Black holes are one of the few phenomena in space that have been theorized about before they were discovered. Black holes are regions of outer space that nothing can escape from. It is widely believed that any matter drawn into these regions is crushed, adding to the density and depth of the black hole.
Discovering or detecting black holes had been a puzzle for astronomers for many years. In general, however, they are discovered by detecting the surrounding materials such as gas that has been directed by gravity as a disc or swirl around the black hole. As a gas approaches the black hole the molecules that make up the gas will increase in speed. Eventually, the particles move so fast that the gas will heat up. As the gas continues to heat, the molecules become more and more excited, and will start to emit x-rays and other types of radiation. It is these x-rays and other radiation that can be detected by astronomers on Earth.
Large black holes can be detected in other ways. If a certain location is suspected to contain a black hole, the speed and curvature of the stars near that location can be measured. By comparing these measurements to what would normally be expected of these bodies, it is possible to determine if something is affecting their orbit. By comparing data from several different stars, it is possible to theorize the presence of a black hole. It is even possible to estimate its size.
It is also possible for smaller black holes to be detected by the intensity of the individual photons that they emit. Photons are essentially packets of light that are emitted by various sources. Since they have no mass, they cannot be sucked into a black hole. In general, a photon’s wavelength is determined by how the photon was generated. Photons that are generated near black holes have very distinctive wavelengths.
Matter that falls into a smaller black hole will be subjected to forces that are strong enough to rip apart its component atoms. This process releases very high-energy photons known as hard gamma rays. Presently, hard gamma rays are only known to be emitted by black holes, so it is assumed that their presence indicates the presence of a black hole.
Black holes typically require years of data to be collected in order to be definitively discovered. Often the data is collected by several groups of scientists, who will all share credit for discovering the black hole.