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Stellar oscillations find
red giants in a spin

Posted: 09 December 2011

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By watching waves from the core of a red giant star crash upon the shore of its surface, astronomers led by Paul Beck of Leuven University in Belgium have determined that the inside of these giant, evolved stars can spin ten times faster than their exterior.

A red giant is an older, evolved version of a Sun-like star. As their central supply of hydrogen becomes exhausted, nuclear reactions slow and their core begins to contract, raising temperatures high enough to ignite helium fusion. This heats the layers outside the core, igniting fusion there where hydrogen is still plentiful, and the star’s outer layers begin to swell up, turning the star into a giant. When the Sun becomes a red giant in five billion years time, it will expand its radius out to almost Earth’s orbit.

An artist’s comparison between the Sun-like star and a red giant (not to scale). The Sun has a core where nuclear fusion occurs and radiative and convective zones through which energy is transported from the centre to the surface. A red giant has a helium-fusing core, a burning shell of hydrogen around the core, and a bloated outer convective layer. It is in the convective layers of both types of stars that disturbances create oscillations within the solar medium, which can be seen on the surface as brightness fluctuations. Image: ESO.

Eventually fusion ceases and the outer layers are cast off to form a beautiful planetary nebula, leaving the contracted core, which has now become a white dwarf star, behind. However, while the broad strokes of this process are known, understanding the details of what occurs inside the star is still a challenge.

Now Beck’s team have managed to look inside a red giant to find out more. Using data from NASA’s Kepler spacecraft, which stares at 150,000 stars in its field-of-view primarily to search for exoplanet transits but also to gather data on stellar magnitude fluctuations, Beck and his colleagues were able to infer the presence of oscillations that were causing the light to fluctuate on three of the red giant stars in the Kepler data.

Disturbances within their turbulent outer convective layers can set stars ringing like a bell, just like seismic waves from earthquakes can be used as a probe of Earth’s interior. “The waves travel hundreds of thousands of kilometres deep and, at a certain depth the stellar material gets too dense to penetrate so the waves bounce back to the surface where they are detectable as rhythmic variations in brightness,” Beck tells Astronomy Now.

Fast core rotation in red giant stars. Credit: Paul beck/KU Leuven, Belgium.

However, Kepler’s ‘red giant working group’ of scientists found the rhythm of a new type of oscillation indicating waves that were travelling deeper, right down into the cores of the red giants. The rate at which parts of the star rotate can affect the frequency of the oscillations (the effect is tiny, says Beck, on the order of a few tens of microhertz) and, for these particular red giants, Beck’s team recorded the period of rotation of their cores as being on the order of a few weeks, whereas the bloated outer layers make about one rotation per year. This has been inferred as the result of the conservation of angular momentum: the core’s rotation speeds up as it contracts, like a spinning ice skater pulling her arms in as she pirouettes. On the flip-side, the swollen outer layers of the star have slowed, as happens when the skater extends her arms.

“The internal rotation is one of the fundamental processes in stars,” says Beck. “Stellar evolution changes the rotational properties while rotation has substantial impact on the stellar evolution. Many processes inside the star are governed by or are connected to rotation and it is crucial to have constraints on the rotation rates in order to understand these processes better.” These properties include magnetic activity – it is well known that faster rotating stars have greater levels of magnetic activity in the form of starspots and flares.

Unfortunately, not much is known about the core rotation of stars in general, including the Sun. To that end Beck’s next step is investigate core contraction at all stages of red giant evolution so as to place this initial result, which has been published in the online version of the 7 December edition of Nature, into proper context. For more on stellar oscillations and the science of ‘asteroseismology’, see the January 2012 issue of Astronomy Now.

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