Hubble spied more than 100 young, bright, compact star clusters known as super star clusters, in M 82's central region. They are seen as white dots in the new image. 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 M 82 and M 81 began colliding some 600 million years ago in a series of events that lasted about 100 million years. The study is discussed in the February 2001 issue of the Astronomical Journal.

This discovery provides evidence linking the birth of super star clusters to 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.

The huge lanes of dust that crisscross M 82's disk are another telltale sign of the flurry of star formation. Below the center and to the right in the image, a strong galactic wind is seen spewing knotty filaments of hydrogen and nitrogen gas.

M 82 wasn't a huge star-making factory before it met up with M 81.

"The last tidal encounter between M 82 and M 81 about 600 million years ago had a major impact on what was probably an otherwise normal, quiescent disk galaxy," said Richard de Grijs of the University of Cambridge, U.K., who led an international team of astronomers in the M 82 study. "It caused a concentrated burst of star formation in the fossil starburst region. The active starburst taking place today is probably related to debris from M 82 itself that has slowly 'rained' back on the galaxy since the interaction with M 81."

The researchers have an idea for what might produce massive super star clusters.

"It is possible that a large fraction of the star formation in starbursts takes place in such concentrated clusters," de Grijs explained. "And we argue that these clusters are in fact very young globular clusters," which are spherically shaped clusters of up to 1 million stars.

So far, only very old globular clusters in our Milky Way have been found. Astronomers once thought that this type of cluster only formed during the early stages of galaxy evolution many billions of years ago.

"Our results support other observations, mostly made with Hubble, that the formation of globular clusters does indeed continue today," said de Grijs. "This is, in our opinion, one of Hubble's main contributions to astrophysics to date."

Astronomers using ground-based telescopes have provided circumstantial evidence supporting the galactic encounter 600 million years ago. Radio observations have shown a cocoon of hydrogen enclosing the two galaxies and about a dozen smaller galaxies belonging to the M 81-M 82 group.

M 82 is a bright galaxy that sits 12 million light-years from Earth in the constellation Ursa Major, the Great Bear. The Hubble pictures were taken in 1997, but only studied more recently.