Before Hubble, astronomers theorized the existence of supermassive black holes, but they had no conclusive evidence. Quasars, extremely bright quasi-stellar objects in remote active galaxies, indirectly hinted at their existence, but direct evidence of supermassive black holes didn’t come until 1994 when Hubble’s Faint Object Camera observed the heart of the giant elliptical galaxy M87. Hubble’s observations found a whirlpool of hot, ionized gas orbiting the heart of the galaxy at a speed of about 1.2 million miles per hour (550 kilometers per second). Only a supermassive black hole would have the gravitational power to create a vortex of hot gas at such velocities.
Supermassive black holes are millions to tens of billions times the mass of the Sun. A Hubble galaxy census showed that a black hole’s mass is dependent on the mass of its host galaxy’s central bulge of stars: the larger the galaxy, the larger the black hole. This close relationship may be evidence that black holes grew along with their galaxies, devouring a fraction of the galaxy’s mass and intrinsically linking the black hole to the galaxy’s evolution.
Black holes aren’t just associated with galaxies. Astronomers estimate that 100 million black holes roam the stars of our Milky Way galaxy alone. In 2022, two teams using Hubble data measured how a suspected isolated black hole’s gravity acted like a lens, warping and deflecting the light from a background star. Their measurements indicate the lensing object’s size is either a black hole or a compact neutron star.