Solar mystery solved: Sun’s rotation has slowed with time — explains surface composition

European Space Agency share footage of ‘solar hedgehog’

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The study was undertaken by astrophysicist Dr Patrick Eggenberger of the University of Geneva (UNIGE) and his colleagues. Dr Eggenberger said: “The Sun is the star that we can best characterise, so it constitutes a fundamental test for our understanding of stellar physics. “We have abundance measurements of its chemical elements, but also measurements of its internal structure, like in the case of Earth thanks to seismology.”

Ideally, all of these real-world measurements would match the Sun’s properties as predicted by the models that physicists have created to explain the star’s evolution.

These models help scientists investigate questions from how the Sun burns hydrogen in its core to how chemical elements move around the star.

Until the early 2000s, the standard solar model held up quite well — until an international team published revised measurements of the chemical abundances on the Sun’s surface that clashed with those predicted by the model.

Since then, the proposed new abundance values have survived various verifications, suggesting that the problem must lie with the solar model itself.

Paper co-author and astronomer Gaël Buldgen, also of UNIGE, said: “The standard solar model we used until now considers our star in a very simplified manner.”

This oversimplification, she explained, applies to both how chemical elements are transported within the Sun’s deepest layers, but also to how the Sun rotates and the nature of the internal magnetic fields that it generates.

Various new models have been proposed over the last two decades that have strived to explain the new chemical abundance measurements.

However, until now, none had managed to adequately reproduce the data obtained from so-called helioseismology — the name given to the analysis of the Sun’s oscillations — specifically as related to the abundance of helium in the Sun’s convective outer layers.

The new model proposed by Dr Eggenberger and his colleagues has reconciled these differences by considering both the evolution of the Sun’s rotation — which they believe was likely faster in the past — but also the magnetic instabilities such creates.

In their model, the team assumed that the Sun was initially rotating around five times faster than its present-day equatorial rotation rate.

As Dr Eggenburger said: “We must absolutely consider simultaneously the effects of rotation and magnetic fields on the transport of chemical elements in our stellar models.

“It is as important for the Sun as for stellar physics in general, and has a direct impact on the chemical evolution of the Universe, given that the chemical elements that are crucial for life on Earth are cooked in the core of the stars.”

According to the team, their new model not only accurately predicts the concentrations of helium in the solar envelope, but also that of lithium, another element whose measured abundances on the Sun’s surface had previously proven challenging to match in models.

Dr Eggenburger explained: “The abundance of helium is correctly reproduced by the new model because the internal rotation of the Sun imposed by the magnetic fields generates a turbulent mixing.”

This, he added, “prevents this element from falling too quickly towards the centre of the star.

“Simultaneously, the abundance of lithium observed on the solar surface is also reproduced because this same mixing transports it to the hot regions where it is destroyed.”

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The new model, however, does not resolve all of the problems.

Paper author and UNIGE astronomer Dr Sébastien Salmon explained: “Thanks to helioseismology, we know within 500 km [311 miles] in which region the convective movements of matter begin, 199,500 km [123,964 miles] below the surface of the Sun.

“However, the theoretical models of the Sun predict a depth offset of 10,000 km [6,214 miles]!”

“Thanks to the new model, we shed light on the physical processes that can help us resolve this critical difference.”

The findings will also have an impact on our understanding of all the other Sun-like stars we have studied to date — as physicists have long drawn inferences about their properties and behaviour based on how we thought the Sun behaved.

Dr Buldgen said: “We are going to have to revise the masses, radii and ages obtained for the solar-type stars that we have studied so far.”

Dr Eggenberger added: “This is particularly important if we want to better characterise the host stars of planets, for example within the framework of the PLATO mission.”

PLATO — short for “PLAnetary Transits and Oscillations of stars” — is a European Space Agency mission, due to launch in 2026.

It will see a space observatory with 26 telescopes sent to a point in space 932,057 miles from Earth to discover and study small planets and analyse the characteristics of their host stars.

The full findings of the study were published in the journal Nature Astronomy.

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