Free Astronomy Magazine July-August 2015

SPACE CHRONICLES 47 Tucanae in ultraviolet, with white dwarfs marked. [NASA, ESA, and H. Richer and J. Heyl (University of British Columbia, Vancouver, Canada)] stars were among the most massive in the cluster, roughly as massive as the Sun,” explained co-author Eli- sa Antolini of the Università degli Studi di Perugia, Italy. “We knew that as they lost mass we would see a migration to the outskirts; that wasn’t a surprise. But, what did sur- prise us was that the youngest white dwarfs were only just embark- ing on their journey. This could be evidence that the stars shed much of their mass at a later stage in their lives than we once thought, which is an exciting find.” About 100 million years before stars evolve into white dwarfs, they swell up and become red giant stars. Many astronomers thought that stars lost most of their mass during this phase. However, if this were the case, the stars would already have been expelled from the centre of the cluster at the red giant stage. “Our observations with Hubble found white dwarfs that are just be- ginning their migration to wider or- bits,” explains team member Harvey Richer, also from the University of British Columbia, Canada. “This re- veals that the migration of the stars from the centre — and the loss in their mass that has caused it — be- gins later in the star’s life than once thought. These white dwarfs are losing a large amount of mass just before they become white dwarfs and not during the earlier red giant phase.” The new results imply that the stars actually lose 40 to 50 percent of their bulk just 10 million years be- fore completely burning out as white dwarfs. Studies into the mass segregation of white dwarfs will continue, and the 47 Tucanae cluster is an ideal place to do them due to its proximity to us and the significant number of stars at the cluster’s core that can be re- solved by Hubble’s crisp vision. n