Cutting edge

July 27, 2001

An international collaboration of astronomers is shining new light on the interior of stars.

We see only the light that escapes from their outermost layers. We have learnt quite a lot about the birth, life and death of stars of various masses by studying their light, but only in the past few decades has stellar astronomy begun to probe the insides of the stars.

This requires techniques that are similar to those that geologists have refined for probing the centre of the Earth. Seismologists deduce the detailed structure of the Earth's interior by how it influences the propagation of seismic waves - waves that are produced by earthquakes and other sources.

In some stars, similar waves induce pulsations - periodic variations in their brightness. Pulsating stars have periods of variations that are determined by their size and their internal properties. The pulsations that we see have periods that range from a few minutes to a few hours. This is remarkable - astronomers in general study objects that are nearly eternal, showing little or no change over many generations. Yet these stars vary their brightness by a percent or so on very short times scales. With the proper instrumentation, one can see the stars changing their brightness during the time it takes to brew a pot of tea.

With sensitive detectors, astronomers can measure minute variations in brightness that signal these internal waves in many different types of stars. These detectors reveal dozens and sometimes hundreds of individual periodicities. Each of the pulsation periods within a star tells us something about the star's inside. The more pulsation periods a star shows, the more information we obtain. But the signature of these many periodicities can be complex - sometimes looking like random fluctuations - as each individual periodicity alternately reinforces or cancels out others.

Eventually, the pattern of variation repeats, allowing astronomers to completely characterise the periods in the star. A complex pulsating star may take weeks to reveal the many periods present. To fully decode the pulsations of a star, we must "listen" to it as continuously as possible for as long as possible.

Achieving this unbroken data stream presents a significant challenge to earth-bound astronomers - stars rise and set daily, introducing blindness on a 24-hour period. Clouds and other forces beyond our control prevent continuous observations, even at night. To surmount these problems, an international band of 50 astronomers belong to a group we call the Whole Earth Telescope (WET).

WET astronomers observe stars in a round-the-world relay. An observer in Texas observes until the target approaches the horizon. By then, the star has risen for an observer farther west in Hawaii. The relay continues with observers in China, Australia, India, Israel, South Africa, the Canaries, Brazil and Chile. Data acquisition and analysis during a WET run is coordinated at a central site - usually at Iowa State University. When all goes well, the sun never rises on the WET.

Since the WET originated in 1986, we have had more than 20 successful observing campaigns. These observations have probed the internal structure of a variety of kinds of stars, concentrating on white dwarf stars. White dwarfs represent the end stage of evolution of stars like our own Sun and give us a preview of 5,000 million years into the future. We have learnt how the products of nuclear fusion have stratified in these stars and how their final transition from being enormous red giants to tiny white dwarfs may be a far more torturous path than expected.

Steven Kawaler is director of the WET and professor of physics and astronomy at Iowa State University.

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