One of the main tools astronomers use to determine what we are seeing through a telescope is a spectrum. A spectrum is a splitting of light into its component colors, just like a rainbow. The picture above shows a spectrum of the sun. If you look closely, you'll see several dark lines. These lines are caused by different materials in the sun, like hydrogen, iron,calcium , magnesium, carbon, oxygen, and so on. Each element has its own unique set on lines that appear in the spectrum, just like a fingerprint. Only in the astronomy, these fingerprints get stacked, one on top of the other, to make the pattern you see here.
Different objects have different spectra. Compare the spectrum of the sun (above) to the spectrum below this paragraph. Notice any differences? (You can click on the spectra to make the images larger.)
The two main differences are: the lower spectrum has many fewer lines, and also has much brighter blue and less obvious red. The star we are looking at (Algol, in the constellation Perseus) is a very hot star. The star appears blue in color (as you can see from the spectrum) and is so hot that the normal lines from metals like iron don't show up -- those metals have lost the electrons that make the characteristic lines!
Tomorrow I'll put up a few more fun examples. But the color representation is just to help you visualize the spectrum -- we astronomers don't use color pictures like this. Instead we make graphs showing how much light we get at each specific wavelength (or color), so that we can use mathematics to uncover even more details than the human eye could ever notice.
Thanks to John Kielkopf at the University of Louisville for writing the program that colorized these spectra!