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absolute magnitude the true brightness of a star based on its actual luminosity, or light output.

apparent magnitude the apparent brightness of a star as observed from Earth.

black dwarf the burned out core of a white dwarf star.

black hole the theoretical remains of a supermassive star that has exploded and collapsed in on itself. No light can escape from a black hole because its gravity is so strong.

brown dwarf a starlike object that does not radiate energy because it has insufficient mass for nuclear fusion.

constellation a group of stars that form a pattern or shape in the night sky. Astronomers use 88 constellations to locate objects and stars.

cosmology a branch of astronomy concerned with the study of the origin and evolution of the universe.

dark matter matter that cannot be detected with current technology. It is too cold and dark to see or measure; scientists believe it makes up about 90 percent of the universe.

fusion a nuclear reaction in the center of a star that releases a tremendous amount of energy in the form of radiation. In most stars, two hydrogen atoms are fused into a helium atom.

galaxy a group of stars, usually numbering in the billions, that orbit a common mass and travel through the universe as a single unit. Our galaxy is called the Milky Way.

gravity the universal force of attraction that pulls all matter together.

H-R Diagram a plot of stars that shows their relative brightness and temperature; first developed by Enjar Hertzsprung and Henry Norris Russell.

light-year the distance a beam of light in a vacuum travels in one year, about 5.88 trillion miles.

Messier object any number of objects originally plotted by French astronomer Charles Messier to help astronomers differentiate them from comets. The Messier catalog contains 110 objects, including nebulae and galaxies.

nebula derived from the Latin word for "cloud," a fuzzy patch of light in the night sky. Nebulae appear in many forms and are thought to be clouds of dust and gas, frequently the remains of exploded stars.

neutron star a small, extremely dense star composed mostly of neutrons, or the remains of a supernova explosion.

nova a stellar explosion of a red giant star, ending in a planetary nebula and a white dwarf.

parsec a distance of 3.26 light-years; used to measure immense distances in space.

planetary nebula a ring of dust and gas blown off a red giant star after it undergoes an explosion.

pulsar a rapidly spinning neutron star that sends out pulses of radiation at regular intervals. Pulsars have extremely strong magnetic fields.

quasar quasi-stellar radio source. A very energetic distant object that may be powered by a black hole; possibly a galaxy in the early stages of formation.

red giant an unstable star whose hydrogen has run out; it burns helium at its core. The surface of the star is much cooler, which produces its red color. A red giant usually becomes a nova and ultimately a white dwarf.

red shift the apparent shift in a spectrum of light coming from a star or distant galaxy to the red end. Red shifts indicate that an object is moving away.

white dwarf a remnant of a red giant star that has exploded.