LIVING WITH A RESTLESS STAR

Imagine you woke up one morning and your planet had been engulfed by the atmosphere of a star. High-energy particles slam into the atmo­sphere, creating shimmering auroras. Sunspots flicker, each one re­leasing more energy than the largest atomic bomb. Great loops of hot gas uncoil from the star’s surface, extending millions of miles. Each whip crack of activity causes mayhem on orbiting satellites, frying their circuit boards and wreaking havoc on their guidance systems. Your planet is assailed by high-energy particles traveling at nearly the speed of light. Looking at the star, its emission doesn’t change detectably from day to day or year to year, but its invisible short wavelength radiation fluctuates wildly and unpredictably.

Where is this place? It’s the Earth, and the star is the Sun. This activity isn’t because the Sun has run out of nuclear fuel and has turned into a red giant; our star is in the dull middle age of its life. But the clean, crisp edge the Sun seems to have in the daytime sky is an illusion. That curved surface—the edge of the photosphere— corresponds to a place where the gas of the Sun has thinned out to the point where photons no longer careen into particles and they can travel freely.1 From that point they travel in straight lines with­out interruption, streaking to the Earth in eight minutes. Inside the photosphere, which is a slender sheath thinner than the skin on an apple compared to its radius, the Sun is opaque and our view is blocked. We see the boundary layer as having a sharp edge, just like a cloud has a well-defined edge.2

The edge is an illusion because there’s no physical surface or barrier. If you could penetrate the Sun in a spaceship, you’d feel no bump. The material of the Sun extends much further into space than its visible edge. In its physical quantities of temperature, pres­sure, and density, the Sun varies continuously and smoothly from its fusion core outward. It possesses a surprising multi-million – degree corona, the influence of which extends past the Earth’s orbit almost to the edge of the Solar System. Invisible forms of high-energy radiation and subatomic particles—ultraviolet waves, X-rays, gamma rays, and relativistic cosmic rays—streak toward us and past us at the speed of light. In a very real sense, we live “inside” the atmosphere of an active star, with profound and sur­prising consequences for life on Earth.