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Black Holes !
A black hole is a spherical region in space in which some mass has been
crushed by gravity to the point that the escape speed from the object
becomes larger than the speed of light.
Any mass can be squeezed into a black hole: stars, planets, & people.
The boundary at which this critical escape speed is reached is called the
Schwarzschild radius, after Karl Schwarzschild, a contemporary of Einstein.
The surface of the sphere whose radius equals the Schwarzschild radius is
termed the event horizon. Information (mass, energy) only crosses the event
horizon in the inbound direction. Nothing comes back, ever.
Near the Schwarzschild radius the intense force of gravity “bends" space,
producing a variety of phenomena. Einstein’s General Theory of Relativity,
relating mass, forces, and the nature of space-time, is required to explain
many of these phenomena.
Our current understanding of physics breaks down at the center of a black
hole, where mass is crushed to infinite density, a condition known as a
Black holes might form during the core collapse of very massive (M > 25
M), or in the coalescence of a neutron star binary.
The Schwarzschild radius for a black hole can be approximated as Rs = 3 
M(BH), where M(BH) is the mass of the black hole expressed in solar units.
A BH equal in mass to the Sun would be 3 kilometers in radius.
The best evidence for black holes in space has been discovered in certain
binary star systems, in which strong, variable X-ray radiation is observed,
and one member of the binary appears to contain at least several solar
masses. The more massive object fits all the conditions to be a solar-mass
black hole.
Astrophysicists use X-ray satellites, like the Chandra Observatory, to search
for black holes. Matter falling into a black hole will be heated to millions of
degrees, and radiate x-rays.
Other evidence suggests that the cores of galaxies may possess
supermassive black holes, with masses exceeding many billions of solar