Star clusters see multiple episodes of star birth

Two distinct populations of stars in a distant cluster formed at different times, Canadian astronomers say, yielding clues to the Milky Way galaxy's early days. Researchers led by Harvey Richer of the University of British Columbia in Vancouver used data from NASA's Hubble telescope to determine the motions of stars in the globular cluster 47 Tucanae, about 16,700 light-years from Earth in the southern constellation Tucana. An analysis enabled researchers to link the movement of stars within the cluster with their ages, allowing them to confirm two populations of stars in 47 Tucanae that differ in age by around 100 million years. "The data offer detailed evidence to help us understand how various stellar populations formed in such clusters," Richer said. The findings suggest multiple episodes of star birth within star clusters, the researchers said. The first population in 47 Tucanae consists of redder stars, reflecting the initial composition of the gas that formed the cluster, they said, while the second population consists of bluer stars, which are younger and more chemically enhanced. After the most massive of the red stars completed their stellar evolution, they expelled gas enriched with heavier elements back into the cluster, the researchers said, which collided with other gas and formed a second, more chemically enriched generation of stars, concentrated toward the cluster center. The astronomers have published their work in The Astrophysical Journal Letters.