BLACK HOLES
Illustration Credit: A. Feild (http://www.stsci.edu/)
Science Credit: NASA and Felix Mirabel (Institute for Astronomy and Space Physics of Argentina and the French Atomic Energy Commission)
Black hole history
What is a black hole, really?
In 1916, when general relativity was new, Karl Schwarzschild worked out a useful solution to the Einstein equation describing the evolution of spacetime geometry. This solution, a possible shape of spacetime, would describe the effects of gravity outside a spherically symmetric, uncharged, nonrotating object (and would serve approximately to describe even slowly rotating objects like the Earth or Sun). It worked in much the same way that you can treat the Earth as a point mass for purposes of Newtonian gravity if all you want to do is describe gravity outside the Earth's surface.
What is a black hole, really?