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Studying galactic interactions is like sifting through the forensic evidence at a crime scene. Astronomers wade through the debris of a violent encounter, collecting clues so that they can reconstruct the celestial WATCH: HubbleMinute Video HubbleMinute: When Galaxies Collide  crime to determine when it happened. Take the case of M82, a small, nearby galaxy that long ago bumped into its larger neighbor, M81. When did this violent encounter occur? New infrared and visible-light pictures from the Hubble telescope reveal for the first time important details of large clusters of stars, which arose from the interaction.

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Q & A: Understanding the Discovery

1. 1. What did Hubble see?




Hubble's sharp eye spied more than 100 young, bright, compact star clusters, known as super star clusters, in M82's central region. Each cluster contains about 100,000 stars. These stars act like clocks: Their ages tell astronomers when the wreck occurred. Sampling clusters of stars in an older, "fossil starburst" region, astronomers concluded that the galactic violence between M82 and M81 began some 600 million years ago and lasted about 100 million years. This discovery provides evidence linking the birth of super star clusters with a violent interaction between galaxies. These clusters also provide insight into the rough-and-tumble universe of long ago, when galaxies bumped into each other more frequently.

2. 2. What are these massive super star clusters?




The researchers believe that these clusters are very young globular clusters (spherically shaped clusters of up to one million stars). So far, astronomers have observed only very old globular clusters in our Milky Way. Astronomers once thought that this type of cluster only formed during the early stages of galaxy evolution many billions of years ago. This view of M82 supports other observations, mostly made with Hubble, that the formation of globular clusters does continue today.