Do globular clusters ‘adopt’ new stars?

The presence of different generations of stars within the same globular cluster could be explained by the clusters ‘adopting’ other stars rather than creating them, according to a new study.

Shown here in a new image taken with the Advanced Camera for Surveys (ACS) on board the NASA/ESA Hubble Space Telescope, is the globular cluster  NGC 1783. This is one of the biggest globular clusters in the Large Magellanic Cloud, a satellite galaxy of our own galaxy, the Milky Way, in the southern hemisphere constellation of Dorado. First observed by John Herschel in 1835, NGC 1783 is nearly 160 000 light-years from Earth, and has a mass around 170 000 times that of the Sun. Globular clusters are dense collections of stars held together by their own gravity, which orbit around galaxies like satellites. The image clearly shows the symmetrical shape of NGC 1783 and the concentration of stars towards the centre, both typical features of globular clusters. By measuring the colour and brightness of individual stars, astronomers can deduce an overall age for a cluster and a picture of its star formation history. NGC 1783 is thought to be under one and a half billion years old — which is very young for globular clusters, which are typically several billion years old. During that time, it is thought to have undergone at least two periods of star formation, separated by 50 to 100 million years. This ebb and flow of star-forming activity is an indicator of how much gas is available for star formation at any one time. When the most massive stars created in the first burst of formation explode as supernovae they blow away the gas needed to form further stars, but the gas reservoir can later be replenished by less massive stars which last longer and shed their gas less violently. After this gas flows to the dense central regions of the star cluster, a second phase of star formation can take place and once again the short-lived massive stars blow away any leftover gas. This cycle can continue a few times, at which time the remaining gas reservoir is thought to be too small to form any new stars. A version of this image was entered into the Hubble's Hidden Treasures image pr

Globular cluster NGC 1783, captured here by the Hubble Space Telescope, is one of the clusters observed by the team. Credit: ESA/Hubble & NASA
. Acknowledgement: Judy Schmidt (geckzilla.com)

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Recent discoveries of young stars in old globular clusters have upset a long-held theory that globular clusters created all their stars in bulk around the same time.

Astronomers studying this phenomenon have suggested that the discrepancy may be because the clusters are incorporating new, younger stars into the mix from outside, rather than creating them from materials that exist with the cluster.

The results of the study, if true, show how globular clusters can create second or third generations of new stars throughout their lifespan.

Globular clusters are collections of hundreds of thousands, sometimes millions of stars, although the exact nature of their formation and evolution remains unclear.

“This study offers new insight on the problem of multiple stellar populations in star clusters,” says study lead author Chengyuan Li, an astronomer at the Kavli Insititute for Astronomy and Astrophysics at Peking University.

According to the study, globular clusters do not actually incorporate fully formed stars, but rather the external, cosmic material from which they can create new stars themselves.

The study used Hubble observations of globular clusters NGC 1783 and NGC 1696 in the Large Magellanic Cloud, along with NGC 411 in the Small Magellanic Cloud.

“Our explanation that secondary stellar populations originate from gas accreted from the clusters’ environments is the strongest alternative idea put forward to date,” says Richard de Grijs, also an astronomer at KIAA.

“Globular clusters have turned out to be much more complex than we once thought.”

A contrasting theory suggests that new stars are formed from material left behind within that cluster, but co-author of the new study Licai Deng of NAOC believes this is not the case.

“The most massive stars that form in a globular cluster only live about 10 million years before exploding as supernovae, which blow away the remaining gassy, dusty fuel required for making new stars,” he says.

“We have now finally shown that this idea of clusters forming new stars with accreted gas might actually work,” says de Grijs, “and not just for the three clusters we observed for this study, but possibly for a whole slew of them.”

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The team now intend to extend their study to other globular clusters in the Magellanic Cloud and the Milky Way.