Our sun is a ball of self-gravitating gas whose core is heated by nuclear fusion. Radiation and convection move energy down a temperature gradient, from the core at 1.6 × 107 K outward to the 5800-K photosphere, the visible surface that you see through your sunglasses (though it is unsafe to stare). The hydrostatic balance between pressure P and gravity (with acceleration constant g⊙) at density ρ implies a surface thickness of only P/ρg⊙ = 174 km. Dense enough to be opaque, the photosphere resembles a solid surface, shaped by gravity to a near-perfect sphere.
Throughout human history, though, the Sun has offered glimpses of its complex structure and variability. In 364 BC, Chinese astronomer Gan De observed sunspots; we now know those to be the most visible signatures of 0.1-T magnetic fields. The solar corona, which extends from the surface far into space,...
