The concept of a black hole, a space in which so much matter was packed that the gravitational pull prevents the escape of light, was known as far back as the 18th century. But it was seen as more of a theoretical possibility than an actual phenomenon. The possibility of an actual black hole emerged as a result of Einstein’s theory of general relativity, which gave a detailed explanation of subtleties of gravitation that had eluded Newton. A copy of this theory made its way to the Russian Front during World War I, to Karl Schwarzschild, a physicist and astronomer serving in the German army.
Einstein put forth his theory in the form of a system of equations. These equations were extremely difficult to solve, but Schwarzschild managed to find a solution to them in the midst of the carnage of a war. Not only that, but he also showed that for any given quantity of matter, there was a sphere so small that if all that matter were packed inside it, it would become a black hole. The radius of the sphere is known as the Schwarzschild radius. (There is no single Schwarzschild radius; it’s a different size for every possible mass.)
Popular treatments leave us with the impression that black holes are ominously small, dense and black. For example, the Schwarzschild radius for a mass the size of the earth is only about 1 centimeter. But surprisingly, much larger black holes can be diffuse. If an entire galaxy’s mass were distributed evenly within its Schwarzschild radius to create a black hole, the black hole’s density would be about 0.0002 the density of the earth’s atmosphere.
via Popular Mechanics.