Researchers have discovered that the mysterious overweight stars known as blue stragglers are the result of 'stellar cannibalism' where plasma is gradually pulled from one star to another to form a massive, unusually hot star that appears younger than it is. The process takes place in binary stars - star systems consisting of two stars orbiting around their common centre of mass. This helps to resolve a long standing mystery in stellar evolution.

The research, which is part funded by the UK's Science and Technology Facilities Council (STFC) and carried out by scientists at Southampton University and the McMaster University in Canada, is published in the journal Nature on Thursday 15 January.

Blue stragglers are found throughout the Universe in globular clusters - collections of about 100, 000 stars, tightly bound by gravity. According to conventional theories, the massive blue stragglers found in these clusters should have died long ago because all stars in a cluster are born at the same time and should therefore be at a similar phase. These massive rogue stars, however, appear to be much younger than the other stars and are found in virtually every observed cluster.

Dr Christian Knigge from Southampton University, who led the study, comments: "The origin of blue stragglers has been a long-standing mystery. The only thing that was clear is that at least two stars must be involved in the creation of every single blue straggler, because isolated stars this massive simply should not exist in these clusters."

Professor Alison Sills from the McMaster University explains further: "We've known of these stellar anomalies for 55 years now. Over time two main theories have emerged: that blue stragglers were created through collisions with other stars; or that one star in a binary system was 'reborn' by pulling matter off its companion."

The researchers looked at blue stragglers in 56 globular clusters. They found that the total number of blue stragglers in a given cluster did not correlate with predicted collision rate - dispelling the theory that blue stragglers are created through collisions with other stars.

They did, however, discover a connection between the total mass contained in the core of the globular cluster and the number of blue stragglers observed within in. Since more massive cores also contain more binary stars, they were able to infer a relationship between blue stragglers and binaries in globular clusters. They also showed that this conclusion is supported by preliminary observations that directly measured the abundance of binary stars in cluster cores. All of this points to "stellar cannibalism" as the primary mechanism for blue straggler formation.

Dr Knigge says: "This is the strongest and most direct evidence to date that most blue stragglers, even those found in the cluster cores, are the offspring of two binary stars. In our future work we will want to determine whether the binary parents of blue stragglers evolve mostly in isolation, or whether dynamical encounters with other stars in the clusters are required somewhere along the line in order to explain our results."

This discovery comes as the world celebrates the International Year of Astronomy in 2009.

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Notes for editors:

The research uses data taken with Hubble Space Telescope

'A binary origin for 'blue stragglers' in globular clusters' is scheduled for publication in Nature on 15 January.

Images

NASA picture of globular cluster http://nix. ksc. nasa. gov/ info;jsessionid= 231cpmuf6ne9b?id= GL-2002-001076&orgid= 6

NASA picture of blues stragglers in a globular cluster http://nix. ksc. nasa. gov/ info;jsessionid= 231cpmuf6ne9b?id= GL-2002-001163&orgid= 6

Contacts

Julia Short

STFC Press Office

Tel: +44 (0)1793 442 012

Email: julia.short@stfc.ac.uk

Glenn Harris

Public Relations and Media Officer

University of Southampton

Tel: +44 (0)23 8059 3212

Email: G.Harris@soton.ac.uk

Christian Knigge

School of Physics & Astronomy

University of Southampton

Tel: +44-(0)23-8059-3955

Email: christian@astro.soton.ac.uk

Science and Technology Facilities Council

The Science and Technology Facilities Council ensures the UK retains its leading place on the world stage by delivering world-class science; accessing and hosting international facilities; developing innovative technologies; and increasing the socio-economic impact of its research through effective knowledge exchange.

The Council has a broad science portfolio including Astronomy, Particle Physics, Particle Astrophysics, Nuclear Physics, Space Science, Synchrotron Radiation, Neutron Sources and High Power Lasers. In addition the Council manages and operates three internationally renowned laboratories:

The Rutherford Appleton Laboratory, Oxfordshire

The Daresbury Laboratory, Cheshire

The UK Astronomy Technology Centre, Edinburgh

The Council gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Laboratory for Particle Physics (CERN), the Institute Laue Langevin (ILL), European Synchrotron Radiation Facility (ESRF), the European organisation for Astronomical Research in the Southern Hemisphere (ESO) and the European Space Agency (ESA). It also funds UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, and the MERLIN/VLBI National Facility, which includes the Lovell Telescope at Jodrell Bank Observatory. The Council distributes public money from the Government to support scientific research. Between 2008 and 2009 we will invest approximately £787 million.

The University of Southampton is a leading UK teaching and research institution with a global reputation for leading-edge research and scholarship across a wide range of subjects in engineering, science, social sciences, health and humanities.

With over 22,000 students, around 5000 staff, and an annual turnover of more than £370 million, the University of Southampton is acknowledged as one of the country's top institutions for engineering, computer science and medicine. We combine academic excellence with an innovative and entrepreneurial approach to research, supporting a culture that engages and challenges students and staff in their pursuit of learning.

The University is also home to a number of world-leading research centres, including the National Oceanography Centre, Southampton, the Institute of Sound and Vibration Research, the Optoelectronics Research Centre, the Web Science Research Initiative, the Centre for the Developmental Origins of Health and Disease, the Mountbatten Centre for International Studies and the Southampton Statistical Sciences Research Institute.