The Sun's magnetic midlife crisis

Abstract

The Sun is just one of a hundred billion stars in the Milky Way galaxy. Our front-row seat on Earth allows us to observe it in much greater detail than we can for other stars. However, those observations provide only one snapshot in the life story of stars like the Sun. To piece together the entire tale, astronomers need to study other stars that are younger and older. Their observations have revealed something unexpected that never could have come to light from studies of our Sun in isolation.

Figures (1)

• Figure 1: THE DYNAMIC SUN. A solar eclipse (a) makes visible the solar wind, material that follows magnetic field lines as it leaves the Sun's surface. (Photo courtesy of Miroslav Druckmüller.) (b) In this plot relating the length of the stellar magnetic cycle to the rotation period, the solid black lines show the two distinct relationships observed by Erika Böhm-Vitense about a decade ago. Points are colored according to the star's surface temperature; hotter F-type stars are blue, G-type stars like the Sun are orange, and cooler K-type stars are red. The colored lines show representative evolution paths that deviate from Böhm-Vitense's lower relationship when the rotation period corresponds to the critical Rossby number heralding the shutdown of magnetic braking. (For a possible interpretation of the upper relationship, see T. Metcalfe, J. van Saders, https://arxiv.org/abs/1705.09668.) Eventually stars evolve to have constant chromospheric emission, which appears to indicate that they have completed the transition from predominantly large-scale to smaller-scale global magnetic fields. The Sun, indicated by the $\odot$ symbol, falls to the right of the orange line because it is slightly cooler than the other indicated stars and thus reaches the critical Rossby number at a longer rotation period. As older stars lie higher along the colored lines than younger stars, the observations suggest that the 11-year sunspot cycle may grow longer before it eventually disappears. The three stars at the top of the graph are indicated with arrows because they show no detectable cycle period during 25 years of observations.