According to a new study published this week in the journal Nature, globular clusters make new stars by adopting stray cosmic gas and dust.
This Hubble image shows the globular star cluster NGC 1783. It is one of the biggest globular clusters in the Large Magellanic Cloud, a satellite galaxy of our Milky Way Galaxy. First observed by the British astronomer John Herschel in 1835, this star cluster is approximately 160,000 light-years from our Solar System, and has a mass around 170,000 times that of the Sun. Image credit: NASA / ESA / Hubble / Judy Schmidt, geckzilla.com.
Stars in clusters are thought to form in a single burst from a common progenitor cloud of molecular gas.
However, old and massive globular clusters – with ages greater than 10 billion years and masses several hundred thousand times that of the Sun – often harbor multiple populations of stars, indicating that more than one star-forming event occurred during their lifetimes.
Now the new study, led by Dr. Chengyuan Li of the Kavli Institute for Astronomy and Astrophysics, might explain these puzzling stellar generations.
“This study offers new insight on the problem of multiple stellar populations in star clusters,” Dr. Li said.
Using data collected by the NASA/ESA Hubble Space Telescope, Dr. Li and co-authors have for the first time found young populations of stars within globular clusters that have apparently developed courtesy of star-forming gas flowing in from outside of the clusters themselves.
“Our study suggests the gaseous fuel for these new stellar populations has an origin that is external to the cluster, rather than internal,” the scientists said.
The team used Hubble observations of the globular star clusters NGC 1783 and NGC 1696 in the Large Magellanic Cloud, along with NGC 411 in the Small Magellanic Cloud.
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.
“What is the most straightforward explanation for these unexpectedly differing stellar ages? Some globular clusters might retain enough gas and dust to crank out multiple generations of stars, but this seems unlikely,” said co-author Dr. Licai Deng, from the Chinese Academy of Sciences’ National Astronomical Observatories.
“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.”
The scientists propose that globular clusters can sweep up stray gas and dust they encounter while moving about their parent galaxies.
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Chengyuan Li et al. 2016. Formation of new stellar populations from gas accreted by massive young star clusters. Nature 529, 502-504; doi: 10.1038/nature16493
