"It's not a boring white disk," says solar physicist Bernhard Fleck. Certainly not as seen by SOHO, the Solar and Heliospheric Observatory launched by the European Space Agency and NASA in 1995. SOHO captured this image in extreme-ultraviolet wavelengths, color-coded by temperature, with red showing the hottest. Why is the halo-like corona, visible from Earth only during a total eclipse, hundreds—even thousands—of times hotter than the surface? That's one of the questions that keep scientists looking straight at the sun.

Magnetism made visible: That describes virtually every feature on the sun, from sunspots to soaring structures called loops. Loops easily reach the height of ten Earths. Energy generated by the dynamics of smaller loops is likely the source of the corona's