Stars run internal dynamos that flip their magnetic poles on a schedule
Every 11 years the Sun's main magnetic field turns upside down, so a full cycle takes about 22 years. The flip is the work of a dynamo inside the star, where churning, electrically charged gas twists magnetic fields into ropes that burst out as spots, loops and flares.
Inside a star, heat rises by convection, physically hauling hot plasma upward. Because that plasma conducts electricity, its motion generates magnetism. Differential rotation, with each latitude turning at its own pace, winds the field into flux ropes wrapped around the star. A magnetised patch is buoyant, rising until it breaks the surface as a cooler starspot, often crowned by coronal loops that heat the thin outer atmosphere beyond a million kelvin. Flares and coronal mass ejections can drive plasma to tens of millions of kelvin.
The currents behave like alternating rather than direct current, so fields weaken, reverse and rebuild. On the Sun, the weak field around reversal coincides with peak sunspot activity and bursts of hot plasma into space. Earth's field, produced by turbulent flows in its viscous outer core, flips far less regularly. Evidence stretching back millennia, from tree-ring carbon and ice-core beryllium to 150 years of magnetometer readings, shows the Sun's magnetism varies over decades, centuries and longer.
Astronomers read stellar fields through the Zeeman effect, in which a magnetic field splits a dark spectral line into several closely spaced lines and polarises the light. The first instrument dedicated to this work, NARVAL, sat on the Bernard Lyot Telescope at Pic du Midi in the French Pyrenees. Activity tracks age and spin: young, fast-rotating stars are stormy, while slow middle-aged stars like the Sun are calmer and cyclic. Magnetism also acts as a brake, as the star's magnetosphere drags on its wind and slowly bleeds away spin.
Extreme cases abound. Small M-type flare stars erupt in flares spanning up to a fifth of their circumference. The coolest measured ultracool dwarf, at 800 to 900 kelvin, holds a field about 3,000 times stronger than Earth's. And some 80 percent of planetary nebulae are not round, perhaps because magnetic poles channel the escaping gas.
Source: Stellar magnetic field
