A black hole is a theoretical entity predicted by the equations of general relativity.
How do black holes form?
Black holes are thought to form from stars or other massive objects if and when they collapse from their own gravity to form an object whose density is infinite (a singularity). During most of a star’s lifetime, nuclear fusion in the core generates electromagnetic radiation, including photons, the particles of light. This radiation exerts an outward pressure that exactly balances the inward pull of gravity caused by the star’s mass. As the nuclear fuel is exhausted, the outward forces of radiation diminish, allowing the gravitation to compress the star inward. The contraction of the core causes its temperature to rise and allows remaining nuclear material to be used as fuel. The star is saved from further collapse. So: a black hole is formed when a star of sufficient mass undergoes gravitational collapse, with most or all of its mass compressed into a sufficiently small area of space, causing infinite spacetime curvature at that point (a “singularity”). Such a massive spacetime curvature allows nothing, not even light, to escape from the “event horizon,” or border.
An area of space so dense the degenerate neutronic forces/pressures can not withstand the combined force per unit area of the gravitons from the stellar mass. This creates an implosion whereby the collapse only stops when the body occupies a space of infinitesimal dimension.
The mass needed to overcome the degeneracy pressure must be above the Oppenheimer-Volkoff limit. The area around the degenerate mass above the Oppenheimer-Volkoff is warped in a manner consistent with Einstein’s General Theory of Relativity (1916) whereby space times folds about the object. The physicist Karl Schwartzchild produced a solution to Einstein’s equation for a spherical mass (called the Schwartzchild metric). The term expressing the radius had a disturbing feature. This radius, known as the Schwartzchild radius, rs, is defined as:
rs = 2GM/c2
G is the gravitational constant, M is the mass, and c is the speed of light.
An object whose entire mass M lies within rs is considered to be a black hole. The Schwarzschild surface, the sphere at one Schwarzschild radius, is also called the event horizon of a black hole (Event horizon is the name given to rs).
For stellar masses less than about 1.44 solar masses, the energy from the gravitational collapse is not sufficient to produce the neutrons of a neutron star, so the collapse is halted by electron degeneracy to form white dwarfs. This maximum mass for a white dwarf is called the Chandrasekhar limit (it’s 1,44 solar masses).
Hawking radiation (also called Bekenstein-Hawking radiation) is a theoretical prediction from British physicist Stephen Hawking. Smaller primordial black holes can actually emit more energy than they absorb, which results in them losing net mass. Larger black holes, such as those that are one solar mass, absorb more cosmic radiation than they emit through Hawking radiation.
Astronomers in England have discovered a singing black hole in a distant cluster of galaxies.
“Starlight” from Muse.