In 1572, the Danish nobleman Tycho Brahe observed a "new star" in the sky in the constellation Cassiopeia. He published a small book on his observations of this new star called "De Nova Stella," and thus coined the term nova. We now know that Tycho's star was actually a supernova and it is still being studied today. Both novae and supernovae involve explosions by stars that cause them to become much brighter than normal.
This is a composite image from NASA's Chandra X-ray Observatory and the Spitzer Space Telescope. The green and yellow are from x-rays emitted by the expanding shell of gas from the original explosion more than 400 years ago. The blue represents the shock wave from the explosion, also from x-ray emissions. The red is dust observed in infrared wavelengths.
You can get this image and more from the Chandra Observatory's web site. There are desktop patterns and wallpapers in several sizes, and a lot of other cool stuff. Take a look at the photo album for more details about this image.