The stellar atmosphere is divided into several regions of distinct character:
Thephotosphere, which is the atmosphere's lowest and coolest layer, is normally its only visible part.[1]Light escaping from the surface of the star stems from this region and passes through the higher layers. TheSun's photosphere has atemperature in the 5,770–5,780 K (5,500–5,510 °C; 9,930–9,940 °F) range.[2][3]Starspots, cool regions of disruptedmagnetic field, lie in the photosphere.[3]
Above the photosphere lies thechromosphere. This part of the atmosphere first cools down and then starts to heat up to about 10 times the temperature of the photosphere.
Above the chromosphere lies thetransition region, where the temperature increases rapidly on a distance of only around 100 km (62 mi).[4]
Additionally, many stars have a molecular layer (MOLsphere) above the photosphere and just beyond or even within the chromosphere.[5] The molecular layer is cool enough to contain molecules rather than plasma, and may consist of such components as carbon monoxide, water vapor, silicon monoxide, and titanium oxide.
The outermost part of the stellar atmosphere, or upper stellar atmosphere, is thecorona, a tenuousplasma which has a temperature above one million Kelvin.[6] While all stars on themain sequence feature transition regions and coronae, not allevolved stars do so. It seems that only somegiants, and very fewsupergiants, possess coronae.An unresolved problem in stellarastrophysics is how the corona can be heated to such high temperatures. The answer is believed to lie inmagnetic fields, but the exact mechanism remains unclear.[7]
Theastrosphere, which is in the case of the Sun theheliosphere,[8] can be in a broader understanding considered the furthest part of a stellar atmosphere,[9][10] beforeinterstellar space begins at theheliopause. The astrosphere is not to be confused with theSolar System and its outermost region theOort cloud, which extends much further than the astrosphere, therefore far into interstellar space.
During a totalsolar eclipse, the photosphere of the Sun isobscured, revealing its atmosphere's other layers.[1] Observed during eclipse, the Sun's chromosphere appears (briefly) as a thin pinkisharc,[11] and its corona is seen as a tuftedhalo. The same phenomenon ineclipsing binaries can make the chromosphere of giant stars visible.[12]
^ab""Beyond the Blue Horizon" – A Total Solar Eclipse Chase". 1999-08-05. Retrieved2010-05-21.On ordinary days, the corona is hidden by the blue sky, since it is about a million times fainter than the layer of the sun we see shining every day, the photosphere.
^abLang, K. R. (September 2006). "5.1 MAGNETIC FIELDS IN THE VISIBLE PHOTOSPHERE".Sun, earth, and sky (2nd ed.). Springer. p. 81.ISBN978-0-387-30456-4.this opaque layer is the photosphere, the level of the Sun from which we get our light and heat
^Mariska, J. T. (1992).The solar transition region. Cambridge University Press. p. 60.ISBN978-0-521-38261-8.100 km suggested by average models
^"The Sun's Corona – Introduction".NASA. Retrieved2010-05-21.Now most scientists believe that the heating of the corona is linked to the interaction of the magnetic field lines.
^Lewis, J.S. (2004-02-23).Physics and chemistry of the solar system (Second ed.). Elsevier Academic Press. p. 87.ISBN978-0-12-446744-6.The dominant color is influenced by theBalmer radiation of atomic hydrogen
