Star near Southern Cross is ringing like a bell
ANGLO-AUSTRALIAN OBSERVATORY NEWS RELEASE
Posted: January 1, 2006
Astronomers have used the Anglo-Australian Telescope in Australia and European Southern Observatory's Very Large Telescope in Chile as a 'stellar stethoscope' to 'hear' -- more clearly than ever before -- a star that is ringing like a bell.
"The trick was to use two telescopes at essentially the same time."
The researchers, led by Bedding and Dr Hans Kjeldsen (Aarhus University, Denmark), studied the star alpha Centauri B, one of the stars of the 'Pointers' near the constellation of the Southern Cross.
Over a week they observed the star with both the 3.9-m Anglo-Australian Telescope near Coonabarabran in New South Wales, Australia, and Kueyen, one of the four 8.2-m telescopes that make up the European Southern Observatory's Very Large Telescope at Cerro Paranal in Chile.
The team measured the rate at which the star's surface is heaving in and out, getting clues about the density, temperature, chemical composition and rotation of its inner layers -- information that could not be obtained in any other way.
The research was published in the Astrophysical Journal.
Alpha Centauri is the brighter of the two 'Pointers' near the constellation of the Southern Cross. It's not one star but three: they are the closest stars to Earth, about 4.3 light-years away. 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 tiny amounts -- just a dozen meters or so every four minutes.
Because the surface is moving, the light coming from it is very slightly altered in wavelength, by the effect called the Doppler shift. Astronomers can detect such a change and use it to measure the surface's movement.
The researchers sampled the light from alpha Centauri B once a minute for seven nights in a row, making more than 5,000 observations in all.
A star's surface can oscillate in many different patterns, or modes, simultaneously. The researchers were able to determine 37 modes of oscillation in alpha Centauri B. They 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. These kinds of oscillations have been studied in only about a dozen stars other than the Sun.
The stars' surfaces move slowly: in the case of alpha Centauri B, at the tortoise-like speed of 9 cm [3.5 inches] a second, or about 300 meters [300 yards] an hour.
And then there's the sheer distance to the stars. Although alpha Centauri B is one of the closest stars to us, it's still more than 280,000 times further away from Earth than the Sun is. If the Sun were shrunk to the size of a pea, on the same scale alpha Centauri B would be another pea 160 km [100 miles] away.
The astronomers borrowed their high-precision measurement technique from the planet-hunters, who also look for slight Doppler shifts in starlight. Renowned planet hunters Paul Butler and Geoff Marcy were members of the team studying alpha Centauri B.
"So much of what we think we know about the Universe rests on the ages and properties of stars," said Bedding. "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 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 analyzed the star's light."