The globular star cluster Messier 54 in the constellation of Sagittarius
Wide-field view of the sky around the globular star cluster Messier 54
* * * * * * * * * * *
Videos
This new image from the VLT Survey
Telescope at ESO’s Paranal Observatory in northern Chile shows a vast
collection of stars, 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 is part of a small satellite galaxy, the
Sagittarius Dwarf Galaxy. 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.
The Milky Way galaxy 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.
For more than two hundred years after its discovery Messier 54 was
thought to be similar to the other Milky Way globulars. But in 1994 it
was discovered that it was actually associated with a separate galaxy —
the Sagittarius Dwarf Galaxy.
It was found to be at a distance of around 90 000 light-years — more
than three times as far from Earth as the galactic centre.
Astronomers have now observed Messier 54 using the VLT as a test case to try to solve one of the mysteries of modern astronomy — the lithium problem.
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, despite several decades of work [1].
Up to 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.
This new image of the cluster was created from data taken with the VLT Survey Telescope
(VST) at the Paranal Observatory. As well as showing the cluster itself
it reveals the extraordinarily dense forest of much closer Milky Way
stars that lie in the foreground.
Notes
[1] There are several possible proposed
solutions to the riddle. The first is that the calculations of the
amounts of lithium produced in the Big Bang are wrong — but very recent
tests suggest that this is not the case. The second is that the lithium
was somehow destroyed in the earliest stars, before the formation of the
Milky Way. The third is that some process in the stars has gradually
destroyed lithium during their lives.
More information
This research was presented in a
paper, “The cosmological Lithium problem outside the Galaxy: the
Sagittarius globular cluster M54”, by A. Mucciarelli et al., to appear
in Monthly Notices of the Royal Astronomical Society (Oxford University Press).
The team is composed of: A. Mucciarelli (University of
Bologna, Italy), M. Salaris (Liverpool John Moores University,
Liverpool, UK), P. Bonifacio (Observatoire de Paris, France), L. Monaco
(ESO, Santiago, Chile) and S. Villanova (Universidad de Concepcion,
Concepcion, Chile).
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”.
Links
Contacts
Alessio Mucciarelli
University of Bologna
Bologna, Italy
Tel: +39 051 20 95705
Email: alessio.mucciarelli2@unibo.it
Lars Lindberg Christensen
Head of ESO ePOD
Garching bei München, Germany
Tel: +49 89 3200 6761
Cell: +49 173 3872 621
Email: lars@eso.org
Source: ESO
