The Sun is less magnetically active than other stars

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Our Sun is the source of life on Earth. Its calm glow through billions of years has allowed life to evolve and flourish in our world. This does not mean that our Sun has no active side. We have observed massive solar flares, such as the Carrington event of 1859, which produced the northern lights as far as the Caribbean and caused electric currents in the telegraph lines. If such an eruption happened in the direction of Earth today, it would devastate our electrical infrastructure. But luckily for us, the sun is mostly calm. Unusual calm compared to other stars.

Astronomers have only recently studied the activity of the Sun. The oldest study, undertaken since the 1600s, follows the count of the number of spots seen on the surface of the Sun. He showed us that the Sun goes through cycles of active and calm periods. A study of four centuries is long in human terms, but is hardly a moment in cosmic time.

A core of ice from the West Antarctic ice sheet. Credit: Heidi Roop, NSF

Longer studies have examined carbon isotopes and other elements in ice cores and tree rings. When the Sun is particularly active, high-energy protons can strike atoms in the upper atmosphere, converting them to radioactive isotopes. They can then find themselves trapped in ice and wood. This gives us an idea of ​​solar activity over almost ten millennia.

It’s still only a fraction of the Sun’s lifespan. Are the last thousands of years a good sample of solar activity? What happens if the Sun goes through an unusually calm period and is generally much more active? To answer this question, a team of astronomers compared our Sun to similar stars, and the results are surprising.

Using data from the Gaia spacecraft, the team searched for stars very similar to the Sun. They found stars of similar mass, age and surface temperature. Among them, they chose stars which not only had a similar metallicity, but also a similar rotation speed. They ended up with 369 stars who are almost twins of our Sun.

Variations in luminosity of the Sun compared to the star KIC 7849521. Credit: MPS

The team then compared the change in activity of the Sun over four years to the activity of these other stars. They found that the activity of the Sun was much lower than the others. The variability of other stars is five times greater than our Sun. Solar flares such as the Carrington event are much more common on other stars.

This could mean that our Sun has been generally calm for the duration of human civilization. If so, he could become more active in the future, which could have serious consequences for our civilization. It is also possible that there is an unknown factor that keeps our sun so calm.

For the moment, nothing indicates that the Sun could enter an hyperactive period. For now and for the foreseeable future, we can continue to enjoy the calm of the sun.

Reference: Reinhold, Timo et al. “The Sun is less active than the other solar-type stars. ” Science 368.6490 (2020): 518-521.

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