Researchers at Aarhus University have helped discover that the red-hot core in the interior of a giant star rotates ten times faster than the surface. The results have just been published in the recognised journal Nature. (Graphics: Paul G. Beck)

Astronomers at Aarhus University have studied the interiors of red giant stars and can ascertain that they behave quite differently from the Sun.

2011.12.09 | Christina Troelsen

The fact that the stars rotate has been known for a long time, but normal observations are only able to tell something about the rotation on a star’s surface.

New observations using the Kepler Satellite can ‘see’ right into the interior of the stars, and researchers have now discovered that the so-called red giant stars have an inner core that rotates ten times faster than the surface.

Measurements that have just been published in Nature show that the inner core in red giant stars spins at such a speed that it manages to rotate one full circle in a month, while it takes a whole year for the surface to do so.

See animation of the rotation of red giant stars:

Red giant stars are old stars

When stars get old, they develop into red giant stars that can be five to a hundred times bigger than our own star – the Sun – which will reach this stage in approximately five billion years.

Observations of sunquakes, some of which have been analysed in Aarhus, show that the interior of the Sun rotates at virtually the same speed as the surface. However, researchers know that this will change the day the Sun becomes a red giant star. Kepler’s observations of starquakes on the surface of a red giant star can ‘see’ the very compact core of the star and determine its speed of rotation.

Same principle as an ice skater

“When a star turns into a red giant star, the core itself contracts while the outer layers expand to many times their original size,” explains astronomer Søren Frandsen, Aarhus University.

“The interior of a star starts to rotate faster when the core contracts, just like an ice skater, who spins faster if she pulls her arms tightly to her body,” he continues.

The inner core also reaches a much higher temperature than the surface, which cools down as the star expands.

The new results very clearly show how the researchers’ theories and calculations can be observed in practice using the Kepler Satellite’s unique precision measurements.

New knowledge to be studied closer

“As yet, we don’t fully understand what it means for the star’s structure and development that the rotation is so different on the surface compared with the core,” says Professor Jørgen Christensen-Dalsgaard, Department of Physics and Astronomy, Aarhus University.

“One of our greatest challenges will be to make detailed models of this development,” he goes on.

More information

Associate Professor Søren Frandsen, Department of Physics and Astronomy, Aarhus University, +45 8715 6352, srf@phys.au.dk

Professor Jørgen Christensen-Dalsgaard, Department of Physics and Astronomy, Aarhus University, +45 2338 2374, jcd@phys.au.dk

Associate Professor Hans Kjeldsen, Department of Physics and Astronomy, Aarhus University, +45 2338 2160, hans@phys.au.dk

FACTS Red giant stars Stars convert hydrogen to helium, hereby producing energy. When the star has used up the hydrogen in its interior, it swells up to become a so-called red giant star. Red giant stars can be five to a hundred times bigger than the Sun. Our own Sun will develop into a red giant star in approximately five billion years. Giant stars are colder on the surface than other stars and thereby take on a reddish colour, which is why they are also called red giants. NASA’s Kepler Satellite – an ultra-precise photometer The Kepler Satellite was launched by the USA in March 2009 to carry out very precise measurements of the brightness of many thousands of stars. These very precise light measurements were intended to reveal the existence of planets that pass in front of stars during their orbit around them, and shadow a small part of their luminous surface. In this way, the Kepler Satellite has found a number of new planets and, within the next few years, the satellite’s measurements are expected to reveal the existence of planets the size of Earth. Kepler almost continuously observes the same field in the sky in the constellations of the Swan and the Lyre. It must surely be described as one of the most successful missions undertaken by the American space organisation NASA. Asteroseismology – looking into the stars Ever since Kepler was launched, astronomers all over the world have used the measurements to look for starquakes on the surface of the many different stars observed by the satellite. By measuring periodical light variations from starquakes, the astronomers can determine which physical conditions prevail deep inside the stars via a type of investigation called asteroseismology. The asteroseismic studies of the Kepler Satellite’s measurements are led by a group of researchers at Aarhus University. In the last two years, they have succeeded in revealing more and more details of the inner structure and development of the stars. The satellite’s measurements show us the stars in a previously unknown way, and make it possible to carry out in-depth studies that would have been impossible to dream of before the satellite was launched.

A new research centre

As of 2012, Professor Christensen-Dalsgaard will be director of the new Stellar Astrophysics Centre at Aarhus University. The Danish National Research Foundation has granted DKK 55 million (approximately EUR 7.5 million) to set up this research centre, with a focus on using measurements from sources such as the Kepler Satellite.

“We’ve now got an opportunity to make full use of the measurements. We won’t just be looking at stars, but will also study planets around the stars, and the conditions found on the surface of these planets,” concludes Professor Christensen-Dalsgaard.

The three researchers Søren Frandsen, Hans Kjeldsen and Jørgen Christensen-Dalsgaard, who are co-authors of the article on the inner rotation of red giant stars, work at the Kepler Asteroseismic Science Operations Centre, Department of Physics and Astronomy, Aarhus University.

Illustration showing the difference in size between the Sun and a red giant star. (Graphics: Paul G. Beck)