The heartbeats of pulsing stars

By listening to the beating hearts of stars, astronomers have for the first time identified a rhythm of life for a class of stellar objects that had until now puzzled scientists.

The international team of researchers used data from NASA’s Transiting Exoplanet Survey Satellite (TESS), a space telescope mainly used to detect planets around some of the nearest stars to Earth. It provided the team with brightness measurements of thousands of stars, allowing them to find 60 whose pulsations made sense.

“Previously we were finding too many jumbled up notes to understand these pulsating stars properly,” says Professor Tim Bedding from the University of Sydney, who led the research. “It was a mess, like listening to a cat walking on a piano.

“The incredibly precise data from NASA’s TESS mission have allowed us to cut through the noise. Now we can detect structure, more like listening to nice chords being played on the piano.”

The team’s findings have been published in Nature.

Hear the rapid beat of HD 31901, a Delta Scuti star in the southern constellation Lepus. The sound is the result of 55 pulsation patterns TESS observed over 27 days sped up by 54,000 times. Delta Scuti stars have long been known for their apparently random pulsations, but TESS data show that some, like HD 31901, have more orderly patterns. Credit: NASA’s Goddard Space Flight Center and Simon Murphy, University of Sydney

Unravelling the history of the Milky Way

UNSW Sydney’s Associate Professor Dennis Stello was a co-author on the paper. He uses asteroseismology – the ringing inside stars from star quakes – to estimate the physical properties of stars. In this project, he created models of the stars and matched these to data from the TESS mission, allowing the team to best estimate the true ages of the stars.

“Some of the big questions in astronomy are about how our own Milky Way formed and evolved over time. Like archaeologists, we unravel its complicated history by age-dating the stars that form the various parts of the Milky Way – its original building blocks.

“With the ages determined from asteroseismology, we gain a firmer picture of the merging events of smaller dwarf galaxies into the Milky Way.”

The intermediate-sized stars in question – about 1.5 to 2.5 times the mass of our Sun – are known as delta Scuti stars, named after a variable star in the constellation Scutum. When studying the star pulsations of this class of stars, astronomers had previously detected many star pulsations, but had been unable to determine any clear patterns.

The Australian-led team of astronomers has reported the detection of remarkably regular high-frequency pulsation modes in 60 delta Scuti stars, ranging from 60 to 1400 light years away.

“This definitive identification of pulsation modes opens up a new way by which we can determine the masses, ages and internal structures of these stars,” Bedding says.

Daniel Hey, who also contributed to the research from the University of Sydney, designed the software that allowed the team to process the TESS data.

“We needed to process all 92,000 light curves, which measure a star’s brightness over time. From here we had to cut through the noise, leaving us with the clear patterns of the 60 stars identified in the study,” he says.

“Using the open-source Python library, Lightkurve, we managed to process all of the light curve data on my university desktop computer in a just few days.”

The findings are an important contribution to our overall understanding of what goes on inside the countless trillions of stars across the cosmos.

Using asteroseismology to peer into the insides of stars

The insides of stars were once a mystery to science. But in the past few decades, astronomers have been able to detect the internal oscillations of stars, revealing their structure. They do this by studying star pulsations using precise measurements of changes in light output.

Over periods of time, variations in the data reveal intricate – and often regular – patterns, allowing us to stare into the very heart of the massive nuclear furnaces that power the universe.

This branch of science, known as asteroseismology, allows us to not only understand the workings of distant stars, but to fathom how our own Sun produces sunspots, flares and deep structural movement. Applied to the Sun, it gives highly accurate information about its temperature, chemical make-up and even production of neutrinos, which could prove important in our hunt for dark matter.

“Asteroseismology is a powerful tool by which we can understand a broad range of stars,” Bedding says. “This has been done with great success for many classes of pulsators including low-mass Sun-like stars, red giants, high-mass stars and white dwarfs. The delta Scuti stars had perplexed us until now.”

Stello is excited that asteroseismology – a technique previously shown to be powerful for studying stars like the Sun – now also seems to be relevant for more massive and typically much younger stars.

200515 nasa gif pulsating star pulsating star gif 1
Sound waves bouncing around inside a star cause it to expand and contract, which results in detectable brightness changes. This animation depicts one type of Delta Scuti star pulsations — called a radial mode — that is driven by waves (blue arrows) traveling between the star’s core and surface. In reality, a star may pulsate in many different modes, creating complicated patterns that enable scientists to learn about its interior. Credit: NASA’s Goddard Space Flight Center

“This research has opened the door to study stars that are different to the Sun,” he says.

“This has potential for improving our understanding of stars more broadly and to use stars as age tracers of the building blocks of the Milky Way.”

Isabel Colman, a co-author and PhD student at the University of Sydney, said: “I think it’s incredible that we can use techniques like this to look at the insides of stars.

“Some of the stars in our sample host planets, including beta Pictoris, just 60 light years from Earth and which is visible to the naked eye from Australia. The more we know about stars, the more we learn about their potential effects on their planets.”

Poor ‘social distancing’

The identification of regular patterns in these intermediate-mass stars will expand the reach of asteroseismology to new frontiers, Bedding says. For example, it will allow us to determine the ages of young moving groups, clusters and stellar streams.

“Our results show that this class of stars is very young and some tend to hang around in loose associations. They haven’t got the idea of ‘social distancing’ rules yet,” Bedding says.

George Ricker from the MIT Kavli Institute for Astrophysics and Space Research is Principal Investigator for NASA’s Transiting Exoplanet Sky Survey, from which the study took its data.

He says: “We are thrilled that TESS data is being used by astronomers throughout the world to deepen our knowledge of stellar processes. The findings in this exciting new paper led by Tim Bedding have opened up entirely new horizons for better understanding a whole class of stars.”

This article was first published on Australia’s Science Channel, the original news platform of The Royal Institution of Australia.

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