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Globular Star Clusters:
Not as Simple as Thought for Decades

by Dr. Paul Goudfrooij (STScI)

Astronomers have long thought that globular star clusters are very simple systems, consisting of hundreds of millions or even billions of stars that all formed in one big "boom", so that they all share the same age and chemical composition.

Because of this and the fact that they are very bright, globular clusters are often used in astronomy as 'fossil records' of the important star formation events during their host galaxies' evolution.

Deep images taken by the NASA/ESA's Hubble Space Telescope, however, are showing that this idea may be too simplistic. The analysis of the massive globular cluster NGC 1846 and other similar star clusters in the Magellanic Clouds (two small 'next door neighbor' galaxies of our Milky Way, visible by eye from the southern hemisphere) provides evidence that stars in clusters did NOT form instantly, but during a stretch of a few hundred million years after the 1.8-billion-year-old cluster was first formed.

The team, led by Paul Goudfrooij of the Space Telescope Science Institute in Baltimore, used Hubble's Advanced Camera for Surveys to measure the colors and brightnesses of the cluster stars. Hubble's exquisite depth and resolution allowed the astronomers to sort out the different stellar populations as well as their spatial distribution within the cluster. The Hubble measurements showed evidence for a range of temperatures among stars that are otherwise at the same point in their evolution.

The 'kicker' result of this study was, however, that the team also found that the hotter stars within NGC 1846 and other similar clusters are more centrally located within the cluster than their cooler counterparts, and that the level at which the hotter stars were more centrally concentrated than the cooler stars correlates with the escape velocity of the clusters in their study. This result strongly suggests that the reason for a range of temperatures among those stars is a range of stellar ages.

The escape velocity of a celestial object is the speed a lighter object must have to "break free" from the gravitational pull of the celestial object. For NGC 1846, one of the most massive globular clusters in the team's study, the escape velocity was about 25 kilometers per second - that is 56,000 mph! - shortly after the cluster was formed. This turns out to be high enough for the globular cluster to hang onto gas lost by dying stars of the first generation and bring it back to its central regions, so that new generations of stars can be formed there.

Because the globular clusters studied by Goudfrooij's team are small relative to most globular clusters in our Milky Way when the latter were born, this result suggests that multiple stellar generations in globular clusters should be a widespread phenomenon. Indeed, Hubble is currently observing a large number of globular clusters to hunt for multiple generations with its sensitive Wide Field Camera 3 which was installed in 2009. Stay alert for updates soon!

The team's results on NGC 1846 and 10 other star clusters in the Large Magellanic Cloud have been published in three articles in the Astronomical Journal and the Astrophysical Journal.