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22 December 2005

For immediate use


STAR NEAR THE SOUTHERN CROSS IS 'RINGING'

Astronomers have used telescopes in Australia and Chile as a 'stellar stethoscope' to 'listen' to a star near the Southern Cross that is ringing like a bell.

The researchers, led by Dr Tim Bedding (University of Sydney) and Dr Hans Kjeldsen (Aarhus University, Denmark), studied the star alpha Centauri B, one of the stars of the 'Pointers' near the constellation of the Southern Cross.

Over a week they observed the star with both the 3.9-m Anglo-Australian Telescope near Coonabarabran in New South Wales, Australia, and Kueyen, one of the four 8.2-m telescopes that make up the European Southern Observatory's Very Large Telescope at Cerro Paranal in Chile.

The team measured the rate at which the star's surface is heaving in and out, getting clues about the density, temperature, chemical composition and rotation of its inner layers—information that could not be obtained in any other way.

The research was published this week (20 December) in the Astrophysical Journal.

"These are the most precise and detailed measurements to date of such vibrations in a Sun-like star," said Bedding.

"The trick was to use two telescopes at essentially the same time."

Alpha Centauri is the brighter of the two 'Pointers' near the constellation of the Southern Cross. It's not one star but three: they are the closest stars to Earth, about 4.3 light-years away. Alpha Centauri B is an orange star, a little cooler and a little less massive than the Sun.

Churning gas in the star's outer layers creates low-frequency sound waves that bounce around the inside of the star, causing it to ring like a bell. This makes the star's surface pulsate in and out by tiny amounts—just a dozen metres or so every four minutes.

Because the surface is moving, the light coming from it is very slightly altered in wavelength, by the effect called the Doppler shift. Astronomers can detect such a change and use it to measure the surface's movement.

The researchers sampled the light from alpha Centauri B once a minute for seven nights in a row, making more than 5,000 observations in all.

A star's surface can oscillate in many different patterns, or modes, simultaneously. The researchers were able to determine 37 modes of oscillation in alpha Centauri B. They also measured the mode lifetimes (how long the oscillations last), the frequencies of the modes, and their amplitudes (how far the surface of the star moves in and out).

Such measurements are a huge technical challenge. These kinds of oscillations have been studied in only about a dozen stars other than the Sun.

The stars' surfaces move slowly: in the case of alpha Centauri B, at the tortoise-like speed of 9 cm a second, or about 300 metres an hour.

And then there's the sheer distance to the stars. Although alpha Centauri B is one of the closest stars to us, it's still more than 280,000 times further away from Earth than the Sun is. If the Sun were shrunk to the size of a pea, on the same scale alpha Centauri B would be another pea 160 km away.

The astronomers borrowed their high-precision measurement technique from the planet-hunters, who also look for slight Doppler shifts in starlight. Renowned planet hunters Paul Butler and Geoff Marcy were members of the team studying alpha Centauri B.

"So much of what we think we know about the Universe rests on the ages and properties of stars," said Bedding. "But there is still a great deal we don't know about them."

By using two telescopes at different sites the astronomers were able to observe alpha Centauri B as continuously as possible.

"That's a huge advantage, because gaps in the data introduce ambiguity," said Bedding. "The success of the observations also depended on the very stable spectrographs attached to the two telescopes—UVES at the VLT and UCLES at the AAT—which analysed the star's light."


Team members

Denmark
Hans Kjeldsen and Joergen Christensen-Dalsgaard (both University of Aarhus)

Australia
Timothy R. Bedding and Laszlo L. Kiss (both University of Sydney)
Christopher G. Tinney (Anglo-Australian Observatory)

USA
R. Paul Butler and Chris McCarthy (Carnegie Institution of Washington)
Geoffrey W. Marcy and Jason T. Wright (both University of California Berkeley)


The research was supported financially by the Australian Research Council; the Danish Natural Science Research Council; and the Danish National Research Foundation, through its establishment of the Theoretical Astrophysics Center at the University of Aarhus. The authors further acknowledge support by NSF grant AST-9988087 (RPB), and by SUN Microsystems.

Researcher contacts

Dr Tim Bedding, University of Sydney                
+61-(0)425-310-476 (mob)
bedding@physics.usyd.edu.au
                    
Dr Chris Tinney, Anglo-Australian Observatory          
+61-(0)416-092-117 (mob)
cgt@aaoepp.aao.gov.au
                     
Dr Hans Kjeldsen, University of Aarhus, Denmark        
+45-8942-3779 (w)
hans@phys.au.dk

Background on 'ringing' stars

http://www.physics.usyd.edu.au/~bedding/alphacenb/ringing.pdf

Photos

The Southern Cross and the Pointers, with alpha Centauri highlighted (237 kB). Credit as: Copyright Akira Fujii / David Malin Images.
http://www.physics.usyd.edu.au/~bedding/alphacenb/af1-02_sRGB_arrow_0001.jpg

The Southern Cross and the Pointers (without alpha Centauri highlighted) (116 kB). Credit as: Copyright Akira Fujii / David Malin Images. 
http://www.physics.usyd.edu.au/~bedding/alphacenb/af1-02_sRGB.jpg

A high-resolution version of the same image (26cm x 21cm at 300 dpi) is available on request from Helen Sim, hsim@aaoepp.aao.gov.au or +61-(0)2-0419-635-905.

Pictures, movies and sounds of pulsating stars

http://www.physics.usyd.edu.au/~bedding/animations/
Credit information is given on the webpages.

Publication

Kjeldsen, H. et al. "Solar-like oscillations in alpha Centauri B". Astrophysical Journal 635, 2, p. 1281. Also at http://xxx.lanl.gov/abs/astro-ph/0508609.