Our sun is less active than similar stars

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US, WASHINGTON (ORDO NEWS) — By cosmic standards, the sun is unusually monotonous, researchers from the Max Planck Institute for the Study of the Solar System concluded. For the first time, scientists compared the Sun with hundreds of other stars with similar rotation periods. Most stars produce much stronger variations. This raises the question of whether the sun went through an unusually calm phase over several millennia.

The extent to which solar activity changes (and therefore the number of sunspots and the brightness of the sun) can be reconstructed using various methods, at least for a certain period of time. For example, since 1610 there have been reliable records of sunspots covering the sun. The distribution of radioactive varieties of carbon and beryllium in tree rings and ice cores allows us to draw conclusions about the level of solar activity over the past 9000 years. Over this period of time, scientists have discovered regularly recurring fluctuations in comparable strength, as in recent decades. “However, compared to the entire life of the Sun, 9,000 years is a blink of an eye,” says MPS scientist Dr. Timo Reinhold, the first author of the new study. After all, our star is almost 4.6 billion years old.

Since it is impossible to determine how active the Sun was in primitive times, scientists can only resort to the stars: together with colleagues from the University of New South Wales in Australia and the School of Space Research in South Korea, MPS researchers found out whether the Sun behaves “normally” compared to other stars. This can help classify his current activities.

For this, the researchers selected candidate stars, which in their properties resemble the Sun. In addition to surface temperature, age, and the fraction of elements heavier than hydrogen and helium, the researchers primarily looked at the period of rotation. “The speed with which a star rotates around its axis is a critical variable,” explains Prof. Dr. Sami Solanki, director of MPS and co-author of the new publication.

The rotation of the star contributes to the creation of its magnetic field in the dynamic process inside it. “The magnetic field is the driving force responsible for all fluctuations in activity,” says Solanki. The state of the magnetic field determines how often the Sun emits energy radiation and throws particles at high speed into space with strong emissions, how many dark spots and bright areas on its surface – and,

A complete catalog containing rotation periods of thousands of stars was available only in the last few years. It is based on measurements from NASA‘s Kepler Space Telescope, which recorded brightness fluctuations of approximately 150,000 main sequence stars (that is, those that are in the middle of their life span) from 2009 to 2013. Researchers studied this huge catalog and chose stars that make one revolution around their axis for 20-30 days.

For this, the sun needs about 24.5 days. Researchers were able to further narrow this sample using data from the Gaia European Space Telescope. As a result, 369 stars remained, which also resemble the Sun in other fundamental properties.

An accurate analysis of the brightness changes of these stars from 2009 to 2013 shows a clear picture. While between the active and inactive phases, solar radiation fluctuated on average only 0.07 percent, other stars showed much larger fluctuations. Their vibrations were usually about five times stronger. “We were very surprised that most of the Sun-like stars are much more active,” says Dr. Alexander Shapiro of MPS, who leads the “Combining Solar and Stellar Variations” research group.

However, it is impossible to determine the rotation period of all the stars observed by the Kepler telescope. To do this, scientists must find certain periodically repeating dips in the luminous brightness of the star. These dips can be traced back to star spots, which darken the surface of the star, turn beyond the scope of the telescope, and then reappear after a certain period of time. “For many stars, such periodic darkening cannot be detected; they are lost in the noise of the measured data and in the overlapping fluctuations in brightness, ”Reinhold explains. If you look through the Kepler telescope, even the Sun will not give out the period of its rotation.

Therefore, the researchers also studied more than 2500 Sun-like stars with unknown rotation periods. Their brightness fluctuated much less than that of the other group.

These results allow two interpretations. There may still be an inexplicable fundamental difference between stars with known and unknown rotation periods. “It is also possible that stars with known and similar to the Sun periods of rotation show us the fundamental fluctuations in activity that the Sun is capable of,” says Shapiro. This will mean that our star has been unusually weak for the past 9000 years and that phases with much larger fluctuations are also possible on very large time scales.

However, there is no reason for concern. In the foreseeable future, there are no signs of such solar “hyperactivity.” On the contrary: over the past decade, the Sun has shown itself to be rather weak, even by its low standards. Forecasts of activity for the next eleven years show that this will not change soon.


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