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Nebula, enormous interstellar cloud of gas and dust, often luminous. Diffuse nebulae can measure 100 light-years in diameter. Bright nebulae, such as the Orion Nebula, appear to shine due to nearby bright stars, whose light they either reflect (reflection nebula) or absorb and re-emit (emission nebula). Dark nebulae, such as the Horsehead Nebula, are distant from stars, and hence appear as dark patches in the sky obscuring the light from stars beyond them. Planetary nebulae are cast off by stars that have exploded. They are usually symmetrical, forming an expanding shell around the central star, often still visible within. The Ring Nebula is an outstanding example.

See also: Solar System.

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21st Century Webster's Family Encyclopedia21st Century Webster's Family Encyclopedia - Mudpuppy to Nebula