What are the primary components of dark nebulae that cause them to appear as silhouettes?
Answer
Cold molecular hydrogen and dust
Dark nebulae appear as silhouettes because they are dense enough to completely block or obscure the visible light coming from background sources, such as brighter emission or reflection nebulae, or distant stars. Despite their opacity to visible light, these regions are still fundamentally composed of the standard elemental mix, specifically characterized here as being concentrated regions of cold molecular hydrogen and dust.
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