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ASTROPHYSICS

The difference that equalizes the Sun and its sister stars

Chaotic plasma behavior helps explain variations in the magnetic cycle of solar analog stars

The Galileo Project Sunspots recorded by Galileo Galilei between June 9 and 12, 1613The Galileo Project

An international group of astrophysicists including the Brazilian José-Dias do Nascimento Júnior, a professor at the Federal University of Rio Grande do Norte (UFRN), seems to have found the answer to a question which for almost two decades has intrigued those who study the Sun and similar stars known as solar analogs. Except for their ages, which can vary greatly, these stars resemble the Sun in nearly every way, including mass, size, temperature, and brightness. So many similarities led the researchers to imagine that these stars could help reconstruct the past and project the future of the star that warms and illuminates the Earth and neighboring planets. Until recently, difficulty understanding variations in the duration of the cycle of magnetic activity in these stars made the Sun seem like a star without a match. A study published by Nascimento and his colleagues in July in the journal Science helped dispel this mystery, indicating that nothing in the Sun makes it different from its sister stars.

“The duration of the magnetic cycle in solar analogs varies widely; some are longer and others shorter, but none match the Sun’s,” says the Brazilian astrophysicist, who is also a visiting researcher at the Harvard University Center for Astrophysics. One of the projects that helped strengthen the Sun’s reputation as a unique star in its category was published in 2007 in the Astrophysical Journal by the German astrophysicist Erika Böhm-Vitense (1923-2017), who at that time was a professor at the University of Washington. One figure in the article made the difference clear. The image correlated the time a star takes to complete one revolution around its axis (rotation period) with the length of its magnetic cycle, separating the nearly 30 stars analyzed into two groups: younger stars with a shorter magnetic cycle on one side, and older stars with a longer cycle on the other. In the middle, isolated, was the Sun.

The current study published in Science removes the Sun from this unique position by explaining the origin of variations in magnetic cycles and indicating that it is very unlikely (if not impossible) to find two equal cycles, even if all of the stars’ other characteristics are almost identical. The reason for this improbability is that some phenomena involved in generating stellar magnetic fields appear to follow the rules of what is known as the theory of dynamic systems, or chaos theory. Because the phenomena described by this theory are very sensitive to the initial conditions, even if these are very similar, the results can be quite distinct. Consequently, two magnetic cycles would only be likely to coincide if the stars were equal in every way, something which is extremely rare in nature.

“This chaotic component explains why we are unlikely to find two stars with magnetic cycles of the same length,” says Nascimento. It also allows us to understand why, in the case of the Sun, the length of these cycles may vary. “Our results indicate that the Sun is a normal star, like any other in its category.”

Stars like the Sun are spheres of superheated and electrically charged gas (plasma), and are composed primarily of hydrogen and helium. Their magnetic fields are generated in the upper third of the star by the movement of the plasma, which is transported from the deeper, hotter regions of this layer to the cooler and more superficial ones, while simultaneously being dragged by the rotation of the star. All this movement distorts the lines of the magnetic field and amplifies the field itself. This field occasionally undergoes a polarity reversal: positive becomes negative and vice versa. In the case of the Sun, which takes 28 days to rotate on its axis, the polarity reverses approximately once every 11 years. Another 11 years are needed for the poles to return to their initial magnetic configuration and complete the cycle, adding up to a total of 22 years. However, reversals every 9 to 14 years have been observed. These polarity reversals coincide with the star’s period of minimal activity, while periods of maximum activity are marked by the emergence of spots (dark, cooler regions) on the surface of the Sun, which were first recorded in the seventeenth century by mathematician and astronomer Galileo Galilei (see images).

In 1919, the Irish physicist and mathematician Joseph Larmor proposed the dynamo theory, which stated that the Sun’s magnetic field originated from the movement of electrical particles in its interior, which also applies to other stars. But projects that monitored stellar activity for long periods of time indicated that the behavior of magnetic cycles was likely to be more complex. Magneto-hydrodynamic models, which are more sophisticated and consider stars to be filled with a fluid that conducts electricity, were able reproduce the magnetic field reversals, but did not faithfully generate the complete cycle for many stars. Nascimento and the French astrophysicists Allan Sacha Brun, from the Paris-Saclay Laboratory of Astrophysics, Instrumentation, and Modeling, and Antoine Strugarek of the University of Montreal in Canada improved the predictive ability of these models by adding equations from chaos theory that describe the movement of turbulent plasma.

3D simulations
Using the new model, they performed three-dimensional simulations of the interior of the Sun and 30 solar analogs, obtaining cycles which were very similar to those measured by astronomical observations. They also noticed that the length of the magnetic cycle depends on the star’s rotation speed: stars that rotate faster have shorter cycles. “The tendency we found differs from that obtained from the past models,” said Strugarek, lead author of the Science article, in a press release.

“It was already expected that the magnetic activity of the star would be influenced by its speed of rotation,” says astrophysicist Elisabete Dal Pino of the University of São Paulo (USP), who did not participate in the study. “The results they obtained,” she continues, “are important to show that stars that resemble the Sun but have a different rotation may exhibit magnetic cycles of different lengths.”

Knowing the length of stars’ magnetic cycles is important to identify planets and to guide the search for life around stars like the Sun. “The magnetic activity of the stars generates a signal that can be misinterpreted as a planet existing within their orbit,” says Nascimento. According to the researcher, some of the planets outside the solar system which were found by one of the techniques may not exist, and in some cases may indicate a false result caused by magnetic manifestations.

Scientific article
STRUGAREK, A. et al. Reconciling solar and stellar magnetic cycles with nonlinear dynamo simulations. Science. July 14, 2017.

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