FROM THE MOMENT A STAR'S NUCLEAR dynamo roars to life, a countdown begins. The frenetic rotation at the core creates convection: The hotter, more diffuse plasma rises, and the cooler, denser material sinks. In stars about the Sun's size or smaller, this convection transfers heat and the star's magnetic field up to the surface. Unlike the stable, donut-shaped field around Earth, however, a star's field is in a constant state of flux, shifting and tangling as the plasma in the stellar body sloshes mercilessly about.
“Magnetic fields in stars are constantly forming, twisting, dissipating, and reforming,” says Meredith MacGregor (University of Colorado, Boulder). “They thread all throughout the star.”
In the Sun's outer atmosphere, called the corona, these threads make big loops that stick out from the star. But the…