Sun is ‘less active’ than similar stars

The Sun is less magnetically active and shows less variability in its brightness than similar stars in the galaxy, scientists say. 

To come to this conclusion, published in the journal Science, they analysed 369 candidate stars selected because they resemble the Sun in decisive properties. 

“These stars appear nearly identical to the Sun except for their higher variability. Therefore, we speculate that the Sun could potentially also go through epochs of such high variability,” they write.

In selecting their candidates for comparison, the researchers – led by Timo Reinhold from Germany’s Max Planck Institute for Solar System Research (MPS) – considered surface temperature, age, and the proportion of elements heavier than hydrogen and helium – but most particularly rotation period. 

“The speed at which a star rotates around its own axis is a crucial variable”, says MPS’s Sami Solanki.

A comprehensive catalogue of the rotation periods of thousands of stars now exists. It is based on data from NASA’s Kepler Space Telescope, which recorded the brightness fluctuations of approximately 150,000 main sequence stars (those in the middle of their lifetimes) from 2009 to 2013. 

After sifting through the options, the researchers – from MPS, Australia’s UNSW and the School of Space Research in South Korea – selected those stars that rotate once around their own axis in 20 to 30 days (the Sun needs about 24.5). They further narrowed the sample using data from the European Gaia Space Telescope, leaving the 369.

The exact analysis of the brightness variations of these stars from 2009 to 2013 reveals a clear picture, they say. 

While between active and inactive phases solar irradiance fluctuated on average by just 0.07%, the fluctuations of the other stars were typically five times as strong. 

In their paper, Reinhold and colleagues discuss several potential explanations for their observations, including possibilities that the Sun can have higher variability over long timescales, or differs from similar stars in ways that haven’t yet been recognised. 

“It is just as conceivable that stars with known and Sun-like rotation periods show us the fundamental fluctuations in activity the Sun is capable of,” says MPS’s Alexander Shapiro. 

This would mean, he adds, that our star has been unusually feeble over the past 9000 years and that on very large time scales phases with much greater fluctuations are also possible.

In a related Perspective article in Science, Ângela R. G. Santos, from the Space Science Institute, US, and Savita Mathur, from Spain’s Instituto de Astrofísica de Canarias, consider the implications of this.

“The notion that the Sun might reach higher solar activity levels is quite unpleasant news for technological societies,” they write. 

“The Sun’s strong activity and the associated solar storms not only produce the beautiful northern and southern lights but also might compromise satellites and power lines, threaten the lives of astronauts in space, and affect Earth’s climate. 

“High stellar activity also has a non-negligible impact on the habitability of planets found around active stars because their atmospheres might be swept out into deep space by such high magnetic activity.”

Reinhold and colleagues suggest there is no immediate cause for concern, however, as there is no indication of such solar “hyperactivity” in the immediate future.