Hans Kjeldsen
University of Aarhus, Denmark
Phone: +45 8942 3779
Tim Bedding
University of Sydney, Australia
Phone: +61-2-9351-2680
Chris Tinney
Anglo-Australian Observatory
Phone: +61-2-9372-4849
For Immediate Release: 21 December 2005
ESO Press Release 33/05
Allo, Allo? A Star is Ringing
VLT Helps Measuring Tortoise-like Motion
ESO PR Photo 40a/05 ESO PR Photo 40b/05
Alpha Centauri and the The Sky Above Paranal
Southern Cross
Captions: ESO PR Photo 40a/05 shows, in a wide-field image
obtained with an Hasselblad 2000 FC camera by Claus Madsen
(ESO), a region around the Southern Cross, seen in the
right of the image (Kodak Ektachrome 200, 70 min exposure
time). Alpha Centauri is the bright yellowish star seen at
the middle left, one of the "Pointers" to the star at the
top of the Southern Cross.
ESO PR Photo 40b/05 shows the sky above Paranal. Some of the
docking stations for the Auxiliary Telescopes of the Very
Large Telescope Interferometer are clearly visible in the
foreground. Alpha Centauri is the star almost at the centre
of the image, the lowest of the two "Pointers" to the
Southern Cross visible just above the "Coal Sack", in the
middle of the image. The Milky Way is clearly visible in all
its majesty, as well as on the right, the Magellanic Clouds.
A "shooting star" was also captured in this image (top
right) obtained on 11 December 2005 by Hännes Heyer (ESO).
The photo is facing the East. (Canon EOS 5D, 40 and 20 sec
exposures).
Astronomers have used ESO's Very Large Telescope in Chile and
the Anglo-Australian Telescope in eastern Australia as a
'stellar stethoscope' to listen to the internal rumblings
of a nearby star. The data collected with the VLT have a
precision better than 1.5 cm/s, or less than 0.06 km per
hour!
By observing the star with two telescopes at the same time,
the astronomers have made the most precise and detailed
measurements to date of pulsations in a star similar to our
Sun. They measured the rate at which the star's surface is
pulsing in and out, giving clues to the density, temperature,
chemical composition and rotation of its inner layers --
information that could not be obtained in any other way.
The astronomers from Denmark, Australia, and the USA [1] used
Kueyen, one of the four 8.2-m Unit Telescopes of ESO's Very
Large Telescope (VLT) at Cerro Paranal in Chile, and the
3.9-m Anglo-Australian Telescope (AAT) in New South Wales
(Australia), to study the star Alpha Centauri B, one of our
closest neighbours in space, about 4.3 light-years away.
Alpha Centauri is the brighter of the two 'Pointers' to the
Southern Cross. Alpha Centauri itself is a triple system and
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 very tiny amounts -- only a dozen metres or so
every four minutes [2]. Astronomers can detect these changes
by measuring the small, associated wavelength shifts.
The researchers sampled the light from Alpha Centauri B for
seven nights in a row, making more than 5 000 observations
in all. At the VLT, 3379 spectra were obtained with typical
exposure times of 4 seconds and a median cadence of one
exposure every 32 seconds! At the AAT 1642 spectra were
collected, with typical exposures of 10 s, taken every 90 s.
"From this unique dataset, we were able to determine as many
as 37 different patterns (or modes) of oscillation", says
Hans Kjeldsen, from University of Aarhus (Denmark) and lead
author of the paper describing the results [3].
The astronomers 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.
Indeed, the star' surface moves slowly, at the tortoise-like
speed of 9 cm a second, or about 300 metre an hour. The
astronomers borrowed their high-precision measurement
technique from the planet-hunters, who also look for slight
Doppler shifts in starlight.
"So much of what we think we know about the universe rests
on the ages and properties of stars," said Tim Bedding,
from the University of Sydney and co-author of the study.
"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 the 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."
Notes
[1]: The team is composed of Hans Kjeldsen and Jørgen
Christensen-Dalsgaard (University of Aarhus, Denmark),
Timothy R. Bedding and Laszlo L. Kiss (University of Sydney,
Australia), Christopher G. Tinney (Anglo-Australian
Observatory), R. Paul Butler and Chris McCarthy (Carnegie
Institution of Washington, USA), Geoffrey W. Marcy and Jason
T. Wright (University of California Berkeley, USA).
[2]: By comparison, the star is more than a million kilometres
wide.
[3]: The research is described in a paper published on December
20 by the Astrophysical Journal ("Solar-like oscillations in
alpha Centauri B.", by H. Kjeldsen et al.). It is available
for download at
http://arxiv.org/abs/astro-ph/0508609
Belgium: Dr. Rodrigo Alvarez, +32-2-474 70 50
Finland: Ms. Terhi Loukiainen, +358 9 7748 8385
Denmark: Dr. Michael Linden-Vørnle, +45-33-18 19 97
France: Dr. Daniel Kunth, +33-1-44 32 80 85
Germany: Dr. Jakob Staude, +49-6221-528229
Italy: Prof. Massimo Capaccioli, +39-081-55 75 511
The Netherlands: Ms. Marieke Baan, +31-20-525 74 80
Portugal: Prof. Teresa Lago, +351-22-089 833
Sweden: Dr. Jesper Sollerman, +46-8-55 37 85 54
Switzerland: Dr. Martin Steinacher, +41-31-324 23 82
United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25
--------------------------------------------------------------
ESO Press Information is available on the WWW at
http://www.eso.org/outreach/press-rel/
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(c) ESO Education & Public Relations Department
Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
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