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Astronomers take the pulse of a Sun-like star

Using the 3.9-m Anglo-Australian Telescope near Coonabarabran, NSW, an
international team of astronomers led by Dr Tim Bedding of the
University of Sydney has precisely measured the 'throbbing' of a
Sun-like star that lies 24 light-years away. The slow 'pulse rate' of
the star, beta Hydri in the southern constellation Hydrus, confirms
astronomers' ideas of what the Sun will be like a few billion years
from now.

Churning gas in the star's outer layers makes sound waves. Like seismic
waves from an earthquake, these make the star's surface pulse in and
out in different places.

Just as geologists learn about the Earth's interior from the way
seismic waves travel, astronomers will use 'asteroseismology' to learn
about the interiors of stars. This will help them check ideas about how
stars evolve and how old they are.

Seismic oscillations were first noticed on the Sun in 1979.

Today the Sun's quivers and shrugs are monitored in detail. "We've
learned from this how the Sun's interior moves around, how deep some of
its layers are, and its chemical composition," says Tim Bedding.

"We'll be able to learn the same things about other stars."

But it's much harder to detect the pulsing of a star such as beta
Hydri, which is 1.5 million times further away than the Sun.

To crack the problem the researchers applied a high-precision technique
that is used to hunt for planets around other stars.

Light comes from a star's surface. As the surface pulses it causes
small changes (Doppler shifts) in the spectrum of the light, which show
how fast the surface is moving. The researchers sampled the light from
beta Hydri every two minutes for five nights in a row, making 1200
observations in all.

"The surface of beta Hydri pulses in and out at a velocity of only half
a metre per second," says team member Dr Paul Butler of the Carnegie
Institution of Washington, who developed the technique. "We can measure
such tiny velocities because of the high precision of our technique -
the best in the world."

Pulsations in the Sun have a period of about five minutes. "As a star
gets older, its 'voice' deepens - the period of its oscillations gets
longer," says Tim Bedding. Beta Hydri has about the same mass and
temperature as the Sun, but is older: about 7 billion years rather than
the Sun's 4.5 billion years. Because of this, astronomers predicted
that beta Hydri's oscillations would take 15 to 20 minutes. In a
triumph for theory, the research team found the oscillation period was
17 minutes.

"We will use the technique to check basic facts about stars," says Tim
Bedding. "So much of what we think we know about the universe rests on
the ages and properties of stars."

"We could find that our current ideas are wrong. For instance, there
are theories about of how convection processes mix gas in the core of a
star. This is supposed to allow the star to 'burn' hydrogen more
efficiently, allowing the star to live longer than it otherwise would,"
he explains.

"The theories of convection are pretty crude at present. We hope this
technique for measuring oscillations can be used to test them. That
sort of finding could change how old we think stars are."

Evidence from the pulsations of the Sun has been extremely important in
constraining theories of how the Sun 'works'. "We still have problems
with the number of neutrinos the Sun produces," says Tim Bedding.
"There aren't enough of them. We'd like to tweak our theories to make
the problem go away. But we can't because that would contradict the
evidence we get from the pulsations of the Sun about what's going on
inside. So the explanation of the neutrino deficit must lie elsewhere,
in theoretical nuclear physics."

The team plans to look next at the star alpha Centauri A, the brighter
of the two 'Pointers' to the Southern Cross. A member of a binary star
system, it is very similar to the Sun, almost its twin. At 4.3
light-years away, it is the second closest star to us. (The closest is
its faint companion, Proxima Centauri.)

Observations of stellar pulsations will take a giant step forward in
2004 with the launch of an Australian-built telescope called MONS
(Measuring Oscillations in Nearby Stars) aboard a Danish satellite.
MONS will observe stars for about one month each over the course of its
two-year mission.

The research on beta Hydri has been accepted for publication as a
Letter in the Astrophysical Journal.

The members of the research team are Tim Bedding (University of Sydney,
Australia), Paul Butler (Carnegie Institution of Washington), Hans
Kjeldsen (University of Aarhus), Ivan Baldry (Anglo-Australian
Observatory), Simon O'Toole (University of Sydney), Chris Tinney
(Anglo-Australian Observatory), Geoffrey Marcy (UC Berkeley), Francesco
Kienzle and Fabien Carrier (both Geneva Observatory). The research was
supported financially by the Australian Research Council (TRB and
SJOT); National Science Foundation grant AST-9988087 (RPB); SUN
Microsystems, and 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 (HK); and
the Swiss National Science Fund (FK and FC).


Dr Tim Bedding, University of Sydney
+61-2-9351-2680 (w)
+61-2-9568-3834 (h)
+61-410-310-475 (mob)

Dr Paul Butler, Carnegie Institution of Washington
+1-202-478-8866 (w)


Dr Chris Tinney, Anglo-Australian Observatory
+61-2-9372-4849 (w)
+61-416-092-117 (mob)



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