Quaking Giants Might Solve the Mysteries of Stellar Magnetism

Our planet is doomed. In a few billion years, the sun will exhaust its hydrogen fuel and swell into a red giant — a star so big it will scorch, blacken and swallow up the inner planets.

While red giants are bad news for planets, they’re good news for astrophysicists. Their hearts hold the keys to understanding a range of stellar bodies, from fledgling protostars to zombie white dwarfs, because deep within them lies an invisible force that can shape a star’s destiny: the magnetic field.

Magnetic fields near the surfaces of stars are often well characterized, but what’s happening in their cores is mostly unknown. That’s changing, because red giants are uniquely suited for studying magnetism deep within a star. Scientists do this by using starquakes — subtle oscillations at a star’s surface — as a portal to stellar interiors.

“Red giants have these oscillations that allow you to probe the core very sensitively,” said Tim Bedding, an asteroseismologist at the University of Sydney who studies red giant stars.

Last year, a team at the University of Toulouse decoded those oscillations and measured the magnetic fields within a trio of red giants. Earlier this year, the same team detected magnetic fields inside a further 11 red giants. Together, the observations showed that the hearts of giants are more mysterious than expected.

Close to a star’s heart, magnetic fields play crucial roles in chemical mixing in the star’s interior, which in turn affects how a star evolves. By refining stellar models and including internal magnetism, scientists will be able to calculate stellar ages more accurately. Such measurements could help determine the ages of potentially habitable faraway planets and pin down the timelines of galaxy formation.

“We don’t include magnetism in stellar modeling,” said Lisa Bugnet, an astrophysicist at the Institute of Science and Technology Austria who developed methods for studying magnetic fields inside red giants. “It’s crazy, but it’s just not there because we have no idea how it looks [or] how strong it is.”

Stare Into the Sun

The only way to probe the heart of a star is with asteroseismology, the study of stellar oscillations.

In the same way that seismic waves rippling through Earth’s interior can be used to map the planet’s subterranean landscape, stellar oscillations open a window into a star’s innards. Stars oscillate as their plasma churns, producing waves that carry information about a star’s internal composition and rotation. Bugnet compares the process to a ringing bell — the shape and size of a bell produces a specific sound that reveals the properties of the bell itself.

To study quaking giants, scientists use data from NASA’s planet-hunting Kepler telescope, which monitored the brightness of over 180,000 stars for years. Its sensitivity allowed astrophysicists to detect minute changes in starlight linked to stellar oscillations, which affect both the radius and the brightness of the star.

But decoding stellar oscillations is tricky. They come in two basic flavors: acoustic pressure modes (p-modes), which are sound waves that move through the outer regions of a star, and gravity modes (g-modes), which are lower in frequency and mostly confined to the core. For stars like our sun, p-modes dominate their observable oscillations; their g-modes, which are affected by internal magnetic fields, are too weak to detect and can’t reach the star’s surface.

In 2011, the KU Leuven astrophysicist Paul Beck and colleagues used Kepler data to show that in red giants, p-modes and g-modes interact and produce what’s known as a mixed mode. The mixed modes are the tool that probes the heart of a star — they allow astronomers to see the g-mode oscillations — and they’re only detectable in red giant stars. Studying mixed modes revealed that red giant cores rotate much more slowly than the star’s gaseous envelope, contrary to what astrophysicists had predicted.

That was a surprise — and a possible indication that something crucial was missing in those models: magnetism.

Stellar Symmetry

Last year, Gang Li, an asteroseismologist now at KU Leuven, went digging through Kepler’s giants. He was searching for a mixed-mode signal that recorded the magnetic field in the core of a red giant. “Astonishingly, I actually found a few instances of this phenomenon,” he said.

Typically, mixed-mode oscillations in red giants occur almost rhythmically, producing a symmetric signal. Bugnet and others had predicted that magnetic fields would break that symmetry, but no one was able to make that tricky observation — until Li’s team.

Li and his colleagues found a giant trio that exhibited the predicted asymmetries, and they calculated that each star’s magnetic field was up to “2,000 times the strength of a typical fridge magnet” — strong, but consistent with predictions.

However, one of the three red giants surprised them: Its mixed-mode signal was backward. “We were a bit puzzled,” said Sébastien Deheuvels, a study author and an astrophysicist at Toulouse. Deheuvels thinks this result suggests that the star’s magnetic field is tipped on its side, meaning that the technique could determine the orientation of magnetic fields, which is crucial for updating models of stellar evolution.

A second study, led by Deheuvels, used mixed-mode asteroseismology to detect magnetic fields in the cores of 11 red giants. Here, the team explored how those fields affected the properties of g-modes — which, Deheuvels noted, may provide a way to move beyond red giants and detect magnetic fields in stars that don’t show those rare asymmetries. But first “we want to find the number of red giants that show this behavior and compare them to different scenarios for the formation of these magnetic fields,” Deheuvels said.

Not Just a Number

Using starquakes to investigate the interiors of stars kicked off a “renaissance” in stellar evolution, said Conny Aerts, an astrophysicist at KU Leuven.

The renaissance has far-reaching implications for our understanding of stars and of our place in the cosmos. So far, we know the exact age of just one star — our sun — which scientists determined based on the chemical composition of meteorites that formed during the birth of the solar system. For every other star in the universe, we only have estimated ages based on rotation and mass. Add internal magnetism, and you have a way to estimate stellar ages with more precision.

And age is not just a number, but a tool that could help answer some of the most profound questions about the cosmos. Take the search for extraterrestrial life. Since 1992, scientists have spotted more than 5,400 exoplanets. The next step is to characterize those worlds and determine if they’re suitable for life. That includes knowing the planet’s age. “And the only way you can know its age is by knowing the age of the host star,” Deheuvels said.

Another field that requires precise stellar ages is galactic archaeology, the study of how galaxies are assembled. The Milky Way, for instance, gobbled up smaller galaxies during its evolution; astrophysicists know this because chemical abundances in stars trace their ancestry. But they don’t have a good timeline for when that happened — the inferred stellar ages aren’t accurate enough.

“The reality is, sometimes we are a factor [of] 10 wrong in stellar age,” Aerts said.

The study of magnetic fields within stellar hearts is still in its infancy; there are many unknowns when it comes to understanding how stars evolve. And for Aerts, there’s beauty in that.

“Nature is more imaginative than we are,” she said.

Jackson Ryan’s travel for this story was partly funded by the ISTA Science Journalist in Residence Program.