Professor Tim Bedding is part of a team at the University of Sydney that has started using this new technique, known as asteroseismology, to study the later stages of stellar evolution. "A star is a huge ball of gas held together by its own gravity," he explains, "and many stars undergo oscillations that involve periodical expansion and contraction. By measuring fthe periods of these oscillations we can infer conditions deep inside the star, such as temperature and chemical composition."
So how do the waves manifest themselves? "They're rather like the oscillations inside the tube of a flute or trumpet" continues Bedding, "except that a star is three-dimensional, which makes its oscillations more complicated."
When a wind instrument is played, the air column is limited to oscillating in just one dimension, rippling back and forth and the sound waves settle into stable patterns known as modes. "But while an instrument has a limited number of modes, a star like the Sun can oscillate in hundreds or even thousands of different modes simultaneously."
But there's a problem - while the Sun is on our cosmic doorstep and has been studied in detail by 'helioseismologists' since the 1970s, other stars are many millions of times more distant - mere points of light through even the most powerful telescopes. Directly observing ripples on these stars' surfaces is tar beyond current technology, but fortunately, as Professor Bedding points out the waves also produce another effect that we are able to measure: They affect the light output of the star, causing its brightness to vary in time by small amounts, in a very complicated way."
Thats the theory at least, but putting it into practice is another matter: "Measuring those brightness variations from the ground is very difficult due to the effects of the Earth's atmosphere. What we really need is a space telescope that can make regular and very accurate measurements of stellar brightness."
Fortunately in 2009, NASA launched a satellite that provides just this kind of data. The Kepler mission's main aim was to detect planets around distant stars by measuring the tiny regular dips in their brightness when a planet transits across the face of the star. But its highly accurate measurements are also perfect for asteroseismology. "It's created a revolution in the field over the past few years," enthuses Bedding.
By identifying the complex interacting wave modes influencing a star's brightness, and comparing them to computer models of stellar interiors, it's possible to work out the arrangement of layers with different densities inside the star. Perhaps the best example so far comes from the Sydney team's work on red giants - stars near the end of their lives that have swollen to enormous size as they start to run out of fuel.
Stars like the Sun shine due to nuclear fusion of hydrogen into helium in their cores. But after a few billion years, the core hydrogen is fully converted to helium, and the star starts burning the hydrogen in a shell around the core.
Professor Bedding takes up the story: "When this happens, the outer part of the star expands and cools, creating a red giant. But at the same time, the helium core contracts and gets hotter. Eventually it's hot enough for helium to start fusing into carbon and oxygen. So there are two types of red giants: those that have started burning helium in their cores, and those that have not. From the outside, it's very difficult to tell the difference, but now asteroseismology has given us a way."
A key difference between the two types of red giant, in theory, should lie in the size and density of their cores. In the first phase, the dormant helium slowly shrinks and grows denser, but once it ignites, the pressure of escaping radiation should cause the core to expand again, and grow less dense. The changing core size and density should produce a measurable difference in the seismic oscillations
affecting the two types of star.
"Indeed, it turns out that the red giants divide neatly into two groups based on observed properties known as mixed modes," continues Bedding, "and we can match these with the two groups of red giants. This result is a beautiful confirmation of the theories of stellar structure and evolution. Until now, almost everything we know about the lives of stars has come from theoretical calculations, but as in all science, it's vital to test theory by using observations."
However, while asteroseismology has helped confirm some long-standing ideas about red giants, that's not quite the end of the story. "More extensive analysis of Kepler data by several groups has turned up a puzzle," admits Professor Bedding. "The rotation rates in the cores of these red giants, which we can also deduce from asteroseismology, turn out to be only about ten times faster than the surface rotation. But theory predicts that the core of a red giant should rotate hundreds or even thousands of times faster than the surface. It's a puzzle that theoreticians are currently trying to solve, and is sure to shed new light on our understanding of stars."
What of future research? Hardware failures mean Kepler can no longer point accurately towards its original field of view, but Tim Bedding sees the bright side: "Kepler's new mission, K2, involves looking at a series of different fields for about 80 days each. This has an advantage for asteroseismology because it will allow us to sample oscillating stars in many different directions in the galaxy, rather than just one."
Beyond Kepler, the Sydney team and others will eagerly await planet-hunting missions such as NASA's TESS, due for launch in 2017, and the European Space Agency's PLATO, planned for 2024. And that's not all a Danish-led network of ground-based telescopes called SONG is also being developed. With floods of information telling us more about a star's insides. we could soon know much more about them than ever before.