Neutral atomic gas, usually known as HI (hydrogen + Roman numeral one, for "not ionized"), forms the backbone of the disk in a spiral galaxy. This neutral hydrogen image of a spiral galaxy M81 (Rots, A.M. & Shane, W.W., 1975, Astronomy & Astrophysics, 45, 25) can be compared to an optical image of the galaxy. It's easy to see how the HI contours very efficiently trace the gas in the spiral arms of the galaxy. From this gas, molecular clouds and, in turn, stars form, which is why the spiral arms look bright in the optical image.
Perhaps the most important aspect of the neutral gas is the means by which it emits radio energy, a rare transition of the hydrogen atom which makes the gas very easy to map and trace, even in distant galaxies. This transition is due simply to a flip of the nucleus (proton) of the atom (top to bottom). Recall that a hydrogen atom is simply a proton and an electron. Both of these particles have spins, either up or down. The spins of these particles are related to tiny magnetic moments. If a proton is spinning west to east, its magnetic north points north; it's opposite for the electron, because it has the opposite charge. Now, magnets like to point north-to-south, so ideally the electron and proton would both spin in the same direction. Imagine that the proton has a spin so that the magnetic north is up. If the electron's spin is in the same direction, its magnetic north is down. In that case, proton north faces electron south when the electron is above the proton, and proton south faces electron north when the electron is below the proton. In both cases, north is facing south; and so an atom with aligned spins resides at a slightly lower energy than atoms with misaligned spins.
This energy difference is so small that an atom in the higher energy state will take an average of 10 million years to fall into the lower energy state. Compare this with a few tens of nanoseconds for an electron to change atomic energy level. When the atom finally does get its spins aligned, it emits a photon with a wavelength of 21 centimeters.
The incredibly small radiation coefficient for this transition prevents such
