The Cartwheel Galaxy, 500 million light years away in the constellation Sculptor, is the spectacular result of a head-on collision between two galaxies. It was presumably a normal spiral galaxy, similar to the Milky Way, before a small intruder careered through its very center. One of the two small galaxies to the right of the Cartwheel itself may be the culprit, but it is not clear which. The small blue galaxy is disrupted and has new star formation, which strongly suggests it is the interloper. However, the smoother looking yellowish galaxy has no gas, which is consistent with the idea that gas was stripped out of it during passage through the Cartwheel Galaxy.
The collision triggered a ripple of energy through the Cartwheel that is plowing gas and dust in front of it. Expanding at 200,000 miles per hour, this `cosmic tsunami' leaves bursts of new star formation in its wake. The bright blue knots in the rim of the `wheel' are gigantic clusters of newborn stars. The ring is so large - 150,000 light years across - that the whole of the Milky Way would fit inside it. It contains at least several billion new stars that would not normally have been created in such a short time span. This HST image also resolves immense loops and bubbles blown off by exploding supernovae. The faint `spokes' between the `rim' and the `hub' suggest that spiral structure is beginning to re-merge.
Camera: WFPC2
Technical Information: Composite of two images taken in blue and near-infrared light.
