More ISM, and the hyperfine structure of hydrogen

The interstellar medium (which I introduced yesterday) is really cool in a lot of ways, so I thought it merited another post.

The ISM is made mostly of hydrogen (something like 90% or so), with a bit of helium (~8%), and some other stuff (around 2% heavier things, which astrophysicists lump into the 'metal' category). It turns out to be a lot easier to tell how much dust (granules of atoms, typically (I think) the heavier atoms) is present in a cloud than, say, how much hydrogen there is. We just look at how opaque a cloud is, which can tell us roughly how much dust is between us and the stars behind the cloud. But we can also get a good measure for the amount of neutral single hydrogen present in a cloud by looking at what's called the 21-cm line. You may have heard of emission/absorption lines. Every element has a characteristic signature, because in essence it can only emit photons with energies equal to energy level differences for its electrons. So, for instance, the photons emitted by neutral hydrogen when an electron drops from the third energy level to the second all have pretty much identical wavelengths of around 656 nanometers, which appears to us as red light. But the 21-cm line isn't from the electron energy levels you learn about in high school chemistry. Instead, imagine a hydrogen atom, just an electron 'orbiting' its proton, in which the spins of the electron and proton are aligned. This state of hydrogen has a slightly higher energy than the state in which the proton and electron have opposite spin. This division of the ground state of hydrogen into two sub-states is called hyperfine structure, and it occurs because of the interactions of the magnetic moment of the electron due to its spin and its orbital angular momentum. When an atom of hydrogen drops from the slightly-excited, spin-aligned state, it emits a photon with a wavelength of 21 cm, which, as a microwave, propagates freely through clouds of gas that are otherwise opaque to visible and infrared light.

The hyperfine energy levels of the ground state of hydrogen are so close together that a decay from the spin-aligned state to the spin-unaligned state takes the average atom around 10 million years. That's a rare decay, so it's really hard (impossible?) to observe on Earth - just regular heat-based interactions of materials interfere with it - but it turns out there's a heck of a lot of cold neutral hydrogen out there in space.

The 21-cm line is uniquely narrow, which means that it's really easy to pick up any Doppler shifting caused by the movement of the emitting hydrogen. That allows us to do things like map out how fast the various arms of the Milky Way are moving away from us. Pretty neat.