Stellar Black Holes
Can astronomers see a black hole? Not directly. The only way to find one is to
use circumstantial evidence. Observations must imply that a sufficiently large
amount of matter is compressed into a sufficiently small region of space so that
no other explanation is possible. For stellar black holes, this means observing
the orbital acceleration of a star as it orbits its unseen companion in a double
or binary star system.
Illustration of Binary Star Orbit
Searching for black holes is tricky business. One way to locate them has been to
study X-ray binary systems. These systems consist of a visible star in close
orbit around an invisible companion star which may be a neutron star or black
hole. The companion star pulls gas away from the visible star.