corona

The term corona indicates glowing plasma surrounding a star. The surface of the star is the layer that produces the visible light that reaches us (photosphere), the corona being a bit above it but transparent so we see through it despite its glow. The term was first used for the glow seen around the Sun during solar eclipses. The Sun's corona (solar corona if not evident from context) is "active", changing visibly with time. It is considered a portion of the Sun's stellar atmosphere, external to the chromosphere, with a thin transition region between. Its temperature is much higher than the surface of the Sun.

Coronagraphs were first developed to study the Sun's corona at times other than eclipses, but the term has come to mean any telescope feature designed to block out the light of the Sun or a star in order to see radiation from near the star that is much less intense.

The term corona is also incorporated in terms for glows surrounding other astronomical objects, such as galactic corona. Another example use is used for plasma presumed to be in the vicinity of an active galactic nucleus (an AGN corona, possibly localized to its rotational axis) as the source of photons producing observed Compton reflections, and possibly heated in a manner somewhat-related that of the solar corona, such as through magnetic reconnection.