I'd been wondering about emission spectra for a while. To summarise it, what happens is this: electrons exist in "orbits" round atoms. You can think of these orbits simply, like planets orbiting the Sun, or in a more sophisticated manner, as a "wave of probability" of where the electron will be at any given time. There are fixed orbits for the electrons (imagine Mercury being able to jump into Venus or Earth's orbit, but not to a random place in between), and being struck by a photon can boot an electron up into a higher one. This is an unstable, i.e. higher-than-minium-energy state, so at some point later the electron will fall back down again, re-emitting a photon as it does so.
My dilemma was this: if the electron needs a certain amount of energy to move up one or more orbits, and emits exactly the same amount when it returns to its ground state, shouldn't the two cancel out? I.e. why should we see any absorption and emission spectra at all?
I asked our wonderful Chemistry tutor, Tim, this in Brighton two years ago. He told me to think of it as light travelling in particular directions. In the case of absorption spectra, it absorbed it at a particular angle (the light travelling between the source and your eye via it) and radiated it out in a halo, so you didn't really see the re-emitted radiation. And with emission spectra it's vice versa.
Quite by accident I came across some information about hydrogen in space. The interstellar medium is full of hydrogen. Even in regions of "empty" space, i.e. not clouds, hydrogen is present in individual atoms (HI in astronomy notation; a new language to learn after many years of thinking of that as an H-radical). When the temperature is below 100K, or in a denser region, the atoms form molecular hydrogen, or H2, as we find it on Earth. In hotter regions, with starforming activity, the hydrogen is likely to become ionised. In chemistry terms, it is H+ (+ should be superscript!); in astronomy notation, it is HII, or ionised hydrogen. Ionised means that a photon has hit the electron with enough energy that it has knocked it clean off the atom.
So that's the HII galaxies Tom and the zooites are collecting. Hang on. HII emission lines? Emission lines from a proton all by itself, without the electron to be booted up and down any orbit at all? I don't know why I didn't think of it before, but there you go. I asked in the thread, and mukund vedapudi soon came up with the answer. In short, in regions that are hot enough (the vicinity of massive blue stars you get in bright blue galaxies), hydrogen nuclei (protons) and their electrons are constantly recombining as well as ionising. The recombining releases a characteristic emission line just like the Balmer or Lyman series (when the electron drops down to the second and innermost orbit respectively).
But we also detect hydrogen in space with radio waves. How? Photons which fire electrons up into higher orbits need (to the best of my knowledge) to be more energetic than radio waves. Take this picture, from an article about it.
(Credit: sciencedaily.com - the detection of 5 hydrogen clouds in space.)
You can detect hydrogen in clouds and out. In fact there's a specific way to detect cold (but above 100K) lonely HI atoms, because they emit radio waves of 21cm.
I just found out the way they do this. It's to do with spin. I learned enough about that to establish that the Pauli exclusion principle states that no two electrons can exist in the same orbit with the same spin. They can spin one way or the other. That's why you get the s-orbital in hydrogen, and then another s-orbital and three p-orbitals in lithium to neon in the Periodic Table. Each orbit contains room for 2 electrons, provided they're spinning opposite ways.
I hadn't consciously realised that atomic nuclei have a spin as well. But they do. And in the simplest atom we know, hydrogen, which is just a proton and an electron, it's simple enough to keep track of the spin of each. And just like electrons in orbits, when more than one state is available, the system will prefer the one with the lower energy. That is when the electron and the proton's spin are opposite, not equal.
In a moderately warm environment, not cold or dense enough for H2 to form, hydrogen atoms can collide, the energy of which knocks them into the state of parallel spins. They will later revert to opposite spins, emitting radiation with a wavelength of 21cm. This allows us not only to detect that there is warm-ish, thin, atomic HI about, but also (using redshift) to measure the radial velocity - i.e. whether it is coming towards us, or heading away.
Why doesn't this work for molecular hydrogen? Basically because molecular hydrogen works quite differently. Both electrons are in a hazy cloud of probability around both protons, and the molecule has a great deal more symmetry. This makes dense hydrogen clouds harder to detect (which is probably why it made the news when 5 were detected in the story above). This is also true of most gases, such as oxygen, in space, because they tend to exist in pairs.
Except, for instance, in the important case of carbon monoxide.
Carbon and oxygen have a different number of protons, neutrons and electrons, which makes them behave differently. Oxygen, for instance, is just dying to grab two electrons to fill up its remaining p-orbitals; we call it "electronegative". (In water it is quite greedy and the electrons spend more overall time around it than around hydrogen, which is why water molecules are dipoles.) This means that carbon monoxide will not have that symmetry which insulates it from our beady eyes.
Carbon monoxide undergoes rotational transitions like atomic hydrogen, HI, so we can detect radio waves from it, at the higher frequencies than HI of 1.3 and 2.6mm. It is relatively common in space. Astronomers believe that it is also found pretty much where hydrogen clouds are, so it's a useful tracer for them. For molecules to form, gas has to be cold. And for a cloud to become dense, it has to be cold. And for stars to form, a cloud has to be dense. Isn't it ironic that star formation can't take place unless the environment around it is cold enough?
That is why radio waves, the longest wavelength of the electromagnetic spectrum and its lowest energy, can forewarn us of some very energetic events in the future. It is because of some of the smallest particles in the Universe altering the way they move around.
Update, 14th May: EigenState on the forum has had a chance to read this post, although I promise I did warn him it might make his hair stand on end. He kindly gave me some great science feedback, well worth a read - he knows a lot more than I do. Here it is.