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 arcs don't sit still like the petals on a daisy; they ripple and crisscross one another, becoming more and more tangled until they suddenly snap back into orderly loops. Meanwhile, various forces within the stellar body cause channels to form that connect the plasma in the interior to the corona. When the fields snap in magnetic reconnection, star-stuff can be ejected up through these channels in a spectacular explosion.
These outbursts come in two main flavors: flares and coronal mass ejections (CMEs). Flares are flashes of multiwavelength light, including ultraviolet and X-rays, that often begin in the concentrated fields that produce a sunspot. CMEs are eruptions of plasma, primarily electrons and protons, which are ejected from the star in a spray. The two are related, though we don't know exactly how, and they frequently (but not always) occur together.
Flares and CMEs are extremely useful to scientists because they tell us about the daily life of a star, its activity, and its impact on its surrounding planets. “For the longest time, the only star we could study in detail was our Sun,” says
