A black hole is a region of spacetime whose gravitational field is so strong that nothing which enters it, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return. It is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics.Quantum mechanics predicts that black holes emit radiation like a black body with a finite temperature. This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of stellar mass or greater.

Objects whose gravity field is 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 not fully appreciated for another four decades. 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 a star of more than 5 solar masses runs out of energy fuel. This results in the outer layers of gas being thrown out in a supernova explosion. The core of the star collapses and becomes super dense where even the atomic nuclei are squeezed together.The energy density at the core goes to infinity. 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 may form. There is general consensus that supermassive black holes exist in the centers of most galaxies. In particular, there is strong evidence of a black hole of more than 4 million solar masses at the center of our galaxy, the Milky Way.

Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with light and other electromagnetic radiation. From stellar movement, the mass and location of an invisible companion object can be calculated. A half-dozen or so binary star systems have been discovered by Astronomers where one of the stars is invisible, yet must surely exist since it pulls with enough gravitational force on the other visible star to make it orbit around their common center of gravity. Therefore these invisible stars are thought to be good candidate black holes. Astronomers have identified numerous stellar black hole candidates in binary systems by studying the movement of their companion stars in this way.

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