A black hole is a region of space within which the gravitational force is so strong that nothing, not even light, can go through it.
The existence of such objects was first suggested as early as the late eighteenth century. However, it was Karl Schwarzschild (1873–1916), a German astronomer, who basically developed the modern idea of a black hole. Using Einstein's theory of general relativity, Schwarzschild discovered that matter compressed at a point (now known as a singularity) would be surrounded by a spherical region of space from which nothing could escape. The boundary of this region is called the event horizon, a name that means that it is impossible to observe any event that takes place inside it (since the information cannot leave).
For a non-rotating black hole, the radius of the event horizon is known as the Schwarzschild radius and marks the point at which the escape velocity of the black hole is equal to the speed of light. In theory, any mass can be compressed enough to form a black hole. The only requirement is that its physical size be less than the Schwarzschild radius. For example, our Sun would become a black hole if its mass were contained within a sphere about 2.5 km in diameter.
Well within the event horizon is the heart of the black hole: the singularity. Everything within the event horizon is irreversibly attracted to this point where the curvature of spacetime becomes infinite and gravity is infinitely strong. An interesting dilemma for astrophysicists is that physical conditions close to a singularity result in the complete breaking of the laws of physics. However, there is nothing in the theory of general relativity that prevents isolated or "naked" singularities from existing. To avoid the situation where we could actually see this collapse of physics occur, the conjecture of cosmic censorship was proposed. This states that each singularity must have an event horizon that hides it from view, exactly what we find for black holes.
According to the classical theory of general relativity, once a black hole is created, it will last forever, since nothing can escape it. However, if quantum mechanics is also considered, it turns out that all black holes will eventually evaporate as they slowly lose Hawking radiation. This means that the lifespan of a black hole depends on its mass, and smaller black holes evaporate faster than larger ones. For example, a black hole of 1 solar mass takes 1067 years to evaporate (much more than the current age of the Universe), while a black hole of only 1011 kg will evaporate in 3 billion years.
Observational evidence of black holes is, of course, not easy to obtain. Since radiation cannot escape the extreme gravitational pull of a black hole, we cannot detect it directly. Instead, we infer its existence by observing high-energy phenomena such as X-ray emission and jets, and the movements of nearby objects in orbit around the occult mass. An additional complication is that similar phenomena are observed around less massive neutron stars and pulsars. Therefore, identification as a black hole requires astronomers to estimate the mass of the object and its size. A black hole is confirmed if no other object or group of objects could be so massive and compact.