A black hole is a region of spacetime exhibiting such strong gravitational effects that nothing not even particles and electromagnetic radiation such as light can escape from inside it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime. The boundary of the region from which no escape is possible is called the event horizon. Although the event horizon has an enormous effect on the fate and circumstances of an object crossing it no locally detectable features appear to be observed. 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 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. Black holes were long considered a mathematical curiosity it was during the 1960s that theoretical work showed they 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 of millions of solar masses may form. There is general consensus that supermassive black holes exist in the centers of most galaxies.

Despite its invisible interior the presence can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light. Matter that falls onto external accretion disk heated by friction forming some of the brightest objects in the universe. If there are other stars orbiting their orbits can be used to determine the 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 Sagittarius at the core of our own Milky Way galaxy contains a supermassive of about 4.3 million solar masses.

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