An international team using the Kepler space telescope has spied oscillations in the atmospheres of 500 Sun-like stars, a twenty-fold increase in the number of such objects previously studied. This bonanza of stellar data provides a new way of testing our understanding of how stars evolve, and could even help in the search for a second Earth.

Scientists have long known that the atmosphere of the Sun oscillates. In the same way seismologists use earthquakes to model the interior of the Earth, helioseismologists use vibrations observed on the Sun to probe deeper into our star. Oscillations of about 25 other stars have also been studied in the emerging field of asteroseismology. Now, thanks to the Kepler space telescope, a team led by Bill Chaplin, at the University of Birmingham, UK, has increased this figure significantly.

Launched by NASA in 2009, Kepler is better known for its planet-hunting endeavours, and the mission has identified over 1000 possible exoplanets. These are planets orbiting stars other than the Sun. In order to determine the density of an exoplanet, astronomers must know the mass and radius of its host star. These two parameters can be determined by asteroseismology and Kepler was designed with this in mind.

Continuous star gazing

"Kepler looks at the same stars continuously, it isn't chopping and changing to different parts of the sky," Chaplin told physicsworld.com. "This has allowed us to find small oscillations in 500 stars, compared to only a handful known previously. We can now get great coverage of the different flavours of Sun-like stars and their different evolutionary stages," he adds.

With our own Sun, the simplest oscillations occur with periods of approximately five minutes and effectively cause the Sun to "breathe" in and out. However, this expansion and contraction only amounts to tens of metres over the 1.4 million kilometres of the Sun's diameter, which is why finding the same phenomena in other stars had proved so difficult. "These are really tiny effects; they are tricky to measure. We've had to wait for a sophisticated satellite like Kepler to be able do it," Chaplin explains.

The oscillations observed with Kepler were of comparable periods to those on our Sun, ranging from three minutes up to 25. These differences in period, and therefore frequency, provide important information about the individual properties of the stars. For example, when it comes to radius, the smaller the star the more high-pitched its song. "It is similar to musical instruments. A piccolo trumpet is small and resonates at a higher frequency than a much larger tuba. The same is true for stars: they resonate like musical instruments because they have sound trapped inside," says Chaplin.

Models come up short

Such large ensembles of stars allow statistical analysis of their properties, which include mass and age as well as radius. When it comes to mass, variations in the star's density alter the way sound waves permeate through the star and this leaves a tell-tale fingerprint in the observed oscillations. Using this technique Chaplin was able to measure the masses of the Kepler stars and compare this mass distribution to that predicted from theoretical models. It turns out the models come up short. "We found there are more low-mass stars, ones with around one solar mass, than predicted by theory," Chaplin explains.

These results excite asteroseismology researcher Don Kurtz, at the University of Central Lancashire who was not involved with this work. "For decades we've been trying to find stars like this, and now we have 500 of them," Kurtz told physicsworld.com. "This is one of the big results from the Kepler asteroseismic effort," he adds.

Kurtz also believes that it could be an important stepping stone to finding a planet like the Earth. "An Earth-like planet needs to be in the 'habitable zone' where water can be liquid. To pin down the habitable zone for each individual star you need to know its properties accurately, something Kepler's asteroseismology work is helping to do," he says.

And the work is set to continue according to Chaplin. "As we add to the data, we will be able to not just measure the masses and radii more accurately, but we'll be able to do the equivalent of a CT scan and peel away the surface layers and look in a lot more detail at what is going on inside these stars," he explains.

The research is outlined in Science.