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Binary star
Binary star
A double star that occurs when two stars are so close that they continue to be held close together by their own gravity. Binary stars are very common, making up about half of all known stars.
 Binary star—This NASA Hubble Space Telescope image shows a newborn binary star with a long thin nebula pointing toward a companion object bottom left that could be
a planet within another solar system.
Black body
A surface that absorbs all of
the energy that it receives from radiation. It appears black because all of the light falling on it is absorbed, and none is reflected.
Black dwarf
A star in the last stages of its life. It has collapsed and cooled down so much that it no longer gives out light and so cannot be seen.
Black hole
Stars owe their size to the way they continually use up their fuel. While they remain as fireballs, they can
be very large; but when the fuel
is finally used up, very large stars will blow up (to form a supernova) or collapse inward (implode). An imploding star becomes denser
and denser as its gravity becomes stronger and stronger.
Nobody knows what these bodies look like because the pull of gravity is so great, no light ever leaves them. Black holes are the most mysterious features of the Universe.
The center of a black hole is called a singularity. A star ten times as massive as our Sun may collapse into something less than 30km across.
Black holes were predicted
by Einstein’s general theory
of relativity. Although they are difficult to see because no light comes from them, they can be detected through the effect their massive gravity fields have on things around them. If one part of a binary star (one with two centers) becomes a black hole, it begins to suck matter from the other star, causing it to become so hot it sends out huge amounts of x-rays. One star in the galaxy Cygnus X-1 is
believed to be a black hole. Some black holes may not be
collapsed stars, but a collapse of large masses of interstellar gas (see: Stellar). The collapse of this matter is unbelievably large and is thought to account for the formation of quasars. There is one of these supermassive black holes in the center of the M87 galaxy (see: M). Its mass is equal to two to three billion suns, but it only occupies a space the size of the Solar System.
Stars that are less massive than three times our Sun cannot develop enough gravity to become black holes, but instead simply shrink and become white dwarfs or neutron stars.
Blue giant
A very large star that has used
up all of the hydrogen fuel in its core and is cooling. The surface temperature of a blue giant is about 30,000°C. When it cools further, it will become a red giant.
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