Study provides new insight into the problem of multiple star populations within star clusters.
Massive groups of stars known as globular clusters are one of the most phenomenal objects ever found in the universe. These are a glittery and dense swarm of stars which are tightly bound by gravity.
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It has been long thought that these globular clusters produced their millions of stars in bulk at the same time with all having similar ages. But recently astronomers have discovered a young generation of stars in old globular clusters, which is a huge surprise indeed.
Globular clusters can produce second or third sets of thousands of sibling stars instead of giving stellar birth at once.
To find out how massive globular clusters produce sibling stars, a group of researchers led by Institute for Astronomy and Astrophysics (KIAA) use Hubble Space Telescope observations and for the first time it was found that these young populations of stars with globular clusters are formed with gases flowing in outside the clusters. That is contrasting to conventional idea where stars shed gas as they grow older to form new generations of star.
"This study offers new insight on the problem of multiple stellar populations in star clusters," said Chengyuan Li, an astronomer at KIAA and lead author of the study. "Our study suggests the gaseous fuel for these new stellar populations has an origin that is external to the cluster, rather than internal."
Researchers suggest that globular clusters seem capable of ‘adopting’ stray cosmic gases in order to form new stars instead of ‘biological’ manner like humans.
“Globular clusters have turned out to be much more complex than we once thought,” said co-author Richard de Grijs, an astronomer at KIAA. “Our explanation that secondary stellar populations originate from gas accreted from the clusters’ environment is the strongest alternative idea put forward to date.”
Globular clusters are spherical group of highly glittering stars that orbit the outer part of a galaxy. Our own galaxy Milky Way also hosts several hundreds of such massive clusters but most are quite old. For the study, researchers observed three relatively young clusters in nearby dwarf galaxies: NGC 1783 and NGC 1696 in the Large Magellanic Cloud while NGC 411 in the Small Magellanic Cloud.
Researchers estimated the ages of the star found them by looking at their colors and brightness. Within NGC 1783, for example, they identified an initial population of stars aged 1.4 billion years, along with two newer populations that formed 890 million and 450 million years ago.
The most straightforward explanation for their differing stellar ages could be some globular clusters might retain enough gas and dust to crank out multiple generations of stars but it is highly unlikely.
“Once the most massive stars form, they are like ticking time bombs, with only about 10 million years until they explode in powerful supernovae and clear out any remaining gas and dust," said co-auhor Aaron Geller from Northwestern University. "Afterwards, the lower-mass stars, which live longer and die in less violent ways, may allow the cluster to build up gas and dust once again."
The idea that globular clusters sweep up stray gas to form new stars was first proposed in 1952 and latest study suggests that theoretical explanation is possible in these massive star clusters.
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"We have now finally shown that this idea of clusters forming new stars with accreted gas might actually work," said de Grijs, "and not just for the three clusters we observed for this study, but possibly for a whole slew of them."