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An emission nebula is a nebula that emits EMR, as opposed to an absorption nebula or reflection nebula. The term bright nebula refers to an emission or reflection nebula. Emission nebulae consist of gas heated until it is ionized and glows, invariably by one or more hot stars. The two main types are HII regions and planetary nebulae, a third, less frequent type consisting of visible supernova remnants. Red is the most common visible-light color, e.g., from Balmer emission lines. They tend to be associated with star-forming regions, due to the young massive stars still around to produce ionizing radiation, and because a means by which a region can have multiple planetary nebulae is that a bunch of such stars formed together are reaching the end of their life.