Radio

The Basics
A radio signal is simply low energy light; it has a low frequency and a long wavelength. Unlike what we see with our eyes, radio light acts more like a wave than a particle. The high energy light we can see acts like a particle such as a rock, and can be collected with cameras or anything else that detects how much energy enters a telescope. Collecting light this way is called (with perhaps a little after-the-fact tongue-in-cheek) incoherent detection. Radio waves are detected coherently, preserving the phase of the wave, or where in its waving (crest, trough, what-have-you) it is as it enters the telescope. This way, if we have to, we can mix the incoming signal with a wave created at the telescope, increase its strength, and finally send it to the astronomers adding very little static. This setup is called a heterodyne system, and uses the fact that waves can be combined to make different waves that are easier to detect.

Pretty Pictures After All
Since radio signals can be combined easily, we can do something called interferometry. This technique lets us use many small telescopes together to act as one big one. Big telescopes are good because they let us see more detail and observe faint objects. There are a bunch of radio interferometers around, the most famous of which is probably the Very Large Array (VLA) in New Mexico. Interferometry is how radio astronomers make pretty pictures, but with even better accuracy than the fanciest Hubble Space Telescope image.

But Pictures of What?
At radio wavelengths, we look at gas rather than stars. In the days before radio, people knew about gas, but only because it got in the way of the stars! Although only a small fraction of the mass of the Milky Way is gas (it's mostly stars) stars are born from gas clouds and stars make gas clouds when they die. So if you think stars are important, it's the gas that tells us where they came from and where they are going. In fact, the whole process of star formation is still a mystery, and radio astronomers are helping to attack it.

In the radio you can see the light given off by the gas itself, just like light from mercury in a fluorescent bulb or from sodium in a yellow street lamp. The light from interstellar gas, practically all hydrogen, tells us where it is, how it is moving, what it is made of, how hot it is, how much is there, its density, and many other things. It is used to find the spiral structure of the Milky Way, and to determine the age of the universe.

Other radio signals come from the sky too. The leftover radiation from the Big Bang that created our universe is brightest in the radio. Really hot gas around supernova explosions and massive young stars is very bright in the radio. Also, unlike visible light, most radio emission easily passes through things. This is why you get the TV, but not the sunshine, inside your house - radio waves go through walls! So we can see into the hearts of galaxies, to the center of forming solar systems, and even through our atmosphere. Yes, radio astronomy is possible during the day, through clouds, in fog, whatever. Only rain stops us at high radio frequencies, and the Earth's ionosphere cuts off the really low frequencies. Ever get an AM station from across the country? This happens because the signal bounces off the ionosphere - it can't pass through. So signals from space can't get in either.