Stellar corona

A stellar corona is the hot, diffuse outer atmosphere above a star’s chromosphere. It emits strongly at ultraviolet and X-ray wavelengths and can extend millions of kilometers from the stellar surface.

Coronal heating problem: The unresolved question of how stellar coronae reach temperatures far above their photospheres. The corona’s extreme temperature demands an energy source that ordinary surface heating cannot explain.
Stellar atmosphere: The gaseous outer layers of a star, including the photosphere and higher atmospheric regions. The corona is the outermost atmospheric region in this broader structure.
X-ray astronomy: The study of celestial objects through X-rays, which Earth’s atmosphere largely absorbs. Hot stellar coronae emit X-rays, making this a principal way to study them.
Solar corona: The Sun’s hot, diffuse outer atmosphere, observed in detail during eclipses and with space-based instruments. The Sun’s corona is the nearest and most thoroughly observed example of a stellar corona.
Magnetic reconnection: A process in which magnetic-field configurations change and release stored magnetic energy as heat and accelerated particles. Reconnection can convert tangled coronal magnetic energy into heat and explosive events.
Stellar chromosphere: A relatively thin atmospheric layer above a star’s photosphere and below its transition region and corona. The chromosphere lies directly beneath the corona and forms their atmospheric boundary.
Ultraviolet astronomy: The study of astronomical objects using ultraviolet radiation, mostly observed from space. Ultraviolet spectral lines trace coronal temperatures, densities, and chemical composition.
Solar eclipse: An astronomical event in which the Moon blocks the Sun, allowing the faint solar corona to become visible. Total eclipses historically exposed the corona against the bright solar disk.
Alfvén wave: A magnetohydrodynamic wave that travels along magnetic field lines in a conducting fluid. Alfvén waves can carry energy upward from a star’s surface into its corona.
Transition region: A narrow atmospheric layer where temperature rises sharply between a star’s chromosphere and corona. This layer marks the abrupt thermal change from chromosphere to corona.

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