This is a portrait of the massive globular cluster NGC 1783 in the Large Magellanic Cloud, taken by the Hubble Space Telescope. This dense swarm of stars is located about 160,000 light years from Earth and has the mass of about 170,000 Suns. A new study by astronomers from the Kavli Institute for Astronomy and Astrophysics at Peking University (KIAA), the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), Northwestern University and the Adler Planetarium suggests the globular cluster swept up stray gas and dust from outside the cluster to give birth to three different generations of stars. Credit: ESA/Hubble and NASA. Acknowledgement: Judy Schmidt (geckzilla.com)
Astronomers have for the 1st time found old globular clusters with young populations of stars that developed from star-forming gas flowing in from outside of the clusters themselves. This method stands in contrast to the conventional idea of the clusters’ initial stars shedding gas as they age in order to spark future rounds of star birth. Astronomers had long thought globular clusters formed their millions of stars in bulk at around the same time, with each cluster’s stars having very similar ages.
Instead of having all their stellar progeny at once, globular clusters can somehow bear 2nd or even 3rd sets of thousands of sibling stars by outside gas accretion. Globular clusters are spherical, densely packed groups of stars orbiting the outskirts of galaxies. Our home galaxy, the Milky Way, hosts several hundred.
KIAA-led research team used Hubble observations of globular clusters NGC1783 and NGC1696 in the nearby Large Magellanic Cloud, along with NGC411 in the Small Magellanic Cloud. Scientists routinely infer the ages of stars by looking at their colors and brightnesses. Within NGC 1783, for example, Li, de Grijs and colleagues 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? “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,” Geller said. “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 team proposes globular clusters can sweep up stray gas and dust they encounter while moving about their respective host galaxies. The theory of newborn stars arising in clusters as they “adopt” interstellar gases actually dates back to a 1952 paper. More than a half-century later, this once speculative idea suddenly has key evidence to support it.
In the study, KIAA analyzed Hubble data of these star clusters, and then Geller and his NW colleague Faucher-Giguère carried out calculations that show this theoretical explanation is possible in the globular clusters this team studied. “We have now finally shown that this idea of clusters forming new stars with accreted gas might actually work,” de Grijs said, “and not just for the three clusters we observed for this study, but possibly for a whole slew of them.” Future studies will aim to extend the findings to other Magellanic Cloud as well as Milky Way globular clusters. http://www.northwestern.edu/newscenter/stories/2016/01/globular-clusters.html
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