What is a simple description of a black hole?
A black hole is an area in space where matter has been compressed so tightly and compactly that the relative gravity is extremely powerful. The accepted source of most black holes are stars, which live millions of years and then collapse in on themselves, concentrating the heavy metals that are created in the fusion process. This enormous mass can be small, but has a gravity so strong it can trap light, which travels at 186,000 miles a second; the photons of light cannot escape a black hole's gravity well, and so vanish. The result is that black holes cannot be seen or detected with conventional tools; telescopes see the reflected light of an object, so a black hole seems simply to be nothing. The main method of observing or tracking black holes is to observe the matter around it. Stars and other solar objects can be observed to alter their orbits or shape if they pass near a black hole. It is commonly accepted by astronomers that galaxies have supermassive black holes at their centers; the Milky Way Galaxy, which contains the Earth, could have a black hole in its center the mass of four million solar systems.