Scientists have revealed one of the secrets of the red giants. How did they get stains?

Among the most striking features of the Sun are its sunspots, relatively dark regions, some of the

which are visible from Earth even without magnification.Many other stars, which, like the Sun, are in their prime, are also covered in spots. On the other hand, in red giants that are at an advanced stage of stellar evolution, such spots were previously considered rare. The reason for this difference can be found deep inside stars. The interaction of conductive plasma currents and rotation generates the star's magnetic field, which is then washed out onto its surface. In some places, particularly strong magnetic fields prevent hot plasma from flowing upward. These areas appear dark and make up starspots.

Until now, researchers have suggested that almostall red giants rotate quite slowly around their axis. In the end, stars expand dramatically when they turn into red giants by the end of their lives. As a result, their rotation slows down. A new study led by scientists from MPS and the University of New Mexico (USA) is currently changing the way scientists think about this process.

The research team examined measurement data from about 4,500 red giant stars recorded by NASA's space telescope from 2009 to 2013 to identify the sunspots.

At the second stage, scientists investigated the question of why spotted giants rotate so quickly. How do they collect the necessary energy?

To answer this question, scientists neededdetermine as many properties of the stars as possible, and then make up the big picture, explains lead author of the publication, Dr. Patrick Golm. For example, at Apache Point Observatory in New Mexico (USA), researchers studied how the wavelengths of starlight from some stars change over time. This allows us to draw conclusions about their exact movement. The team also examined fast brightness fluctuations that overlap with slower ones caused by star spots. Faster fluctuations are an expression of pressure waves propagating through the interior of a star to its surface.

The analysis showed that approximately 15% of the detectedgiants belong to close binary stellar systems, usually consisting of a red giant with a small and less massive satellite. In such systems, the rotation speeds of both stars are synchronized in time until they rotate in unison. Thus, the slower red giant is gaining momentum and spinning faster than without a companion star.

Three paths to the formation of red giants with spots. Provided by: MPS / hormesdesign.de

Other red giants with starspots,about 85% are without a star pair, and yet they rotate rapidly. Those whose mass is approximately equal to that of the Sun likely merged with another star or planet during their evolution and thus gained speed.

A few heavier red giantswhose mass is two to three times the mass of the Sun, there was another development. During the heyday of their life, their internal structure prevented the creation of a global magnetic field, which gradually carries away particles from the star. Their rotation probably never slowed down significantly. Even as red giants, they still spin almost as fast as in their youth.

In total, three groups of rapidly rotating stars were found, each of which has a completely different explanation of the spots.

Studies like this shed light onthe evolution of the rotation and magnetic activity of stars. As well as their complex interactions, including the impact on the habitability of their possible planetary systems. This is one of the main goals of the mission of the European Space Agency in PLATO (PLAnetary Transits and Oscillations of stars), the launch of which is expected by the end of 2026.

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