Stellar-Mass Black Holes

The most common black holes form from the deaths of massive stars. Stellar-mass black holes typically weigh between 5 and 100 solar masses. They are scattered throughout galaxies, usually invisible unless they have a companion star. When gas from a nearby star spirals into a stellar-mass black hole, it heats up and emits X-rays that telescopes can detect.

The first strong black hole candidate, Cygnus X-1, was identified in 1964 as a powerful X-ray source. A massive blue star orbits an invisible companion with about 21 solar masses — far too heavy to be a neutron star.