Galaxies consist of billions of stars, as well as dust and gas. Some of these clouds of dust and gas can be seen, illuminated by nearby stars or emitting light on their own. These clouds are called nebulae. There are so-called reflection nebulae that are lit up by starlight, emission nebulae that glow on their own due to ultraviolet radiation from nearby stars, and dark nebulae that absorb light and are seen as black silhouettes against the background of stars.
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