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Astronomers take the pulse of a Sun-like star
Posted: January 31, 2001

A "cut-away" illustration of pulsating star. Photo: University of Sydney
Using the 3.9-m (150-inch) Anglo-Australian Telescope near Coonabarabran, Australia, a team of astronomers from Australia, the U.S., Denmark and Switzerland 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 team leader Dr.Tim Bedding of the University of Sydney, Australia.

"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, which lies near the Southern Cross constellation. 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).