The globular star cluster NGC 6752 in the constellation of Pavo
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New VLT observations create major headache for stellar theories
Astronomers expect that stars like the Sun will blow off much of their atmospheres into space near the ends of their lives. But new observations of a huge star cluster made using ESO’s Very Large Telescope have shown — against all expectations — that a majority of the stars studied simply did not get to this stage in their lives at all. The international team found that the amount of sodium in the stars was a very strong predictor of how they ended their lives.
The way in which stars evolve and end their lives was for
many years considered to be well understood. Detailed computer models
predicted that stars of a similar mass to the Sun would have a period
towards the ends of their lives — called the asymptotic giant branch,
or AGB [1] — when they undergo a final burst of nuclear burning and puff off a lot of their mass in the form of gas and dust.
This expelled material [2]
goes on to form the next generations of stars and this cycle of mass
loss and rebirth is vital to explain the evolving chemistry of the
Universe. This process is also what provides the material required for
the formation of planets — and indeed even the ingredients for organic
life.
But when Australian stellar theory expert Simon Campbell
of the Monash University Centre for Astrophysics, Melbourne, scoured
old papers he found tantalising suggestions that some stars may somehow
not follow the rules and might skip the AGB phase entirely. He takes
up the story:
“For a stellar modelling scientist this suggestion was
crazy! All stars go through the AGB phase according to our models. I
double-checked all the old studies but found that this had not been
properly investigated. I decided to investigate myself, despite having
little observational experience.”
Campbell and his team used ESO’s Very Large Telescope
(VLT) to very carefully study the light coming from stars in the
globular star cluster NGC 6752 in the southern constellation of Pavo
(The Peacock). This vast ball of ancient stars contains both a first
generation of stars and a second that formed somewhat later [3].
The two generations can be distinguished by the amount of sodium they
contain — something that the very high-quality VLT data can be used to
measure.
“FLAMES, the multi-object high-resolution spectrograph
on the VLT, was the only instrument that could allow us to get really
high-quality data for 130 stars at a time. And it allowed us to observe
a large part of the globular cluster in one go,” adds Campbell.
The results were a surprise — all of the AGB stars in the
study were first generation stars with low levels of sodium and none of
the higher-sodium second generation stars had become AGB stars at all.
As many as 70% of the stars were not undergoing the final nuclear
burning and mass-loss phase [4] [5].
“It seems stars need to have a low-sodium “diet” to reach the
AGB phase in their old age. This observation is important for several
reasons. These stars are the brightest stars in globular clusters — so
there will be 70% fewer of the brightest stars than theory predicts. It
also means our computer models of stars are incomplete and must be
fixed!” concludes Campbell.
The team expects that similar results will be found for other star clusters and further observations are planned.
Notes
[1] AGB stars get their odd
name because of their position on the Hertzsprung Russell diagram, a
plot of the brightnesses of stars against their colours.
[2] For a short period of time this
ejected material is lit up by the strong ultraviolet radiation from the
star and creates a planetary nebula (see for instance eso1317).
[3] Although the stars in a globular
cluster all formed at about the same time, it is now well established
that these systems are not as simple as they once thought to be. They
usually contain two or more populations of stars with different amounts
of light chemical elements such as carbon, nitrogen and — crucially
for this new study — sodium.
[4] It is thought that stars which skip the AGB
phase will evolve directly into helium white dwarf stars and gradually
cool down over many billions of years.
[5] It is not thought that the sodium itself is the
cause of the different behaviour, but must be strongly linked to the
underlying cause — which remains mysterious.
More information
This research was presented in a
paper entitled “Sodium content as a predictor of the advanced evolution
of globular cluster stars” by Simon Campbell et al., to appear online
in the journal Nature on 29 May 2013.
The team is composed of Simon W. Campbell (Monash
University, Melbourne, Australia), Valentina D’Orazi (Macquarie
University, Sydney, Australia; Monash University), David Yong
(Australian National University, Canberra, Australia [ANU]), Thomas N.
Constantino (Monash University), John C. Lattanzio (Monash University),
Richard J. Stancliffe (ANU; Universität Bonn, Germany), George C.
Angelou (Monash University), Elizabeth C. Wylie-de Boer (ANU), Frank
Grundahl (Aarhus University, Denmark).
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
Simon CampbellMonash University
Melbourne, Australia
Tel: +61 3 9905 4454
Email: simon.campbell@monash.edu
John Lattanzio
Monash University
Melbourne, Australia
Tel: +61 3 9905 4428
Email: john.lattanzio@monash.edu
Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
