Star cluster shows lithium shortfall is widespread
Taking a new look at an old object, astronomers have found a long standing mystery is more prevalent than they thought. The team, who used the VLT Survey Telescope in northern Chile to observe the globular star cluster Messier 54, report that this object has unexpectedly low levels of the element lithium. This deficit, which contradicts ideas about the formation of elements in the early universe, was previously only seen in stars in our own Galaxy, the Milky Way, but the new work suggests it could be much more widespread. The scientists publish their results in Monthly Notices of the Royal Astronomical Society.
The Milky Way is orbited by more than 150 globular star clusters, which are balls of hundreds of thousands of old stars dating back to the formation of the galaxy. One of these, along with several others in the constellation of Sagittarius (the Archer), was found in the late eighteenth century by the French comet hunter Charles Messier and given the designation Messier 54.
This unusual parentage has now allowed astronomers to use the Very Large Telescope (VLT) to test whether there are also unexpectedly low levels of the element lithium in stars outside the Milky Way.
Most of the light chemical element lithium now present in the Universe was produced during the Big Bang, along with hydrogen and helium, but in much smaller quantities. Astronomers can calculate quite accurately how much lithium they expect to find in the early Universe, and from this work out how much they should see in old stars. But the numbers don’t match — there is about three times less lithium in stars than expected. This mystery remains unsolved, despite several decades of work.
Until now it has only been possible to measure lithium in stars in the Milky Way. But now a team of astronomers led by Alessio Mucciarelli (University of Bologna, Italy) has used the VLT to measure how much lithium there is in a selection of stars in Messier 54. They find that the levels are close to those in the Milky Way. So, whatever it is that got rid of the lithium seems not to be specific to the Milky Way.
As well as showing the cluster itself, the new VST image reveals the extraordinarily dense forest of much closer Milky Way stars that lie in the foreground.
Lars Lindberg Christensen
Images and captions
Caption: This image from the VLT Survey Telescope at ESO’s Paranal Observatory in northern Chile shows the globular cluster Messier 54. This cluster looks very similar to many others, but it has a secret. Messier 54 doesn’t belong to the Milky Way, but actually is part of a small satellite galaxy, the Sagittarius Dwarf Galaxy. This unusual parentage has allowed astronomers to use the Very Large Telescope (VLT) to test whether unexpectedly low levels of the element lithium in stars are also found in stars outside the Milky Way. Credit: ESO
This research has been published in Mucciarelli A. et al., "The cosmological Lithium problem outside the Galaxy: the Sagittarius globular cluster M54", Monthly Notices of the Royal Astronomical Society, vol. 444, pp. 1812-1820, published by Oxford University Press. A preprint version is available on the arXiv.
Notes for editors
ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning the 39-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.
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