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Program contact: John Graham

National Science Foundation

Australian telescope measures Beta Hydri’s solar-like oscillations

An international team of astronomers has precisely measured the oscillations of a sun-like star. The measurements provide clues to the star’s internal structure that will help scientists test models and theories of stellar evolution.

Beta Hydri is a nearby star in the constellation Hydrus, or Southern Water Snake. It is similar to the sun in mass and temperature, but is estimated to be about seven billion years old, compared to about 4.5 billion years for the sun.

“Beta Hydri gives us a good idea of what the sun will look like in a few billion years,” said researcher Tim Bedding of the University of Sydney. Bedding, Paul Butler of the Carnegie Institution of Washington and colleagues from the United States, Australia, Denmark and Switzerland studied the star with the 3.9 meter Anglo-Australian Telescope at Siding Spring, Australia. The research is supported by the National Science Foundation.

By learning about the structure of such stars, scientists can test theories about the creation, structure and eventual demise of stars like our sun. These theories are tested by comparing actual data with predictions generated by computer models. Today’s computers can simulate billions of years of aging of a star in minutes.

Bedding’s and Butler’s team precisely measured tiny, periodic variations, or oscillations, in the velocity of material moving on or near the surface of the star. Twelve hundred measurements were taken over five nights in June 2000. The results will be published in an upcoming issue of the Astrophysical Journal Letters.

Ever since the discovery in 1979 of tiny oscillations -with periods of as little as five minutes — on the surface of the sun, scientists have looked for the same minute motions in stars similar to the sun. Since Beta Hydri is older than the sun, the oscillation periods were expected to be longer, on the order of 15 to 20 minutes. The astronomers found, indeed, that the star’s oscillation period was 17 minutes.

Just as seismologists use sound waves to probe the interior of the earth, astronomers use similar techniques to probe the interior of a star and determine details such as temperature, rotation and composition. Because oscillation periods increase as stars get older, these techniques also allow scientists to estimate the ages of stars.

“Detecting these seismic waves on Beta Hydri is like feeling the pulse of the star,” said Butler. “Just as a person’s pulse reveals information about the heart, these oscillations allow us to peer deep into the center of the star to tell us about conditions there.”

The team used a spectroscopic technique developed by Butler and Geoff Marcy, of the University of California at Berkeley, to detect tiny velocity variations in astronomical objects. Butler, Marcy and other colleagues have used the technique to find new planets, including three recently detected with the Anglo Australian telescope.

Studies of another star, Procyon, have also revealed evidence of oscillations similar to those of the sun. However, Bedding’s and Butler’s team believes the measurements of Beta Hydri provide the clearest evidence to date.


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