Our Sun is an ordinary star after all

Our Sun is an ordinary star after all

The Sun is a solar-type star, a new study claims — resolving an ongoing controversy about whether the star at the centre of our Solar System exhibits the same cyclic behaviour as other nearby, solar-type stars.

The results also advance scientists’ understanding of how stars generate their magnetic fields. The Sun’s activity — including changes in the number of sunspots, levels of radiation and ejection of material – varies on an eleven-year cycle, driven by changes in its magnetic field.

Understanding this cycle is one of the biggest outstanding problems in solar physics, in part because it does not appear to match magnetic cycles observed on other solar-type stars — leading some to suggest the Sun is fundamentally different.

Here, by carrying out a series of simulations of stellar magnetic fields, Antoine Strugarek and colleagues show that the Sun’s magnetic cycle depends on its rotation rate and luminosity. This relationship can be expressed in terms of the so-called Rossby number; they show that the magnetic cycle of the Sun is inversely proportional to this number.

Comparing the results of their simulations with available observations of cyclic activity in a sample of nearby solar-type stars, the authors further find that the cycle periods of the Sun and other solar-type stars all follow the same relationship with the Rossby number.

The results demonstrate that the Sun is indeed a solar-type star.

In this video you are placed inside a simulation of the interior of a solar-type star. The red and blue shapes represent the turbulent convective motions that animate the external shell of the Sun, which covers 30% of its radius. Inflow (blue) and outflows (red) change properties from the equator to the pole due to the influence of the global rotation of the star.

The turbulent motions inside the Sun generate a large-scale magnetic field, which is represented by the white-blue tubes (colour represents the polarity of the magnetic field). Two main structures emerge, one in each hemisphere. While the convective cells typically live for about a month, this magnetic field reverses (i.e. changes sign) on a decadal time-scale, as shown by the time-lapses in the video.

This reversal is regular and occurs because of the non-linear modulation by magnetic torques of the large-scale flows inside the turbulent convection zone. By carrying out a series of such simulations, we show that the magnetic cycle directly depends on the rotation rate and luminosity of the star, which can be expressed as a function of only one parameter, called the Rossby number.

The research is published in the journal Science.

Source: American Association for the Advancement of Science

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Tags assigned to this article:
magnetic cyclesmagnetic fieldssolar activitysun

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