Black holes are cosmic objects of such intense gravity that nothing, not even light, can escape. A black hole is formed by the death of a massive star. When such a star exhausts its internal thermonuclear fuel at its core.
Lacking life, the core becomes unstable and gravitationally collapses in on itself, blowing away the outer layers of the star. The crushing weight of incoming material from all sides compresses the dead star into a point of zero volume and infinite density, called a singularity.
The details of the formation of black holes are calculated from Albert Einstein’s theory of general relativity. The singularity forms the center of a black hole and is hidden by the “surface” of the object, the event horizon. The escape velocity (that is, the velocity required for matter to escape the gravitational field of a cosmic object) within the event horizon exceeds the speed of light, so that even light rays cannot penetrate into space. The radius of the event horizon is called the Schwarzschild radius, named after the German astronomer Karl Schwarzschild, who in 1916 predicted the existence of collapsed galaxies that emit no radiation. The Schwarzschild radius is proportional to the mass of the collapsed star. For a black hole with a mass 10 times that of the Sun, the radius would be 30 km (18.6 mi).
Only the most massive stars those with masses greater than three solar masses become black holes at the end of their lives. Low-mass stars evolve into less compressed objects, either white dwarfs or neutron stars.
Black holes are usually not directly observable due to their small size and lack of light emission. However, they can be “observed” through the effects of their enormous gravitational field on nearby matter. For example, if a black hole is part of a binary star system, the material flowing into it from its companion is intensely heated and then, emitting a large amount of X-rays, enters the event horizon of the black hole and disappears forever. One of the components of a binary X-ray system is a star. Cygnus X-1 is a black hole. Discovered in 1971 in the constellation Cygnus, this binary consists of a blue supergiant and an invisible companion with a mass of 14.8 times that of the Sun, which orbit each other in 5.6 days. Some black holes are of clearly non-stellar origin. Various astronomers have hypothesized that quasars and supermassive black holes at the centers of galaxies collect and collapse large amounts of interstellar gas. It is estimated that the mass of gas falling rapidly into a black hole releases more than 100 times the energy released by nuclear fusion of the same mass. Accordingly, the gravitational force of millions or billions of solar masses of interstellar gas falling into a supermassive black hole would be the cause of the enormous energy production of quasars and some galactic systems.
One such supermassive black hole, Sagittarius A*, is located at the center of the Milky Way Galaxy. Observations of stars orbiting Sagittarius A* indicate the presence of a black hole with a mass greater than 4,000,000 solar masses. (For these observations, American astronomer Andrea Ghez and German astronomer Reinhard Genzel were awarded the 2020 Nobel Prize in Physics.) Supermassive black holes have also been detected in other galaxies. In 2017, the Event Horizon Telescope obtained an image of the supermassive black hole at the center of the galaxy M87. This black hole has a mass equivalent to six and a half billion suns but is only 38 billion kilometers (24 billion miles) in diameter. It is the first black hole to be directly imaged. The existence of even larger black holes, each with a mass equivalent to 10 billion suns, can be inferred from the energetic effects of the extremely high-velocity gas swirling around their centers. NGC 3842 and NGC 4889, galaxies near the Milky Way.
The existence of other types of non-stellar black holes was proposed by British astrophysicist Stephen Hawking. According to Hawking’s theory, numerous tiny primordial black holes, perhaps of the same mass as or less than that of an asteroid, could have been created during the Big Bang, the state of extremely high temperature and density in which the universe originated 13.8 billion years ago. These so-called tiny black holes, like larger black holes, lose mass over time through Hawking radiation and disappear. If some theories that require extra dimensions for the universe are correct, the Large Hadron Collider could create a significant number of tiny black holes.
