hundreds to hundreds of thousands of solar masses, and supermassive black holes contain millions to billions of solar masses. These categories describe mass scale, not fundamentally different objects.

Invisible massive companions in X-ray binaries reveal stellar black holes, gravitational waves measure merging black-hole masses, stellar orbits constrain the Milky Way's central black hole, and active galactic nuclei plus stellar/gas dynamics reveal supermassive black holes in other galaxies.

Stellar black holes form when no stable neutron-star configuration can support a collapsing core or compact-object merger remnant. Supermassive black holes must grow through some combination of massive seeds, gas accretion, black-hole mergers, and long cosmic timescales; their earliest formation pathways remain an active research problem.

Dynamical mass measurements, X-ray binaries, microlensing, gravitational-wave mergers, stellar orbits near Sagittarius A*, and active nuclei reveal black holes across mass scales.