The calls black holes They are places with a very large, huge gravitational field. No electromagnetic or light radiation can escape, so they are black.
They are surrounded by a border spherical called "event horizon" that allows light to enter, but not out.
There are two types of black holes: high density bodies and low mass concentrated in a very small space, and low density bodies, but very large mass, as happens in the centers of the galaxies.
If the mass of a star is more than twice that of the Sun, there comes a time in its cycle when not only neutrons can withstand gravity. The star collapses and becomes a black hole.
Stephen Hawking and the light cones
British scientist Stephen W. Hawking He devoted much of his work to the study of black holes. In his book History of Time It explains how, in a star that is collapsing, the light cones it emits begin to curl on the surface of the star.
As it becomes small, the gravitational field grows and the light cones are tilted more and more, until they can no longer escape. The light goes out, as in the black hole CO-0.40, located 200 light years from the center of the Milky Way.
If a component of a binary star becomes a black hole, it takes material from its partner. When the whirlpool approaches the hole, it moves so fast that it emits X-rays. Thus, although it cannot be seen, it can be detected by its effects on nearby matter.
Black holes are not eternal. Although no radiation escapes, it seems that some atomic and subatomic particles can.
Someone who observed the formation of a black hole from the outside, would see an increasingly small and red star until, finally, it would disappear. Its gravitational influence, however, would remain intact.
As in the Big Bang, there is also a singularity in black holes, that is, physical laws and predictability fail. No outside observer, if any, could see what happens inside.
The equations that try to explain a singularity, such as that given in black holes, must take into account space and time. The singularities will always be placed in the observer's past (such as the Big Bang) or in his future (such as gravitational collapses), but never in the present. This curious hypothesis is known as cosmic censorship.
The first detection of gravitational waves, on September 14, 2015, came from the clash of two black holes, which melted releasing an energy equivalent to about three times the mass of our sun.
More information on the page What is a black hole?.