Looking into the interior of the Earth or the Sun is a bit similar to examining a baby in its mother’s womb using an ultrasound scan. Light cannot penetrate the area, so we make pictures in these cases using sound waves, which human ears cannot hear. With SOHO, ESA has probed deeply into the Sun using the sound-waves principle, and with great success. The future missions, Solar Orbiter and Eddington, will look inside our Sun and other stars, respectively, in a similar way.

Here on Earth, when scientists recorded slight shakes, or seisms, coming from earthquakes even on distant continents, they began to estimate the routes and the changing speeds of the waves passing through the Earth’s interior. This revealed our planet’s molten core. Nowadays, oil prospectors routinely thump the ground to get seismic echoes from deep-lying strata. Scientists combine earthquake records from seismometers worldwide, to make 3D pictures of the rocks far beneath our feet.

Seismology is the study of earthquake waves. Studying solar sound waves is called helioseismology, from helios, a Greek word for Sun. When you transfer your focus onto the stars, as Eddington will do, you are studying asteroseismology. Although the Sun and stars are made of very hot gas rather than rocks, basic principles about deducing the routes and speeds of internal waves still work.

Sound waves do not travel through space, but scientists can register their activities by subtle changes in the light of the Sun and the stars. The reverberating waves make the visible surface of the Sun rise and fall roughly every few minutes, and the motions affect the wavelength and brightness of the light. The simplest helioseismic telescopes observe the whole Sun oscillating. They detect sharply defined ‘notes’ rather like a musical chord. From these, scientists can deduce the internal layers of the Sun with amazing precision, all the way down to its superheated core.

SOHO’s MDI instrument registers the waves at a million points across the visible surface. It has detected vast streams of hot gas flowing unseen beneath the surface. Most remarkably, MDI can look right through the Sun to observe stormy sunspot regions forming on the far side, which swing into view when the Sun rotates on its axis.

When the next generation of solar spacecraft looks more closely at selected parts of the Sun, helioseismologists are sure to make more sensational discoveries. For ESA’s Solar Orbiter, due for launch around 2012, key targets will be regions near the poles.

“Our special chance to detect sub-surface flows near the poles is one of the most exciting aspects of the Solar Orbiter mission,” says Bernhard Fleck, ESA’s Project Scientist for SOHO and study scientist for the Solar Orbiter. “We think these hidden streams of gas have a strong influence on the Sun’s magnetic behaviour, and so can affect its storminess.”

For astronomers, the Sun is a fairly typical middle-aged star seen in close-up. All the other stars are so far away that asteroseismologists can observe oscillations only of the whole star. However, as with the Sun, these can provide information never available before about the internal make-up of the stars. Much of what astronomers think they know about the Universe’s structure and evolution depends on their understanding of how stars work. Knowing a star’s age is an important part of this study.

Canadian, Danish, and French satellites will pioneer the field in the next few years, probing between them hundreds of stars. ESA’s Eddington spacecraft, due to fly in 2008, will therefore not be the first space mission to study asteroseismology. However, it will go much farther by examining as many as 50 000 stars, from the smallest to the largest, and from the oldest to the youngest, with an accuracy never seen before. For example, previously, if you wanted to state the age of a 100-million-year-old star, you would have to estimate in the range 80 to 120 million years. Eddington allows you to specify it to be almost exactly 104 million years old.

The sound waves in the Sun and the stars are pitched far too low for human beings to hear, but it is fairly simple to convert them to audible frequencies. To listen to the ‘Song of the Sun’, go the following site: http://soi.stanford.edu/results/sounds.html

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