Researchers question assumptions about slow rotation of old stars

by alex

The mechanism of rotation of stars turned out to be not as simple as it was thought for a long time

The stars turned out to be much more dynamic than previously thought. As they rotate, they emit a stream of charged particles that are attracted to the stars' powerful magnetic fields. This process is similar to a ballerina spreading her arms, and once the stellar wind becomes magnetically coupled, it begins to slow down the star's rotation. It has been assumed for many years that older stars rotate more slowly, a hypothesis that has been central to astronomy for recent decades.

However, in recent years, astronomers have begun to reconsider this idea of ​​a “star ballet.” Research has shown that around the middle of their lives, stars begin to maintain a constant rotation rate for billions of years. Now scientists have begun to explore how this transition occurs and propose a hypothesis that the Sun is preparing for this particular transition. This discovery will help answer some long-standing mysteries about the Sun and help in the search for life beyond the solar system.

Initially, this idea faced opposition from astronomers, but with observational evidence, confidence in its correctness is increasing. Evidence is becoming increasingly compelling, highlighting gaps in our current understanding of the evolution of stellar magnetic fields over time.

In the 1970s, astronomers discovered that a star's wind interacts with its magnetic field to slow the star's rotation over time, a phenomenon called magnetic braking. In 2003, researchers began using this relationship to estimate the age of a star by measuring its rotation. This process is called gyrochronology and has become widespread. However, all this time, scientists assumed that this relationship would be true for stars of all ages, although estimates and observations were verified only for stars that are younger than the Sun.

In 2016, Jennifer van Saders of the University of Hawaii discovered an unexpected feature in the rotation of stars, based on data from 21 stars collected by the Kepler Space Telescope. The rotation of stars slowed down until they reached the current age of the Sun, and then stabilized.

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«It was absolutely not what we expected. Almost halfway through their lives, stars fundamentally change their rotation, — says van Saders.

After subsequent studies using various methodologies and large samples, this trend was confirmed. Scientists now believe that there is a certain speed at which the star's rotational force is not strong enough to redistribute magnetic energy. This leads to the loss of the ability to create large-scale magnetic fields, and, as a result, the star stops slowing down and maintains a constant rotation speed.

The team's new and unpublished research includes extensive data from the sun-like star 51 Pegasi. The researchers used observations from various telescopes to measure the star's magnetic field and the amount of material being ejected. The results showed that the strength of the star's magnetic field and the amount of material in the wind were significantly lower than predicted by standard models. This suggests that it is impossible to unambiguously determine the age of a star simply by observing its rotation speed.

It was also discovered that stars can be divided into two categories: those that slow down their rotation and those that do not. The Sun is on the verge of this transition and is experiencing some kind of crisis, losing its stable state. This may explain the periodic periods of quiescence in the Sun's magnetic activity, which have raised questions among astronomers for many years.

«The calm of the Sun will continue longer and more often, until, ultimately, the Sun becomes completely calm», — says co-author of the new paper, astronomer Travis Metcalf.

When stars are in a quieter state with less radiation and plasma emissions, this creates a more stable and viable environment for life to arise. Scientists believe future space missions could increase their chances of finding habitable planets by focusing their searches on stars beyond this transition.

In the future, astronomers plan to study how quickly stars transition to a state of more stable rotation and how this affects their magnetic fields.

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