Epping, NSW, Australia
FOR MORE INFORMATION
Dr Tim Bedding, University of Sydney
+61-2-9351-2680 (w), +61-410-310-475 (mob)
Dr Paul Butler, Carnegie Institution of Washington
30 Jan 2001
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,
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).
FOR ADDITIONAL COMMENT
Dr Chris Tinney, Anglo-Australian Observatory
+61-2-9372-4849 (w), +61-416-092-117 (mob)
MOVIES SHOWING HOW STARS PULSATE
AUDIO RECORDINGS OF PULSATING STARS