Why do interstellar molecular clouds appear as dark patches against the Milky Way background?
Answer
They absorb and block the light from stars situated behind them
Unlike terrestrial clouds, which are visible because suspended water droplets or ice crystals effectively scatter visible light, interstellar molecular clouds are overwhelmingly composed of cold gas and extremely fine dust particles. This composition means they are exceptionally effective at absorbing and blocking electromagnetic radiation, particularly visible light, coming from objects located beyond them in space. Consequently, when viewed against the brighter background of the Milky Way disk, these structures appear distinctly dark, creating the recognizable silhouette effect observed by astronomers.
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