Hydrogen is the most abundant gas in the Universe, and provides important clues about its properties. The cosmic microwave background was created through interactions with ionized hydrogen, and emissions from hydrogen ions help us identify energetic events like star formation. They do, that is, if the events are far enough away. Ironically, the Universe conspires to keep us from seeing events in our own galaxy. Or, more accurately, the Sun has kept us from seeing them until very recently, when the Voyager probes finally got far enough from the Sun to see what our galaxy is up to.

Hydrogen's lone electron normally resides in the ground state, but various energetic events can move it to higher states. Two types of emission result from these processes. Balmer-alpha occurs when the electron drops from the second excited state to the first (and has nothing to do with Steve or thrown chairs). Lyman-alpha emissions occur when the electron drops from the first excited state back to the ground. The Lyman emissions, which occur in the UV, rarely reach us directly. There's so much hydrogen out there that it's often absorbed and re-emitted multiple times before getting very far from its source.

And that's precisely what leaves us blind to emissions from anywhere nearby. The sun, having lots of hot hydrogen, is a prodigious Lyman emitter, and hydrogen scattered throughout the solar system captures and reemits many of these photons, creating a bright fog of radiation at this wavelength. If this fog were consistent, it would be possible to subtract it and look at what other local sources are up to. Unfortunately, the solar weather and other changes mean that it fluctuates so much that this sort of background subtraction has been impossible.

Ironically, Lyman alpha radiation is easy to detect in distant objects. The expansion of the intervening space causes a red shift that brings the Lyman alpha radiation down to longer wavelengths, shifting it away from the fog of our local solar system. So, we can see distant sources of Lyman alpha emissions, and have assigned them to things like areas of active star formation, but we can't actually confirm that these assignments are accurate based on equivalent observations of star formation in our local galaxy.

Or couldn't, up until very recently. Once the Voyager probes neared the orbits of Pluto, they got far enough away from the Sun that emissions were much lower. Not only that, but they started to smooth out to a constant background, as distance evens out the fluctuations in the solar wind, and most photons get absorbed and re-emitted several times between leaving the sun and reaching that distance.

The Voyager probes carried UV spectrometers that could identify Lyman alpha emissions and, as luck would have it, researchers starting using them to scan their surroundings all the way back in the early '90s. (They were actually looking for something else: a spot where the solar system's hydrogen piles up against pressure from the interstellar medium.) These scans continued sporadically for most of the rest of the decade.

Researchers have now gone back and reanalyzed this data, incorporating information on the Voyager's position and the orientation of their observations. The resulting image clearly shows the plane of our galaxy as a bright area, as you'd expect, and there are obvious regional variations in the signal.

This probably isn't detailed enough to provide a real sanity check on our models of distant objects, and, unfortunately, the Voyagers have a bit of a catch-22 when it comes to running their UV spectrometers. The data from last decade includes a lot of noise generated by their nuclear power supply that had to be subtracted. Now, this background should be fading, but the amount of power they're generating isn't high enough to run the spectrometers for these sort of detailed scans. Voyager 2's has been shut off entirely, and Voyager 1's may face a similar fate soon.

The patient astronomers have a bit of hope on the way, though. New Horizons is already over half-way to Pluto, and it, too, carries a UV spectrometer.

Science, 2011. DOI: 10.1126/science.1197340 (About DOIs).