Nebula (Encyclopedia of Science)
Bright or dark clouds hovering in the interstellar medium (the space between the stars) are called nebulae. Nebula, Latin for "cloud," is a visual classification rather than a scientific one. Objects called nebulae vary greatly in composition. Some are really galaxies, but to early astronomers they all appeared to be clouds.
Some categories of bright nebulae include spiral, planetary, emission, and reflection. Others are remnants of supernova explosions.
(The entire section is 800 words.)
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