What causes the characteristic deep red glow often seen in massive, star-forming diffuse nebulae?
Answer
The signature of ionized hydrogen ($ ext{H} ext{ extalpha}$) excited by massive young stars.
The energetic ultraviolet light from massive young stars strips electrons from hydrogen gas in the cloud, causing it to re-emit light predominantly as the reddish $ ext{H} ext{ extalpha}$ signature.
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