Music of ancient stars reveals stellar surprise

By Ellen Rykers 13 March 2017
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The stars are alive with the sound of music – and astronomers are detecting their characteristic notes to uncover valuable new insights.

ASTRONOMERS STUDYING SOUND waves inside ancient stars rocked by starquakes have discovered the stars do indeed align – or at least their axes of rotation do.

Starquakes, analogous to our planet’s earthquakes, make stars “ring like a bell”. An international team of researchers, including Australian astronomer Dr Dennis Stello, studied the distinctive ‘musical notes’ of stars in two ancient clusters – one 2 billion years old and the other 8 billion years old. What they found was totally unexpected.

“Most of the stars in the two open clusters spun around axes that were strongly aligned,” said Dennis, from the University of New South Wales in Sydney. “It was a surprise result.”

Scientists previously thought stars in a cluster would spin in random directions due to “massive turbulent motion” inside the gas cloud where they form.

But this research shows the overall rotation of the “stellar nursery” cloud is transferred to the individual stars.

“It’s remarkable that the imprint of these initial conditions can still be seen billions of years later, by studying tiny oscillations in stars many light years away,” said Dennis.

Like a hammer on a gong

The researchers, led by Dennis’ former PhD student Dr Enrico Corsaro, used an ‘astroseismology’ approach. “Just as seismologists use earthquakes to understand the interior of our planet, we use starquakes to understand the interior of stars,” explained Dennis.

The starquakes generate sound waves inside the star, and make the surface vibrate “like a hammer on a gong,” he said.

Different stars make different sounds based on their physical properties – just like violins and cellos sound different.

“It’s like how at a symphony, you can close your eyes but still recognise different instruments,” Dennis said. “The pitch can tell you about the size of the instrument. Here, the different notes of the stars can tell us their angle of rotation.”

However, astronomers can’t directly hear the sound waves – instead, they look for tiny vibrations on the star’s surface.

“Sound can’t travel in empty space, but the sound waves create small compressions and expansions on the surface that change the brightness of the star,” said Dennis. “These small changes in brightness occur at the same frequency of the sound wave generated by the starquake.”

The research team had a wealth of data to analyse thanks to NASA’s Kepler Space Observatory. Kepler accurately measured the brightness of these stars once every half hour for four years.

The next steps in this research will include observing other star clusters, and updating stellar formation theory to match these new observations.

“The result is such a surprise,” said Dennis. “It indicates that our fundamental understanding of how stars form isn’t complete.”

The research appears on the cover of today’s Nature Astronomy.