Given that nothing from inside a black hole can reach us, we need to look at the impact a black hole has on its surroundings to find one. We’ll use real examples to illustrate how black holes are detected.
A binary star system where one of the stars is not visible is a good place to look for black holes. The first one ever detected was found in the Cygnus region. Here’s a Hubble image of the region. The star visible at the center is called HDE 226868. It’s a blue supergiant star 7300 ly from Earth.
A very strong x-ray source called Cygnus X-1 was also found at this location. But blue supergiant stars cannot generate the volume of x-rays detected. This led astronomers to suspect that the source is a black hole orbiting close to the blue supergiant. We used this system earlier in our segment on accretion disks.
Analysis of the system showed that the distance between the x-ray source and the star is just 1/5 of the distance between the Earth and the Sun. That’s very close. These two objects are orbiting their center of gravity once every 5.6 days. This orbital motion gives us the mass of the two objects. The blue giant is 40 times the mass of the sun and Cygnus X-1 is 21 times the mass of the Sun. With that mass, it cannot be a neutron star because neutron stars cannot exceed three solar masses.
In addition, if the star that collapsed into a black hole had exploded as a supernova, the companion star would have been ejected from the system. That HDE 226868 remained in orbit, indicates that the progenitor may have collapsed directly into a black hole without exploding (or at most produced only a relatively modest explosion). Plus, additional evidence for a black hole comes from the x-ray fluctuations. Observations of Cygnus X-1 found a fading pulse. With all this, astronomers came to accept that Cygnus X-1 is indeed a stellar-mass black hole.