Our sun is a celestial laboratory only 8 light-minutes from Earth. Its close proximity means that we can see details there that we cannot see in other stars. The Sun’s energy, formed by nuclear fusion deep inside, travels to the surface by radiation and convection until it reaches a layer in which the gas is especially transparent. The visible and IR radiation emitted from that photosphere is what heats Earth to habitable temperatures.
Just above the photosphere is the chromosphere, about one-thousandth as bright and composed of spiky structures about 10 000 km high; the name comes from the colorful, mainly hydrogen and helium emission lines seen during solar eclipses when the chromosphere is silhouetted against a dark background. (See the Quick Study by Charles Kankelborg, Physics Today, April 2012, page 72.) Above it is the solar corona, one-thousandth as bright as the chromosphere. The visible light from...
