The stellar atmosphere is the outer region of the volume of a

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

, lying above the stellar core, radiation zone and

convection zone A convection zone, convective zone or convective region of a star is a layer which is unstable due to convection. Energy is primarily or partially transported by convection in such a region. In a radiation zone, energy is transported by radiation ...

Overview

The stellar atmosphere is divided into several regions of distinct character: * The

photosphere The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...

, which is the atmosphere's lowest and coolest layer, is normally its only visible part.

Light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

escaping from the surface of the star stems from this region and passes through the higher layers. The Sun's photosphere has a

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

in the range. Starspots, cool regions of disrupted

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

, lie in the photosphere. * Above the photosphere lies the

chromosphere A chromosphere ("sphere of color", from the Ancient Greek words χρῶμα (''khrôma'') 'color' and σφαῖρα (''sphaîra'') 'sphere') is the second layer of a Stellar atmosphere, star's atmosphere, located above the photosphere and below t ...

. 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 the transition region, where the temperature increases rapidly on a distance of only around . * Additionally, many stars have a molecular layer (MOLsphere) above the photosphere and just beyond or even within the chromosphere. 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 the corona, a tenuous plasma which has a temperature above one million Kelvin. While all stars on the

main sequence In astronomy, the main sequence is a classification of stars which appear on plots of stellar color index, color versus absolute magnitude, brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or d ...

feature transition regions and coronae, not all evolved stars do so. It seems that only some

giants A giant is a being of human appearance, sometimes of prodigious size and strength, common in folklore. Giant(s) or The Giant(s) may also refer to: Mythology and religion *Giants (Greek mythology) * Jötunn, a Germanic term often translated as 'g ...

, and very few supergiants, possess coronae. An unresolved problem in stellar

astrophysics Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of the founders of the discipline, James Keeler, said, astrophysics "seeks to ascertain the ...

is how the corona can be heated to such high temperatures. The answer is believed to lie in

s, but the exact mechanism remains unclear. * The astrosphere, which is in the case of the Sun the heliosphere, can be in a broader understanding considered the furthest part of a stellar atmosphere, before interstellar space begins at the heliopause. The astrosphere is not to be confused with the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

and its outermost region the Oort cloud, which extends much further than the astrosphere, therefore far into interstellar space. During a total

solar eclipse A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of Earth, totally or partially. Such an alignment occurs approximately every six months, during the eclipse season i ...

, the photosphere of the Sun is obscured, revealing its atmosphere's other layers. Observed during eclipse, the Sun's chromosphere appears (briefly) as a thin pinkish arc, and its corona is seen as a tufted halo. The same phenomenon in eclipsing binaries can make the chromosphere of giant stars visible.

Notes

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Atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

Overview

See also

Notes