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Astronomers reveal a rapidly spinning core inside old stars

An international team of astronomers led by PhD student Paul Beck from Leuven University in Belgium have managed to look deep inside some old stars and discovered that their cores spin at least ten times as fast as their surfaces. The result appeared in the renowned journal Nature on December, 7. The surfaces of these stars spin slowly taking a whole year to complete one rotation. The team have now discovered that the cores at the heart of the stars spin much faster with about one rotation per month. The discovery was made possible because of the amazing precision of the data from NASA's Kepler space telescope. Een international team geleid door doctoraatsstudent Paul Beck van de K.U.Leuven slaagde er in om een blik te werpen diep in de binnenkant van oude sterren. Zij ontdekten dat de kern van deze sterren minstens tien keer sneller ronddraait dan hun oppervlak. Deze vondst verschijnt vandaag in het toptijdschrift Nature. Het oppervlak van de ster roteert erg traag: een volledige rotatie duurt al gauw een jaar. Nu blijkt dat de diep gelegen kern van de ster veel sneller draait, zowat één rotatie per maand. Deze ontdekking dankt het team aan de buitengewoon precieze metingen van de Kepler Ruimtetelescoop van NASA.

 

An international team of astronomers led by PhD student Paul Beck from Leuven University in Belgium have managed to look deep inside some old stars and discovered that their cores spin at least ten times as fast as their surfaces. The result appeared today in the renowned journal nature. It has been known for a long time that the surfaces of these stars spin slowly, taking about a whole year to complete one rotation.  The team has now discovered that the cores at the heart of the stars spin much faster with about one rotation per month. The discovery was made possible because of the ultra high precision of the data from NASA's Kepler space telescope.

Beck and his collaborators analysed waves travelling through the stars, which appear at the surface as rhythmic variations in the stars’ brightness. The study of such waves is called asteroseismology, and is able to reveal the conditions deep inside a star which would otherwise remain hidden from view. Different waves probe different parts of the star and by a detailed comparison of the depth to which these waves travel inside the star, the team found evidence of the rotation rate and its dramatic increase towards the stellar core. “It is the heart of a star, which determines how it evolves“, says Beck, “and understanding how a star rotates deep inside helps us to understand how stars like our Sun will grow old.“

The stars studied in the article are so-called red giants. Our Sun will become a red giant in about 5 billion years. Their outer layers have expanded to more than 5 times their original size, and cooled down significantly so that they appear red. Meanwhile, their cores did exactly the opposite, and have contracted to an extremely hot and dense environment. To understand what has happened to a star’s spin consider what happens to an ice skater performing a pirouette. A spinning ice skater will slow down if the arms are stretched far out, and will spin faster if the arms are pulled tightly to the body. Similarly, the rotation of the expanding outer layers of the giant has slowed down, while the shrinking core has spun up.

The Kepler space telescope, is one of NASA’s most successful current space missions. Designed to search for Earth-size planets in the habitable zone of distant stars, the mission has detected numerous planetary candidates, and has confirmed many bona fide planets outside our solar system. Kepler is capable of detecting variations in a star’s brightness of only a few parts in a million, and its measurements are therefore ideally suited to detect the tiny waves mentioned above. The effect of rotation on these waves is so small, that its discovery needed two years of almost continuous data gathering by the Kepler satellite.

Animation

This artist impression illustrates the rotation inside a red giant star. Such stars have radii of more than 5 times the radius of the Sun. Initially the outer layers, which are rotating very slowly, are shown. When these layers are hidden, the hot core of the star, which rotates 10 times faster than the surface, becomes visible. While the surface of this red giant needs about one year to complete a full revolution, it takes the core only a few weeks to rotate once. For better visual effect, the rotation rate is artificially increased. In the animation, 60 seconds correspond to an entire year in real time. Image Credits: Paul G. Beck, KU. Leuven.

Scene 1: Comparison of diameter and rotation rate of a redgiant to the sun.
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Scene 2: The fast rotating core becomes visible, when the convectibe envelope is removed. 
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The Nature Paper

Fast core rotation in red-giant stars as revealed by gravity-dominated mixed modes (DOI: 10.1038/nature 106212)

By Paul G. Beck, Josefina Montalban, Thomas Kallinger, Joris De Ridder, Conny Aerts, Rafael A. Garcıá, Saskia Hekker, Marc-Antoine Dupret, Benoit Mosser, Patrick Eggenberger, Dennis Stello, Yvonne Elsworth, Søren Frandsen, Fabien Carrier, Michel Hillen, Michael Gruberbauer, Jørgen Christensen-Dalsgaard, Andrea Miglio, Marica Valentini, Timothy R. Bedding, Hans Kjeldsen, Forrest R. Girouard, Jennifer R. Hall & Khadeejah A. Ibrahim

 

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Acknowledgements

We acknowledge the work of the team behind Kepler. Funding for the Kepler Mission is provided by NASA's Science Mission Directorate. The research leading to these results has received funding from the European Research Council under the European Community's Seventh Framework Programme (FP7/2007--2013)/ERC grant agreements n°227224 PROSPERITY in support of Conny Aerts and Paul Beck as well as n°267864 ASTERISK in support of Jørgen Christensen-Dalsgaard, Hans Kjeldsen and Søren Frandsen; Joris De Ridder and Thomas Kallinger were supported by the Fund for Scientific Research Flanders. Saskia Hekker was supported by the Netherlands Organisation for Scientific Research. Josefina Montalban and Marica Valentini were supported by the Belgian Science Policy Office. Yvonne Elsworth and Andrea Miglio acknowledge their financial support from the UK Science and Technology Facilities Council. Partially based on observations with the HERMES spectrograph at the Mercator Telescope which is operated at La Palma/Spain by the Flemish Community.