Black holes are regions of space where gravity is so powerful even light cannot escape their grasp. They can form when stars many times more massive than the Sun burn out and collapse in on themselves.
Supermassive black holes are much larger than black holes that form from the death of a single star and are believed to sit at the centre of some galaxies including the Milky Way. Supermassive black holes may have formed when huge clouds of interstellar gas collapsed, but this is not certain.
Image: An Illustration of a black hole surrounded by a disc of gas and dust (credit: NASA/CXC/M.Weiss)
The collapsed remains of massive stars are regions of powerful gravity.
Astronomers search for X-ray emitting stars.
Astronomers began looking for stars emitting bursts of X-rays in order to find evidence of black holes. It's thought that stars emit radiation as nearby black holes strip them of their matter.
Patrick Moore and his guests use magic to explain black holes.
Sir Patrick Moore and his guests use magic tricks to explain some of the strange properties of black holes.
Gamma rays may provide a clue.
Professor Brian Boyle from the Anglo-Australian Observatory explains how observations of a gamma ray burst helped his team discover that it resulted from a supernova and the possible formation of a black hole.
Star death takes place alongside the birth of new stars.
The source of gamma ray bursts, high intensity flashes of gamma ray radiation from the distant Universe, puzzled scientists for many years. However, intense study tracked down the source of these explosions to the formation of black holes.
Patrick Moore and his guests discuss gamma ray bursts.
Sir Patrick Moore and his guests discuss gamma ray bursts, which are thought to be caused by exploding massive stars. Could the star Eta Carinae emit a gamma ray burst one day?
A black hole is a mathematically defined region of spacetime exhibiting such a strong gravitational pull that no particle or electromagnetic radiation can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon. Although crossing the event horizon has enormous effect on the fate of the object crossing it, it appears to have no locally detectable features. In many ways a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe.
Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was first published by David Finkelstein in 1958. Long considered a mathematical curiosity, it was during the 1960s that theoretical work showed black holes were a generic prediction of general relativity. The discovery of neutron stars sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.
Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle. After a black hole has formed, it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses (M☉) may form. There is general consensus that supermassive black holes exist in the centers of most galaxies.
Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light. Matter falling onto a black hole can form an accretion disk heated by friction, forming some of the brightest objects in the universe. If there are other stars orbiting a black hole, their orbit can be used to determine its mass and location. Such observations can be used to exclude possible alternatives (such as neutron stars). In this way, astronomers have identified numerous stellar black hole candidates in binary systems, and established that the radio source known as Sgr A*, at the core of our own Milky Way galaxy, contains a supermassive black hole of about 4.3 million M☉.