One of the most important findings in astrophysics is that when stars die they can collapse to extremely small objects. When their internal fuel sources are exhausted, stars millions of miles across can collapse to roughly 10 kilometers in diameter, known as neutron stars.
In extreme cases, stars can contract even further under their own gravity to an infinitely dense point, or singularity, called a black hole. The discovery of black holes has been a critical confirmation of Einstein's theory of gravity, known as general relativity.
Black holes are among the most commonly studied astrophysical phenomena for the simple reason that they are associated with the brightest known objects and events in the cosmos. Even though black holes are themselves invisible, they tend to capture gas around their margins due to their gravitational pull. The gravity near the black hole is very strong and forces the gas to be compressed into even smaller space. This squeezing increases friction between gas particles, causing the gas to heat up. This heat is observed as radiation by telescopes.
By studying the plasma around black holes, researchers are seeking to test Einstein’s theory of general relativity and discover if there are astrophysical alternatives to black holes in the universe.