Scientists show that the sun’s magnetic activity influences the determination of its seismic age

An international team of astronomers, led by a researcher from the University of Geneva (UNIGE), has shown that the Sun’s magnetic activity has a significant influence on its seismic characterization, contrary to the predictions of previous studies. Important data such as its size, age and chemical composition depend on it.

These results open the way to further research to better understand the nature of magnetic activity and its impact on stellar oscillations. The study is published in the journal Astronomy and astrophysics.

Asteroseismology, or helioseismology in the case of the Sun, is a fascinating branch of astronomy that studies the oscillations of stars.

“To understand it, you have to imagine a star as a big ball of gas in constant motion. Inside this star, there are waves or pulsations that make it vibrate, a bit like the sound that resonates in a musical instrument,” explains Jérôme Bétrisey, postdoctoral researcher in the Department of Astronomy of the Faculty of Science at UNIGE and first author of the study.

“These vibrations cause the surface of the star to move slightly and regularly change in brightness. Thanks to very precise instruments, we can detect these variations in brightness from Earth or from space,” the researcher continues.

Listen to the symphony of the stars

By observing these changes, researchers can learn a lot about the internal structure of the star and determine important characteristics such as its size, age, chemical composition or stage in the stellar life cycle. A precise understanding of the characteristics of stars is essential, among other things, to determine the properties of the planets that surround them or to trace the history of the Milky Way.

Despite some major successes in recent decades, asteroseismology has also shown that there are significant differences between observations and predictions from theoretical models of the internal workings of stars.

Over the years, various methods have been developed to reduce these discrepancies, with varying degrees of success. However, none of the current methods take into account the magnetic activity of stars, its impact on the results being assumed to be negligible.

The international team led by Bétrisey has just demonstrated the opposite. It has established that the age of the Sun, determined by helioseismology, varies significantly depending on the level of activity of the solar cycle.

To give an order of magnitude, the Sun is about 4.6 billion years old, and variations of up to 300 million years have been observed between solar minima. While these variations may seem small compared to the age of the Sun, they are no longer negligible given the level of precision that will be achieved by future space missions.

The seismic age of the sun correlates with the solar activity cycle

To better understand the impact of magnetic activity on the Sun, the scientists analyzed 26.5 years of solar data, covering two complete cycles of activity. They divided this data into about 90 small one-year series, spaced three months apart. For each of these series, a seismic analysis was performed, allowing them to measure the evolution of the fundamental properties of the Sun such as its mass, radius and age over time.

Two independent datasets were used to check the robustness of the results. One comes from the University of Birmingham’s BiSON (Birmingham Solar Oscillations Network) ground-based telescope network, and the other from the GOLF (Global Oscillations at Low Frequencies) instrument on board the Solar and Heliospheric Observatory (SOHO) satellite, which has been orbiting the Sun since the mid-1990s.

Regardless of the configuration tested, the age of the Sun determined by helioseismology was correlated with the level of activity of the solar cycle. They measured variations of about 6% on average between periods of solar minimum and maximum, which is very significant in terms of the precision targeted for future space missions that will analyze other similar stars.

For example, the PLATO (PLAnetary Transits and Oscillations of stars) mission aims for a precision of 10% for the age of a star like the sun.

The study of GOLF and BiSON data also showed that the impact of the activity cycle on the seismic age is more marked for the more active of the two cycles studied. This result is logical and expected from a physical point of view.

“However, the Sun is not a particularly active star, which suggests that the impact of magnetic activity could be very important for more active stars like those that PLATO will detect,” adds Bétrisey.

A bright future for the study of the magnetic activity of stars

The results of this study show that the magnetic activity of stars is a major challenge for future space missions such as PLATO, particularly in terms of characterizing the most active stars. “But this discovery also opens up many exciting research perspectives,” concludes Bétrisey.

The magnetic activity of stars has a considerable influence on stellar oscillations, making it difficult to accurately determine fundamental properties such as the mass, radius and age of stars. For future space missions, this means that more sophisticated methods will have to be developed to account for this magnetic impact.

The challenges posed by magnetic activity will also encourage researchers to explore this phenomenon further. This could lead to a better understanding of stellar physics, including how magnetic fields interact with the internal oscillations of stars. Such research could also improve our understanding of stellar activity cycles, which are similar to solar cycles.