What is a black hole?
Black holes are highly compact objects that are predicted by Einstein's theory of General Relativity. Their extremely strong gravitational field prevents even light from escaping them, causing them to appear black at all wavelengths of the electromagnetic spectrum. While they were considered purely theoretical objects for a long time, astronomical observations in the past decennia have provided an ever increasing amount of support for their actual existence.
There are different types of black holes. The smaller, stellar black holes are formed at the end of the life cycle of a very massive star. When the star is out of fuel, it goes supernova and the core can collapse under its own weight. Another type of black hole is the supermassive black hole at the center of a galaxy, millions to billions times more massive than the Sun. The active supermassive black hole captures gaseous material from its host galaxy, creating a disc around itself that feeds it. Supermassive black holes are expected to lurk in the centers of most galaxies, including the Milky Way. The radio source in the core of our own Milky Way, known as Sagittarius A*, is thought to be associated with a supermassive black hole.
Scientists are researching the precise nature of black holes: what are the effects of the strong gravitational field close to a black hole? What happens to the accreting gas so that it sometimes forms jets? Does General Relativity accurately predict what the direct environment of a black hole looks like?