A nebula, which is Latin for “cloud” or “fog”, is an interstellar cloud which consists of dust, hydrogen, helium and/or other ionized gases.
Most nebulae are enormous in size, some are thousands of lights years in diameter. Many nebulae are visible because of the gases being lit up by embedded hot stars, while others are so diffuse they can only be detected with special filters and very long photographic exposure times collecting light over a longer period of time. Nebulae are often star-forming regions, but others are the remnants of dying stars, a so called nova.
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