Planetary Nebula

Bight Emission Nebula

nebula
immense body of highly rarified gas and dust in the interstellar spaces of galaxies. A diffuse nebula, such as the CRAB NEBULA, is irregular in shape and ranges up to 100 light-years in diameter. A bright emission nebula, composed primarily of hydrogen gas ionized by nearby hot blue-white stars, radiates its own light; a bright reflection nebula, located near cooler stars, reflects the starlight. A dark nebula, which neither emits nor reflects light because it is too distant from any star, appears as an empty patch in a field of stars or as a dark cloud obscuring part of a bright nebula in the background. A planetary nebula consists of a well-defined shell of gaseous material that glows from the radiation emitted by the central hot star it surrounds. The shell, measuring about 20,000 ASTRONOMICAL UNITS in diameter, is slowly expanding, indicating that it was expelled in a nova or SUPERNOVA explosion.