Around 600 million years ago, a nearby starburst galaxy known as M82 collided with its neighbor, M81, igniting a celestial clash of the titans that raged on for nearly 100 million years, according to a recent study. Researchers dated the event based on new infrared and visible-light images from NASA's Hubble Space Telescope. The findings appear in the February issue of the Astronomical Journal.

Specifically, Hubble homed in on more than 100 dense clumps of stars, called super star clusters, in the central region of M82 (right) that were apparently forged in the violent encounter. By determining the ages of clusters in an older "fossil starburst" region, Richard de Grijs of the University of Cambridge and his colleagues were able to assign a date to the start of the galactic interaction and to better understand its effects. "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," de Grijs notes. "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."

De Grijs says that star formation in such starburst regions may take place largely in these super star clusters, adding that the clusters in M82 may actually be very young globular clusters. Researchers once thought that globular clusters, which contain up to a million stars when mature, only formed early on in galaxy evolution¿billions of years ago. The new results, in combination with other Hubble observations, instead indicate that globular cluster formation continues today. This insight, de Grijs remarks, is "one of Hubble's main contributions to astrophysics to date."