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 they can reconstruct the celestial crime to determine when it happened.

This image taken by Hubble shows the heart of the prototypical starburst galaxy M82. The ongoing violent star formation due to an ancient encounter with its large galactic neighbor, M81, gives this galaxy its disturbed appearance. The smaller picture at upper left shows the entire galaxy as taken in December 1994 by the Kitt Peak National Observatory's 0.9-meter telescope. In the Hubble image, the huge lanes of dust that crisscross M82's disk are another telltale sign of the flurry of star formation. Below the center and to the right, a strong galactic wind is spewing knotty filaments of hydrogen and nitrogen gas. Credits for Hubble image: NASA, ESA, R. de Grijs (Institute of Astronomy, Cambridge, UK). Credits for ground-based picture: N.A. Sharp (Association of Universities for Research in Astronomy, National Optical Astronomy Observatories, National Science Foundation)

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 NASA's Hubble Space Telescope reveal for the first time important details of large clusters of stars, which arose from the interaction.

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. The results are published 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.

M82 wasn't a huge star-making factory before it met up with M81.

"The last tidal encounter between M82 and M81 about 600 million years ago had a major impact on what was probably an otherwise normal, quiescent disk galaxy," says Richard de Grijs of the University of Cambridge, UK, who is leading an international team of astronomers in the M82 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 M82 itself that has slowly 'rained' back on the galaxy since the interaction with M81."

But what actually are these 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 explains. "And we argue that these clusters are in fact very young globular clusters [spherically shaped clusters of up to one million stars]!"

Hubble snapped these two views of the heart of the galaxy M82. The image at left was taken in visible light; the picture at right, in infrared light. In the infrared view, the telescope's Near Infrared Camera and Multi-Object Spectrometer peered through thick dust lanes to find some of the galaxy's more than 100 super star clusters. The clusters are the larger pink and yellow dots scattered throughout the picture. They were formed during a violent collision with the galaxy M81 about 600 million years ago. The galaxy is 12 million light-years from Earth in the constellation Ursa Major. Credits: NASA, ESA, R. de Grijs (Institute of Astronomy, Cambridge, UK)

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.

"Our results support other observations, mostly made with Hubble, that the formation of globular clusters does indeed continue today," de Grijs says. "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 M81/M82 group.

M82 is a bright (eighth magnitude), nearby (12 million light-years from Earth) galaxy in the constellation Ursa Major (the Great Bear).

The Space Telescope Science Institute (STScI) is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).