What is the primary cause of the rich, reddish hue observed in most emission nebulae?

Answer

The $ ext{H} ext{alpha}$ line emission

The dominant color is red, which specifically originates from the $ ext{H} ext{alpha}$ line transition, where a hydrogen electron drops from the third to the second energy level, emitting light at $656.3 ext{ nm}$.

What is the primary cause of the rich, reddish hue observed in most emission nebulae?

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