This is the latest press release from the Galaxy Evolution Explorer (Galex), and it is absolutely stunning. The Cartwheel galaxy is a partcularly impressive example of a starbust galaxy, one undergoing an enormous burst of intense star formation. In this case, the star formation was triggered by a direct collision with a smaller galaxy.
Multi-wavelength view of the Cartwheel galaxy CREDIT: NASA/JPL-Caltech/P. N. Appleton
(Spitzer Science Center/ Caltech)
A long time ago, the smaller galaxy involved in this system passed straight through the middle of the larger galaxy, creating a huge shockwave propagating outwards through the interstellar medium of the larger galaxy, like ripples in your tea when you drop in a sugar cube (should you want to do such a thing!). The shock wave causes gas clouds to collapse as it passes which, in turn, causes stars to form. The bigger the collapsing gas cloud, the more massive the stars which form. These stars burn bright and quick, and violently explode as supernovae when they run out of fuel. These explosions leave behind remnants and cause their own shock waves which could trigger further star formation.
Each colour in this picture represents a section of the electromagnetic spectrum. The purple is X-ray emission seen by the Chandra satellite, green is visible light from Hubble, red comes from an infra-red observation made with Hubble, while the blue represents ultra-violet light as seen by Galex itself. This shows that the warm gas and dust (as seen in the infra-red) is concentrated in the centre of the galaxy while the largest concentration of young, hot stars are found in the outermost ring where the ultra-violet and X-ray emission is strongest. This galaxy is also bright when you look with radio telescopes. A group of researchers did just this with the VLA last year and detected many of the supernova remnants, you can find their paper on astro-ph.