Astronomers using the Hubble Space Telescope (HST) have managed to track the motions of over 30,000 stars in a globular cluster orbiting the Milky Way galaxy. Their observations have revealed that there are two distinct populations of stars in the cluster: Those created when the cluster first formed, and a second group that were born a hundred million years later.
The team of astronomers, led by Professor Harvey Richter of the University of British Columbia, combined 8 years worth of Hubble observations of 47 Tucanae to identify and track the individual stars. They then compared each star’s speed and direction with its temperature and brightness. Previous spectroscopic studies had already shown that the stars were split into two distinct age categories, but these new observations revealed that the older population had randomly distributed circular orbits while the younger stars move more rapidly, have elliptical orbits, and are concentrated towards the centre of the cluster.
47 Tuc formed some 10.5 billion years ago when a vast cloud of molecular Hydrogen gas orbiting the Milky Way began contracting. The gas would have begun rotating, thanks to the principal of conservation of angular momentum, and the stars that formed from this contracting gas cloud reflect that initial movement with their cirular orbits. But the largest of those new stars would have ended their lifecycles after only a few tens or hundreds of millions of years, and would have expelled large amounts of gas enriched with the heavier elements forged in their cores. This triggered a second wave of new stars near the centre of the cluster, far richer in heavy metallic elements.
The data was collected using the HST’s Advanced Camera for Survey’s in 2010, and combined those observations with more than 750 archived images of the same cluster to find the proper motions of the individual stars. They used different colouted filters to pick out the differently aged stars (main-sequence stars of a given mass tend to become redder and dimmer with age). Spectroscopic studies of stars from the different populations showed that the redder stars had very low amounts of heavier elements, showing that they formed very early in the clusters age. The newer stars, by contrast, were relatively rich in the heavier elements which would have been created by the largest and shortest lived of the old-population stars.
Understanding exactly how these different populations formed in such clearly defined periods is a bit of a puzzle. Cosmologists hope that the answer, when it’s found, will help them understand the formation of the most distant galaxies, formed shortly after the big bang. Similar distinct populations have been found in other globular clusters, although without the additional data on stellar proper motions.
47 Tuc is visible to the naked eye for observers in the Southern Hemisphere, and appears as a small, dense glowing ball in binoculars. It is found in the constellation Tucana and is a favourite target of amateur astronomers. It is a dense clusters of tens of thousands of stars arranged in a spherical cloud about 120 light years across, and lies about 17000 light years away. It is the second brightest of about 200 globular clusters orbiting the Milky Way galaxy.
The original research was published in the July 1 issue of The Astrophysical Journal Letters, and can be read here.