Black holes are very dense areas in space where gravity has such a large attraction that not even light can escape. Black holes are dense due to the large amount of matter that has been condensed into a tiny space, and this is also the reason why gravity has such a large effect, since a greater mass will have a bigger gravitational attraction.
They were discovered with the help of Einstein’s theory of general relativity, which showed that when a massive star dies, it leaves behind the stellar core. This core has no forces to oppose the gravity acting as an inward pull, unlike the star before it dies, whose nuclear fusion creates a constant outward push to oppose it. According to Einstein’s equations, if the core’s mass is more than 3 times the mass of the Sun, the gravitational forces are strong enough to cause it collapse on itself into a small point, creating a black hole.
The first physical black hole, after just theories of black holes existing, was discovered in 1971, after one of the brightest X-ray sources were discovered in the sky a few years earlier. However, it wasn’t until 1971, when an X-ray Explorer Satellite was launched and suggested that the size of the X-ray emitting region was small and compact, which provided strong evidence for a black hole.
Since black holes don’t let any light out, they are invisible to the human eye, and so they can’t be directly observed, however, their effect on matter nearby can be. Scientists can study stars to find out if they are orbiting a black hole, since a black hole will draw matter inward. They can also study the X-rays these stars release as a result of orbiting.
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