When a giant star—about 10 to 30 times the mass of the Sun—exhausts its fuel, gravity can pull the star's core into a sphere a tiny fraction of its original size. The shrinkage is so drastic that stars millions of kilometers across can collapse to roughly 10 kilometers in diameter. These are 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 gas a black hole captures in this fashion is compressed into a rotating disk called an accretion disk, and friction between gas molecules and atoms heats the gas into plasma. This heat can be observed by telescopes as various wavelengths of electromagnetic radiation, or light.
By studying the plasma around black holes, researchers seek to test general relativity and discover if there are astrophysical alternatives to black holes in the Universe.