Link Between Stars' Ages and Orbits Found

Astronomers have gained a rare look into the Milky Way's early days through the help of the Hubble Space Telescope, determining the orbital motion of two distinct populations of stars in an ancient globular star cluster. In doing so, NASA reports, the scientists have been able to provide proof they formed at different times.

Led by Harvey Richer of the University of British Columbia in Vancouver, the scientists combined recent Hubble observations with data from the telescope's archive collected over the course of eight years' to determine the motions of the stars in the globular cluster 47 Tucanae, which is one of the galaxy's brightest at 10.5 billion years old.

This analysis enabled scientists to link the movement of stars in the clusters with the stars' ages for the very first time, concluding that the two populations in 47 Tucanae differ in age by less than 100 million years.

"When analyzing the motions of stars, the longer the time baseline for observations, the more accurately we can measure their motion," Richer said. "These data are so good, we can actually see the individual motions of the stars in the cluster.

Furthermore, the astronomer explained, the data helped the scientists better understand "how various stellar populations formed in such clusters."

Because the Milky Way's globular clusters are the surviving relics from our galaxy's formation, they offer a wide range of insights into the early history of our galaxy. Previously, spectroscopic studies showed many globular clusters contain stars of varying chemical compositions, suggesting multiple episodes of star birth -- an observation the researchers' analysis supports, adding the stars' orbital motion to the mix.

Richer and his team determined how fast the stars moved by combining observations made using Hubble's Advanced Camera for Surveys back in 2010. They then combined those observations with 754 archival images to measure the change in position of more than 30,000 stars. Using this information the team was also bale to measure the stars' brightness and temperatures.

This stellar archaeology identified the two distinct populations of stars, one of which consists of older, less chemically enriched stars orbiting in random circles. The other contains the exact opposite with younger, more chemically enhanced stars moving in neat elliptical orbits.

This is because after the most massive of the stars in the first group completed their stellar evolution, they expelled gas enriched with heavier elements back into the cluster. This gas then slammed into other gas and formed a second, more chemically enriched generation of stars concentrated toward the cluster center. Over time, these stars second stars moved slowly outward into more elliptical orbits.