Black Hole:
BACK 		An object in the sky that has such strong gravitational force that not even light can escape from it. A black hole may form from the remains of a collapsed star.
COBE satellite:
BACK 		The Cosmic Background Explorer (COBE for short) satellite was put into orbit by a Delta rocket in November, 1989. Its main tasks are to measure the spectrum and the small scale anisotropy of the microwave background radiation, and to map the far infrared background radiation.
Critical Density:
BACK 		The matter density that determines whether the universe will expand forever or will collapse. It is about 10^-29 gram per cubic centimeters. If the average mass density of the universe is larger than this value, the universe will some day collapse back on itself; otherwise, it will expand forever. See the unit, The Fate of Our Universe . The critical density also determines the shape of the universe.
Fluctuation:
BACK 		A small change in density.

Hubble Space Telescope:
BACK

A 2.4 m reflecting telescope launched into Earth's orbit on April 25, 1990 by the American space shuttle Discovery. The telescope is named after American astronomer Edwin Hubble who discovered in 1929 that all distant galaxies are fleeing.
General Theory of Relativity:
BACK 		A theory, developed by Albert Einstein in 1915, that describes the relation between space-time and matter. It is in fact a theory of gravitation.
Kelvin:
BACK 		A unit used to measure temperature -- K, for short. It is an absolute scale of temperature. On this scale, 0K is absolute zero, the coldest temperature possible. At this temperature all atoms and molecules would stop moving. 0K is actually -273.15 degrees Celsius or -459.76 degrees Fahrenheit, another two scales of temperature used in daily life. Room temperature is about 300 degrees Kelvin.
Light Year:
BACK 		A unit of distance in astronomy. A light year is equal to the distance travelled by light in one year, which is 9.46x10^12 kilometers or 0.31 parsec. (See definition, below.)
Mass:
BACK 		The quantity of material a body contains. It is measured by the resistance of the body to the change of motion.
Neutron:
BACK 		A particle that carries no charge, has about the same mass as a proton (see definition, below), and forms part of an atomic nucleus.
Observable Universe:
BACK 		Our universe is about 15 billion years old after the Big Bang, and the speed of light is 3x10^8 meters per second. Therefore, in principle, the farthest event we can see is 15 billion light years away. However, in reality we cannot see that far, because in the early days the universe was so hot that all light was blocked.
Parsec:
BACK 		A unit of distance in astronomy. One parsec is equivalent to 3.26 light years or 3.086x10^16 m.
Planet:
BACK 		A massive body which moves around a star such as the Sun. The planet itself does not emit light; it is illuminated by the star.
Proton:
BACK 		A positively charged particle that forms part of an atomic nucleus.
Quarks:
BACK 		Smallest particles believed to be the building blocks of protons, neutrons, and electrons. They come in six different "flavors" or types -- up, down, strange, charmed, beauty, and truth.


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