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 for about 2000 km, where it merges with the corona. The top of the chromosphere is very dynamic, displaying jagged spicules when seen at the solar limb; these can extend up to about 9000 km. The temperature of the chromosphere rises from 4400 K at 500 km to about 6000 K at 1000–2000 km. There is then a rapid rise to coronal temperatures through the transition region at about 2500 km, the exact height depending on conditions such as the strength of the local magnetic field. At the top of the chromosphere the density can be a millionth of its value at the bottom. The chromosphere can be seen as a thin red layer of glowing gas around the Sun immediately before and after a total solar eclipse; the red coloration is due to Ηα‎ emission, from which the chromosphere (meaning ‘colour sphere’) takes its name. Outside eclipses, the chromosphere is visible in Ηα‎ and calcium K-line filtergrams, and from space in ultraviolet and extreme ultraviolet emission lines. The presence of chromospheres in nearby cool dwarf stars is deduced from similar emissions.