National Geographic Daily News
A pair of star clusters.

Star clusters in the early stages of merging are seen in a Hubble image.

Image courtesy E. Sabbi, STScI/ESA/NASA

Andrew Fazekas

for National Geographic News

Published August 21, 2012

The Hubble Space Telescope has glimpsed the first moments of a head-on collision between two massive star clusters, astronomers say.

The impending cosmic crash is occurring within a 25-million-year-old giant star factory known as the Tarantula Nebula, or 30 Doradus, which is located 170,000 light-years away from Earth.

The Tarantula Nebula is buried inside the Large Magellanic Cloud—a small companion galaxy of the Milky Way.

"We think the merger is undergoing and our simulations indicate that it will take about three million years to be completed," said Elena Sabbi, lead scientist at the Space Telescope Science Institute in Baltimore, Maryland.

The larger cluster, containing 52,000 stars, has dramatically twisted and stretched out the previously unknown smaller cluster containing ten thousand stars to such an extent that it had been unrecognizable until now, Sabbi said.

"Hubble allowed us to observe for the first time the fainter and most numerous stars, revealing the presence of the second cluster," she said.

Sabbi and her team accidentally came across the merging objects while looking for runaway stars—massive suns thrown out of their nursery cluster at speeds of up to 62,000 miles (100,000 kilometers) per hour. The Tarantula Nebula is already known to contain such runaways.

The scientists suggest that the merging clusters' gravitational dance may have launched these high-velocity stars to begin with.

(See "Galaxy Cluster Stuns Scientists—Supermassive and Spewing Out Stars.")

Star Clusters Rethought

Understanding the origin of these runaway stars will help form more accurate computer models of the birth and evolution of star clusters across the cosmos, Sabbi added. (See more star pictures.)

"According to some current models, the giant clouds [of gas and dust] from which star clusters are created fall apart in smaller pieces, and these small parts can later merge into a heavy cluster," she said.

"This is what we think we are observing for the first time in the most active region of star formation in the local universe."

This newfound cluster collision may also lead to a rethinking of the true nature of star clusters, which are found in all the universe's galaxies, noted University of Arizona astronomer Dennis Zaritsky, who is not associated with the study.

"Stellar clusters have been thought of as a single-age, homogeneous population of stars with older clusters gravitationally self-bound, but now, if clusters can be, in some cases, combinations of clusters, then we can no longer expect them to have a single-age or chemical abundance," Zaritsky said.

"Bottom line is that this new evidence for an ongoing merger will affect how we interpret all future cluster observations."

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