| Definition of a
Nebula:
An 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.
What
is an asteroid?
What is a meteor?
About the Solar System
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