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A region of a star's atmosphere above its photosphere. The Sun's chromosphere extends from the temperature minimum, some 500 km above the base of the photosphere, outwards by 9000 km where it merges with the corona. For the first 1500 km the chromosphere is more or less continuous, but above this it breaks up into jagged spicules. The temperature of the chromosphere rises from 4400 K at 500 km to about 6000 K at 1000–2000 km. There is a rapid rise to coronal temperatures at heights of about 2500 km (the transition region), the exact height depending on the strength of the local magnetic field. At the top of the chromosphere, the density is a millionth of its value at the bottom. Immediately before and after a total solar eclipse, the chromosphere is visible either as a crescent or a diamond ring, with red coloration due to Hα emission from which it takes its name (meaning ‘colour sphere’). Outside eclipses it is visible in Hα and calcium K-line filtergrams, and from space in ultraviolet emission lines. The presence of chromospheres in nearby cool dwarf stars is deduced from similar emissions.

Subjects: Astronomy and Astrophysics.

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