An emission nebula is a nebula formed of ionized gases that emit light of various colors. The most common source of ionization is high energy photons emitted from a nearby hot star. Among the several different types of emission nebulae are H II regions, in which star formation is taking place and young, massive stars are the source of the ionizing photons; and planetary nebulae, in which a dying star has thrown off its outer layers, with the exposed hot core then ionizing them.
Glazing at the Sky
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