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Our Sun can expect inner turmoil in old age

Artist's impression of a red giant star

Our Sun will one day become a red giant star (artist's impression), swelling up to be much bigger than it is now. Astronomers have learned that the inside and outsides of such stars are very different.

SCIENTISTS HAVE MADE a new discovery about how old stars called ‘red giants’ rotate, giving an insight into what our Sun will look like in five billion years.

The international team of scientists, including University of Sydney astronomers Professor Tim Bedding and Dr Dennis Stello, has discovered that red giants have slowed down on the outside, while their cores spin at least 10 times faster than their outer layers.

The finding, just published in the prestigious journal Nature, tells us what the Sun will look like in five billion years when it develops into a red giant.

“The heart of a star determines how it evolves, and understanding how a star rotates deep inside helps us to understand how stars like our Sun will grow old,” said Professor Tim Bedding from the University of Sydney’s School of Physics.

Using NASA’s Kepler space telescope, the team “peered” deep inside ageing red giants to make their discovery of the difference in rotation rate between the core and outer layers of the stars.

Stars and the ice skater effect

The team, led by Paul Beck from Leuven University in Belgium, analysed waves inside the stars, which appear as rhythmic variations in the surface brightness of the stars.

The effect of rotation on the frequencies of the waves is so small it took the team nearly two years of almost continuous data gathering from the Kepler satellite to make their discovery.

Cutaway diagram of a red giant star

The cores of red giant stars have been found to spin at least 10 times faster than the outer layers.

“Red giants were once stars like our Sun, but as they age their outer layers expand to more than five times their original size and cool down significantly, so they look red,” explained Dr Dennis Stello, from the University of Sydney’s School of Physics.

“The opposite actually happens to the cores of red giants, as the core contracts and becomes extremely hot and dense,” said Dr Stello.

“We’ve just discovered that the core spins much faster than the outer layers in these old stars, which makes sense when you consider what happens to other spinning things like, say, an ice skater performing pirouettes.”

“A spinning ice skater will slow down if their arms are stretched far out, like the expanded outer layers of the red giants. The ice skater will spin faster if their arms are pulled tightly to the body, like the fast spinning contracted core of red giants.”

Star quakes reveal stellar inner secrets

The Kepler space telescope—one of NASA’s most successful space missions—is searching in the constellation Cygnus for potentially habitable planets by focussing on those similar in size to Earth. It does this by carefully and individually measuring the light coming from over 100,000 stars.

“Kepler is able to detect variations in a star’s brightnessof only a few parts in a million, so its measurements are ideally suited to detect the tiny brightness fluctuations of stars,” explained Dr Stello.

Artist's impression of the Kepler spacecraft

Artist's impression of the Kepler spacecraft

“We study these variations in brightness to work out what’s going on deep inside stars. It’s called asteroseismology—just as geologists use earthquakes to explore Earth’s interior, we use star quakes to explore the interiors of stars,” said Dr Stello.

Different waves reveal information on different parts of the star, and by a detailed comparison of the depth to which these waves travel inside the star the team found the rotation rate dramatically increased towards the stellar core.

In addition to helping us understand how stars age, asteroseismology will help Kepler’s mission of discovering Earth-sized planets outside our Solar System by characterising the host stars around which these planets orbit.

Adapted from information issued by the University of Sydney / ESO / L. Calcada.

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Updated census of Sun-like stars

Artist's impression of a star like our Sun

Artist's impression of a star like our Sun with an orbiting planet in the foreground. The Kepler mission is studying such stars by tracking changes in their brightness.

  • Kepler is a space observatory that measures star brightnesses
  • Brightness oscillations reveal secrets of stars’ sizes, ages and composition

USING NASA’S KEPLER SPACE TELESCOPE, scientists have detected changes in brightness in 500 Sun-like stars, giving a much better idea about the nature and evolution of the stars.

Prior to Kepler’s launch in March 2009, astronomers had identified changes in brightness, or oscillations, of only about 25 stars similar to our Sun in size, age, composition and location within the Milky Way galaxy.

Although Kepler’s primary job is to find Earth-like planets that might be able to support life, it also provides a big boost to ‘asteroseismology’…the study of stars by measuring their natural oscillations.

Those oscillations provide clues about star basics such as mass, radius and age, as well as clues about their internal structure.

“This helps us understand more about the formation of stars and how they evolve,” said Steve Kawaler, an Iowa State University professor of physics and astronomy, a co-author of the research paper and a leader of the Kepler Asteroseismic Investigation.

“These new observations allow us to measure the detailed properties of stars at an accuracy that wasn’t possible before.”

Kepler is orbiting the Sun carrying a photometer, or light meter, to measure changes in star brightnesses. The photometer includes a telescope 94cm in diameter connected to a 95-megapixel CCD camera.

Artist's impression of the Kepler spacecraft

Artist's impression of the Kepler spacecraft

The instrument is pointed at the Cygnus-Lyra region of the Milky Way. It is expected to continuously observe about 170,000 stars for at least three and a half years.

Golden age for star studies

The Kepler Asteroseismic Investigation is using Kepler data to study different kinds of stars.

Kepler has provided astronomers with so much new information, the scientists say they’re “entering a golden era for stellar physics.”

Data from the 500 Sun-like stars gives astronomers a much better understanding of the stars, their properties and their evolution. It also gives astronomers data to test their theories, models and predictions about the stars and the galaxy. And it gives astronomers enough data to make meaningful statistical studies of the stars.

“But this is just the start of things,” Kawaler said. “This is a first broad-brush analysis of the data we’ve seen. This is a preview of this new tool and the kind of detailed census that we’ll be able to do.”

Among the projects to come are studies to determine the ages of all these Sun-like stars, and studies of the host stars of the Earth-like planets.

The investigation is led by a four-member steering committee: Kawaler, Chair Ron Gilliland of the Space Telescope Science Institute based in Baltimore, Jorgen Christensen-Dalsgaard and Hans Kjeldsen, both of Aarhus University in Denmark.

Adapted from information issued by Iowa State University. Illustration by Gabriel Perez Diaz, Instituto de Aastrofisica de Canarias (MultiMedia Service). Kepler illustration courtesy NASA.

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Starquakes reveal stars’ inner secrets

Oscillations in red giant stars

Oscillations in starlight reveal information about the internal structure of stars, in much the same way that seismologists use earthquakes to probe the Earth's interior.

  • Turbulence in stars’ interiors cause continuous ‘starquakes’
  • Long-term monitoring of starlight picks up the quakes
  • Provides a window into the internal life of stars

AUSTRALIAN ASTROPHYSICISTS from the University of Sydney are behind a major breakthrough in the study of stars known as red giants, finding a way to peer deep into their cores to discover which ones are in early infancy, which are fresh-faced teenagers, and which are facing their dying days.

The discovery, published in the latest edition of the journal Nature and made possible by observations using NASA’s powerful Kepler space telescope, is shedding new light on the evolution of stars, including our own Sun.

“Red giants are evolved stars that have exhausted the supply of hydrogen in their cores that powers nuclear fusion, and instead burn hydrogen in a surrounding shell,” said Professor Tim Bedding, the paper’s lead author. Then, “towards the end of their lives, red giants begin burning the helium in their cores.”

The Kepler space telescope has enabled Professor Bedding and colleagues to continuously study starlight from hundreds of red giants at an unprecedented level of precision for nearly a year, giving a window into the stars’ cores.

“The changes in brightness at a star’s surface is a result of turbulent motions inside that cause continuous star-quakes, creating sound waves that travel down through the interior and back to the surface,” Professor Bedding said.

Size comparison of the Sun and red giant

Red giant stars are the focus of University of Sydney research in 'asteroseismology', which aims to probe the internal life of stars.

“Under the right conditions, these waves interact with other waves trapped inside the star’s helium core,” he adds. “It is these ‘mixed’ oscillation modes that are the key to understanding a star’s particular life stage.

“By carefully measuring very subtle features of the oscillations in a star’s brightness we can see that some stars have run out of hydrogen in the centre and are now burning helium, and therefore at a later stage of life.”

Astronomer Travis Metcalfe of the US National Centre for Atmospheric Research, in a companion piece in the same Nature issue which highlights the discovery’s significance, compares red giants to Hollywood stars, whose age is not always obvious from the surface.

“During certain phases in a star’s life, its size and brightness are remarkably constant, even while profound transformations are taking place deep inside,” said Dr Metcalfe.

Starquakes

Professor Bedding and his colleagues work in an emerging field called asteroseismology. “In the same way that geologists use earthquakes to explore Earth’s interior, we use star quakes to explore the internal structure of stars,” he explained.

Professor Bedding said: “We are very excited about the results. We had some idea from theoretical models that these subtle oscillation patterns would be there, but this confirms our models. It allows us to tell red giants apart, and we will be able to compare the fraction of stars that are at the different stages of evolution in a way that we couldn’t before.”

Daniel Huber, a PhD student working with Professor Bedding, added: “This shows how wonderful the Kepler satellite really is. The main aim of the telescope was to find Earth-sized planets that could be habitable, but it has also provided us with a great opportunity to improve our understanding of stars.”

Adapted from information issued by the University of Sydney. Images courtesy ESO / NASA.

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