Sources of Radio Emission – 21 cm
radiation
Neutral hydrogen has a
very weak radio spectral transition. So the Galaxy is transparent to it. On the other hand, there’s a lot of neutral hydrogen. So we can see it everywhere. There are also molecular lines from CO and other molecules.
The transition occurs because electrons and protons
have “spin”. Having the spins aligned is a higher energy state. So in about
10 million
years it will decay to the ground state (anti-aligned). Or a 21-cm photon can
be absorbed and align the spins.
Because the
Galaxy is transparent, it is hard to tell where the emission is
coming from along the line-of-sight. But because we know its precise
wavelength, Doppler shifts in this line can tell us how the gas is
moving.