Talk:Nebula
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The following material is not accurate, so i put it here until I (or someone else) corrects it and reincorporates it into the main text. AN
Stars emanate gas during novae and supernovae, giving rise to planetary nebulae and supernova remnants, respectively. In a nova, the star sloughs off its outer layers of gas as it collapses into a dwarf, and these clouds are propelled outward by the remaining star's stellar windand glow from the heat of the star. In a supernova, the star collapses violently on itself and explodes with an enormous shock wave which lights up all the interstellar gas in the region, and the remaining neutron star or black hole emits highly charged radiation as it sucks up everything around it, keeping the center of the cloud brightly lit.
At this point the the star may die or move on, and the cloud may disperse and merge with other clouds. But when the dust is dense enough, gravity will turn the inevitable knots and perturbances into new stars. These new stars may reflect light off the gas, creating a reflection nebula, or excite the gas with their radiation, creating an emission nebula. Many nebulae display both characteristics, and are referred to as diffuse nebulae. In addition, the lit parts of this kind of nebula may be blocked by unlit gas on the nebula's periphery, creating a dark nebula.