What is a star's corona?
The outermost layer of it's atmosphere.
The corona is the outermost layer of a star's atmosphere, made of very hot plasma (an ionised gaseous state). This plasma is actually hotter than the surface of the sun, and as it cools, matter is thrown into space as solar wind.
Our own star, The Sun, has a corona, but we can't usually see it because the light from The Sun overwhelms it. It is only when The Sun is in The Moon's shadow (a solar eclipse) and the usual light is blocked out, that we can see the corona as a white ring.
I hoped this helped; let me know if I can do anything else:)