Smashed into shape

Figure 1: Collisions of galaxies are usually spectacular, but the latest images of a nearby cosmic pile-up do more than impress the eye.

This colour image from the Hubble Space Telescope (B. Whitmore, STScI) shows the impact region of two merging spirals, the Antennae galaxies. The region is outlined on the ground-based black-and-white image.

Star formation, induced by the collision, is clearly traced by the blue light from young, hot stars. Surprisingly, many compact young clusters of stars are seen in this and other recent observations of colliding galaxies. Previously, stellar groups of this size and shape, such as the globular clusters of our own Galaxy, were thought only to contain much older stars.

The clusters may also be useful as cosmic chronometers: by using the ages of the member stars to tell when a collision took place, it may be possible to investigate how interacting galaxies evolve, perhaps from spiral to elliptical shapes.