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.