VLT Survey Telescope image of the globular cluster Messier 54, part of the Sagittarius dwarf galaxy.
An international team of astronomers using the European Southern Observatory’s (ESO) Very Large Telescope (VLT) at Paranal Observatory in northern Chile have observed the globular star cluster Messier 54 to help solve one of astronomy’s biggest mysteries—the lithium problem. In a paper published by ESO (ESO 1428, 2014), researcher and first author Alessio Mucciarelli at the University of Bologna in Italy observed the lithium abundance in M54, the most distant measurement of this type to date.
The lithium problem arises as a discrepancy between lithium abundance measured in stars and the abundance predicted by standard Big Bang nucleosynthesis calculations. Lithium, a light chemical element believed to have been synthesized during the Big Bang along with all the hydrogen and helium in the universe, is about three times less abundant than expected in even the oldest stars. These old, metal-poor, dwarf stars for which we can measure lithium are typically found in globular clusters that orbit the outer part of our Milky Way galaxy.
In 1994, astronomers discovered that M54 is further away than expected; instead of orbiting the Milky Way, it orbits a neighboring satellite galaxy, the Sagittarius dwarf galaxy. So Mucciarelli and colleagues used the VLT Survey Telescope, the largest telescope in use for visible surveys of the sky, to obtain high-resolution spectra of M54 and determine that the levels of lithium in these very far away stars are the same as those nearer the Milky Way.
The results indicate that the lithium discrepancy may be a universal problem of both the Milky Way and extra-galactic systems. Either the estimates of lithium produced in the early universe are inflated, or a combination of atomic diffusion plus additional mixing during a star’s main sequence may be causing the discrepancy.