This Hubble image from March 2007 shows a cluster of galaxies whose combined gravitational field is so powerful it is ripping apart a galaxy (top left in purple) that is flying through it. The hapless spiral galaxy's odd shape shows that gas and stars are trailing from it as it plows through the cluster, called Abell 2667.

This discovery helps astronomers fill in the missing link between spiral and irregular galaxies--which are rich with gas and new-forming stars--and elliptical galaxies, which are low in gas and star formation. There are roughly equal numbers of the two classes of galaxies today, but in the early universe, spiral and irregular galaxies vastly outnumbered ellipticals.

Scientists now think many galaxies start out as gas-rich and then become gas-poor over time, perhaps due to encounters like this one.