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Why are observations of the $21\,{\rm cm}$ line more useful that emission from other lines? Is it only because of its abundance in the whole Universe, or other reasons as well?

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I only know one particular reason: The transition responsible for the H1 line is highly forbidden and shows an extreme lifetime ($10^7$ years), so the absorption rate in interstellar clouds, which can be very opaque for every other radiation, is very small. Looking at the H1 line allows you to see objects which are for example hidden behind dust clouds that absorb a lot of the radiation of the object behind.

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  • $\begingroup$ Because of the forbidden transition, the 21cm line is an indicator of dilute, cold hydrogen gas. This is the most obvious signal that these hydrogen gas clouds give, and they're everywhere. $\endgroup$ – probably_someone Nov 22 '17 at 15:53
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Another reason beside the one pointed out by Noldig is the fact that you can accurately measure rotational curves of spiral galaxies with it. The hydrogen extends in the plane of the disk of the galaxy way farther than the "visible galaxy" (that is stars), so through doppler measurements you draw its curve of rotation and discover that the mass of the galaxy is not enough to sustain that curve. This indicates that there is extra mass that we can't see, the so called dark matter. Then one can calculate how much dark matter is needed and how it has to be distributed (approximately evenly in an halo around the galaxy) in order to match the observations. This is one of the first evidences of dark matter.

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