German astronomer who predicted the existence of black holes (1916). Schwarzschild showed that a star undergoing gravitational collapse that shrinks below a certain radius—the Schwarzschild radius—develops a gravitational pull so great that not even light can escape. Such objects are called black holes. Schwarzschild also produced the first exact solution to Einstein's field equations (1916). During the 1890s he developed a method for accurately determining the apparent magnitude of stars from photographic plates.