Free Astronomy Magazine July-August 2015

SPACE CHRONICLES has just begun its journey from the dense cluster centre. Another pop- ulation is around 100 million years old and has already arrived at its new position, roughly 1.5 light-years from its starting point, and far from the cluster centre. “Before becom- ing white dwarfs, the migrating T his diagram shows how white dwarfs, the burned-out relics of stars, are distributed in the ancient globular star cluster 47 Tucanae. The youngest white dwarfs are the hottest and bluest stars dwelling mostly in the clus- ter's core, where the most mas- sive stars reside. Shortly before collapsing to become white dwarfs, the stars shed most of their mass. Now, as lightweight white dwarfs, they interact grav- itationally with more massive stars in the core. Through these gravitational encounters, the white dwarfs rob enough speed from their more massive cousins to begin migrating slowly out- ward from their home. During their journey, they become older and cooler white dwarfs, depic- ted in the orange dots. Even- tually, the white dwarfs settle on the outskirts of the cluster, where the oldest, coolest, red- dest white dwarfs reside. [NASA, ESA, and A. Feild (STScI)] White dwarfs are forced out of the densely packed centre of the cluster by gravitational interactions with more massive stars. Whilst astrono- mers knew about this process, they had never seen it in action, until now. Astronomers used Hubble to trace this stellar journey by studying 3000 white dwarfs in the globular star cluster 47 Tucanae, a dense swarm of hundreds of thousands of stars in the Milky Way. “We’ve seen the final picture be- fore: white dwarfs that have migrat- ed and settled into more distant orbits outside the core, determined by their mass,” explained Jeremy Heyl of the University of British Co- lumbia, Canada, first author on the science paper. “But in this study, which comprises about a quarter of all the young white dwarfs in the cluster, we’re actually catching the stars in the process of moving out- ward and distributing themselves appropriately according to mass.” Using the ultraviolet capabilities of Hubble’s sharp-eyed Wide Field Cam- era 3, the astronomers traced popu- lations of white dwarfs with a range of ages. (Although the white dwarfs have exhausted the hydrogen fuel that makes them shine as stars, their brilliant hot cores have been exposed which makes them very lu- minous in ultraviolet light. Only Hubble can detect these stars because ultra- violet light is blocked by Earth’s atmosphere and therefore does not reach ground-based telescopes.) Using the colours of the stars, the astronomers can also estimate the age of each star. One group of six-million-year-old stars T his video shows a pan across the globular cluster 47 Tucanae, taken by the NASA/ESA Hubble Space Telescope. [ESA/Hubble Team]