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Giant star-jet astounds astronomers

Sanduleak's star

Sanduleak's star and the jet of matter shooting out from it at more than 5 million kilometres per hour. The jet is now 400 million million kilometres long.

  • Star shooting out jet of material 400 million million km long
  • Thought to occur due to interaction between two stars
  • Located in the Large Magellanic Cloud galaxy

ASTRONOMERS HAVE FOUND a star spitting matter into a “jet” that stretches for more than 400 million million kilometres across space.

That’s about ten times the distance between the Sun and its nearest neighbouring star (proxima Centauri).

It’s the biggest jet known from a star, and “challenges our current understanding,” said Dr Francesco Di Mille (Australian Astronomical Observatory and the University of Sydney), a member of the team that made the finding.

Theoretical models don’t deal with it, he said, “simply because nobody would ever have bet that such a giant stellar jet could exist”.

In a galaxy not so far away

The star making the jet is called Sanduleak’s star, having been discovered by astronomer Nicholas Sanduleak in 1977.

Sanduleak noted that the star varied in brightness, but didn’t see the jet.

That’s not surprising. The star is shrouded by dust, and it’s not even in our Galaxy—it’s in a small neighbouring galaxy called the Large Magellanic Cloud, about 160 thousand light-years away.

Finding the jet fell to Dr Di Mille’s team, led by Italian astronomer Rodolfo Angeloni (Pontificia Universidad Católica de Chile), which turned the 6.5-m Magellan Telescopes in Chile on the star.

Magellan Telescopes

Observations were made with the Magellan Telescopes in Chile.

Outburst 10,000 years old

Dust surrounding the star makes it hard to tell exactly what’s going on, but it seems that actually two stars are involved: a red giant and a white dwarf, tangoing closely.

The red giant’s hot “breath”—transferred matter—curls into a belt around the white dwarf’s belly. From time to time a jet shoots up and down from this disc of material, along the star’s axis of rotation.

Artist's impression of a system like Sanduleak's star

An artist's impression of a system like Sanduleak's star—a red giant star transferring matter onto a white dwarf star.

Astronomers have worked out that the current outburst has been going on for about ten thousand years, and that the material in the jet is travelling at more than 5 million kilometres per hour (1,500 km per second).

“Because we know the distance to this star we’ll be able to make good estimates of most of the jet’s properties,” Dr Di Mille said.

“It will be the best test-case for understanding jets from stars.”

The researchers have published their finding in The Astrophysical Journal Letters.

Adapted from information issued by AAO. Magellan Telescopes image courtesy Francisco Figueroa. Sanduleak’s star image courtesy R. Angeloni et al. Artist’s impression courtesy Dana Berry (STScI).

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“Vampire star” devouring its companion

ASTRONOMERS HAVE OBTAINED the best images ever of a star that has lost most of its material to a vampire companion.

By combining the light captured by four telescopes at the European Southern Observatory’s (ESO) Paranal Observatory (Chile), they created a virtual telescope 130 metres across with vision 50 times sharper than the Hubble Space Telescope.

Surprisingly, the new results show that the transfer of gas from one star to the other in this double system is gentler than expected.

The video above shows a “zoom in”, through several different images of different resolution, ending with what looks like two blobs—a blue one and a orangeish one. These are the real images of the two stars, with the red giant being the bigger one. Images taken on different dates show how the stars have moved in their mutual orbit around each other.

The astronomers observed the unusual system SS Leporis in the constellation of Lepus (The Hare), which contains two stars that circle around each other in 260 days.

The stars are very close together…separated by only a little more than the distance between the Sun and the Earth. In terms of size, the larger and cooler of the two stars (a red giant) is big enough to extend out to one quarter of this distance — corresponding roughly to the orbit of Mercury.

Because of this closeness, the hot companion star has already cannibalised about half of the mass of the larger star.

The new observations are sharp enough to show that the giant star is smaller than previously thought, making it much more difficult to explain how it lost matter to its companion.

The astronomers now think that, rather than streaming from one star to the other, the gas must be expelled from the giant star as a stellar wind and then captured by the hotter companion.

Adapted from information issued by ESO / Digitised Sky Survey 2 / Nick Risinger (skysurvey.org) / PIONIER / IPAG; music: John Dyson (from the album Moonwind).

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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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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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