Why are cold gas clouds often seen as dark patches against brighter backgrounds?
Answer
They do not emit much visible light.
Gas clouds, particularly the cold molecular clouds, exist at temperatures far too low (e.g., 10-20 K) to generate any substantial amount of visible light through thermal emission. Because they are not luminous in the optical range, they appear dark when situated in front of brighter sources, such as distant stars or glowing emission nebulae. This visual characteristic has historically led to their designation as dark nebulae, necessitating the use of indirect observational techniques, like tracking CO emission, to study their internal structure.
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