The galaxy’s black holes
Because telescopes cannot see inside the event horizon, astronomers must search for a black hole’s impact on its immediate surroundings. Some binary star systems offer a perfect environment. The massive stars that create black holes evolve quickly, typically running through their nuclear fuel in a few million years.
After the star explodes (the companion usually survives), the black hole’s intense gravity may pull material from its neighbor’s outer layers. This gas falls toward the black hole and forms an accretion disk that swirls around the invisible object like water circling a drain. As the material moves ever faster, friction among the atoms heats it to millions of degrees. Gas at this temperature emits lots of X-rays, which Earth-orbiting observatories can detect.
So, to detect a black hole, astronomers look for an X-ray-emitting binary system comprising one normal star and an invisible but massive companion. Lots of these objects exist in the Milky Way, but not all contain black holes. Neutron stars in a binary can produce the same behavior, and because they radiate little light, they can’t be detected across large distances.
To differentiate between the two possibilities, astronomers need to pin down the compact object’s mass. General relativity says that a stable neutron star can’t weigh more than three Suns. Any invisible companion bigger than that must be a black hole — assuming, as almost every scientist does, that relativity accurately describes such strong gravitational fields. To find the object’s mass, astronomers must measure the binary system’s orbit precisely.