“Nebula” is the Latin word for cloud. It was originally used to describe anything in the night sky that had a cloudy or diffuse appearance, and that wasn’t a comet. Early in the 20th Century, however, it was discovered that some of these nebulae were actually “island universes” (galaxies or globular clusters). Since then, the word refers to specific types of objects: Planetary nebulae are usually spherical and are the expanding shells of material ejected from dying stars. They get their name from the way they sometimes look like planets in small telescopes. The other type of nebula is a vast cloud of gas (mostly molecular hydrogen) and dust, and these molecular clouds are usually busy forming new stars deep inside. The name used to describe this type depends on how it looks from Earth: An emission nebula has lots of hot bright new stars inside, and their heat and radiation cause the gas to glow and shine with a pinkish red light. A reflection nebula is usually behind a star, and the blue portion of the starlight reflects off the cloud back to us. Finally, a Dark nebula is one which has no hot bright stars inside it, and no stars in front of it. These dark nebula appear as black empty patches in the sky with no stars in them, because they block the light of the stars behind them.
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