Which specific emission line is primarily used by astronomers to detect the presence of neutral atomic hydrogen ($ ext{HI}$)?
Answer
$21 ext{ cm}$ radio line
The diffuse interstellar gas, primarily composed of neutral atomic hydrogen ($ ext{HI}$), is typically found at moderate temperatures (around $100 ext{ K}$) and densities (1 to 100 atoms/cm$^3$). Because this gas is not hot enough to be ionized like emission nebulae, it is best observed through a specific, characteristic emission signature. Astronomers rely heavily on the $21 ext{ cm}$ radio line, which is generated by a subtle change in the spin state of the neutral hydrogen atom, allowing for widespread mapping of this gas component across the galaxy.
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