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That is, if we were approaching an HI or HII region in a spaceship, would the cloud have a color visible to the naked eye? Of the HI region, Wikipedia says:

These regions do not emit detectable visible light (except in spectral lines from elements other than hydrogen) but are observed by the 21-cm (1,420 MHz) region spectral line. This line has a very low transition probability, so requires large amounts of hydrogen gas for it to be seen.

But I'm not sure what it means by "so requires large amounts of hydrogen gas for it to be seen." What does "except in spectral lines from elements other than hydrogen" entail?

Qmechanic
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alkah3st
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1 Answers1

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"Requires large amounts of hydrogen to be seen" means simply that the light is extremely weak, so you need lots of hydrogen available to get a reasonable number of photons created the 21-cm line. That's a microwave line, by the way, so wouldn't be visible to the eye.

As far as "except in spectral lines from elements other than hydrogen" is just a reminder that clouds of hydrogen gas are generally not composed exclusively of hydrogen. There will be a mix of hydrogen, helium, oxygen, carbon, neon, etc. Your typical cloud of hydrogen will be something like 74% hydrogen, 24% helium, 1% oxygen, 0.5% carbon, 0.1% neon, 0.1% iron, 0.1% nitrogen, etc. This is by number of atoms, not by mass. So even though you won't see any emission lines from the hydrogen itself, you might see emission lines from other elements.

Jared Willson
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