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Seeing spots. Intermediate-aged stellar clusters appear as bright, white dots in this image of galaxy M82.

Middle-Aged Star Clusters Surface

It's not your typical case of middle-child syndrome, but like the middle-born, median-aged swarms of stars called stellar clusters have slipped scientists' scrutiny. Now, a team of astronomers has identified the first stellar clusters that fall into this elusive age group, and find support for the idea that older clusters do evolve from young ones.

A single stellar cluster can contain a million stars, populating our galaxy and galaxies beyond. Astronomers can readily identify young clusters, less than 200 million years old, because they are nearby and in bright galaxies where many new stars are born. They have also been able to find clusters that are almost as old as the universe itself, 10 billion to 12 billion years old, because they are located in our galaxy. But since most galaxies in which middle-aged clusters have formed are likely too far away, astronomers have had little success finding clusters in the range of a few billion of years old.

A serendipitous collaboration led astronomers to discover a new group of middle-aged clusters, explains Henny Lamers of Utrecht University, the Netherlands. The Utrecht group had developed a method for determining the age of stellar clusters in the young Whirlpool galaxy (M51). They based it on the following logic: Young clusters contain many massive, hot stars. But as a cluster ages, the massive stars die out first. The survivors are cooler stars, which radiate lower frequency reddish light. Using images taken by the Hubble Space Telescope, the team measured the amount of high-frequency ultraviolet light emitted by very hot and massive stars and compared it to the other wavelengths emitted by the clusters.

Astronomer Richard de Grijs, working at Cambridge University, teamed with the Utrecht group because he noticed a group of clusters in Hubble images of galaxy M82 that could have formed during a collision with another galaxy during this intermediate time interval. Using their light analysis method, the team confirmed that M82 contained 110 intermediate-aged clusters that were 1.5 billion years old. The mass distribution of these middle-aged clusters is almost identical to that of the older globular clusters. This suggests that old and young clusters have similar origins, the team reports in the 20 January issue of Astrophysical Journal Letters.

This research is notable because it addresses the lingering question of whether young clusters might arise in a different way than old ones, says astronomer Stephen Zepf of Michigan State University in East Lansing. It will be important to test whether similar results are found for other globular cluster systems of similar ages, he says.

Related sites
de Grijs's site
Science's special issue on globular clusters