Every 11 years, these sunspots disappear, and when they return, their magnetism and that of the entire Sun's magnetic field will have reversed. NASA/SDO

Other than the small matter of supporting the only life we know of, our Sun is just an ordinary star. Previous suspicions that there is something unusual about its magnetism have been dismissed. While it might be nice to think of our little patch of space as being a bit more distinctive, the discovery removes one obstacle to the chances of finding life elsewhere in the galaxy.

Science has since walked away from the notion that we inhabit a special place in the universe. Flat-Earthers, or those who think the Sun orbits the Earth, can resist all they like, but the Earth circles a pretty ordinary Sun in a non-descript location in a larger than average but otherwise unexceptional galaxy.

Ordinary, that is, except perhaps for the Sun's magnetic field. The sunspots and solar storms the Sun generates go through an 11-year cycle, after which there is a reversal of magnetic polarity. Other stars also have such cycles, but their length and intensity varies.

When astronomers tried to find a pattern in the length of these cycles, comparing them with the size and brightness of the stars, they instead came up with two groupings, with the Sun fitting into neither and instead lying somewhere in between. This made some astronomers wonder if there wasn't something special about it after all. It might be an excessively long bow to link this distinctive status to the presence of life, but the coincidence was at least intriguing.

However, Dr Antoine Strugarek of the University of Montreal modeled the turbulent convection that creates stars' magnetic fields and found a pattern into which the Sun fits perfectly well.

In Science, Stugarek and co-authors model the behavior of stars somewhat fainter than the Sun and with rotation periods between 14 and 29 days. The work examined the relationship between Rossby numbers and the length of the magnetic cycle. Rossby numbers, which also apply to the atmospheres and oceans of planets, measure the forces created by an object's rotation. “We find that the magnetic cycle period is inversely proportional to the Rossby number,” the authors report.

When the model was compared with actual observations of the cycles of nearby stars, mostly measured by the Mount Wilson Observatory, there was broad agreement. Three stars, whose outer shells appear to resemble the Sun, have periods close enough to the Sun's to support the theory.

The fit between estimated Rossby numbers and stellar period isn't perfect, and other factors, such as luminosity, may have an influence. Nevertheless, the authors believe they have not only confirmed the Sun's normality, but made a contribution to explaining how the magnetic period occurs at all.

A still from a video modeling the magnetism inside a star. Blue lines are convection inflows and orange are outflows. The white-blue tubes represent magnetic fields. The convective cells last around a month, but it takes 11 years before reversals. Stugarek et al/Science



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