1. The outermost layer of the sun and many other stars; a faint halo of extremely hot (million-degree) gas.
2. A faint halo of hot (estimated to be approximately 2,000,000°C or 3,600,000°F) and tenuous gas around the sun, which boils from the surface.

It is visible at solar eclipses or through a coronagraph, an instrument that blocks light from the sun's brilliant disk.

Gas flows away from the corona to form the solar wind.