Photosphere

photosphere, luminous, apparently opaque layer of gases that forms the visible surface of the sun or any other star. The photosphere lies between the dense interior gases and the more attenuated gases of the chromosphere. The incandescent gases of the photosphere, estimated to be at temperatures near 6,000°K;, are so much brighter than the other layers of the sun that they seem to form a surface. These gases are in a constant state of agitation due to convection currents that reach down to 150,000 mi (241,000 km) below the photosphere. Differences in the density of the gases result in a grainy appearance of the photosphere; the small bright patches, or granules, are several hundred miles in diameter and are constantly shifting. Another feature of the photosphere, observed only near the sun's edge, is the appearance near sunspots of bright, veinlike regions known as faculae.

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photosphere

Visible surface of the Sun, about 250 mi (400 km) thick. It emits most of the Sun's light that reaches Earth directly. Temperatures range from about 18,000 °F (10,000 °C) at the bottom to 8,000 °F (4,000 °C) at the top; its density is about 1/1,000 that of air at the surface of Earth. Sunspots are photospheric phenomena. The photosphere has a granular structure. Each grain (cell), a mass of hot gas several hundred miles in diameter, rises from inside the Sun, radiates energy, and sinks back within minutes to be replaced by others in a constantly changing pattern.

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photosphere [′fōd·ə‚sfir]

(astronomy)

The intensely bright portion of the sun visible to the unaided eye; it is a shell a few hundred miles in thickness marking the boundary between the dense interior gases of the sun and the more diffuse cooler gases in the outer portions of the sun.

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Photosphere 

the deepest and densest layer of a stellar atmosphere, including the solar atmosphere, from which most of the stellar radiant energy escapes.

A large part of the continuous spectrum of stars, chiefly the visible spectrum, and most of the Fraunhofer absorption lines arise in the photosphere. The photosphere is generally in radiative equilibrium. It is easier for radiation to escape from the higher layers of a stellar atmosphere, and consequently the temperature of the star decreases as the outer layers are approached. On the average, the temperature is close to the effective temperature of the star. The size of the photosphere of the main-sequence stars (on the Hertzsprung-Russell diagram) relative to the radius of the stars is 10^–4–10^–3, of white dwarfs of the order of 10^–6, and of giants and supergiants 10^–3–10^–2. The average gas densities of the photospheres of various stars vary from 10^–9 g/cm³ for hot stars of the main sequence to 10^–6 g/cm³ for white dwarfs.

The photosphere of the sun, which coincides with its apparent surface, has been studied in greatest detail. It is 200–300 km thick, and its temperature ranges from 4500° to 8000°K; the pressure of the gas varies from 10^–5 to 10^–3 dyne/cm². The photosphere is the only region of the sun with relatively weak ionization of the sun’s predominant chemical element—hydrogen—the degree of ionization of which is about 10^–4. In stars similar to the sun, the strong opacity of the photospheric gases is due to a small impurity of negative hydrogen ions.

By using a photosphere telescope it is possible to observe the fine structure of the solar photosphere—granulation—consisting of small round (about 1,000 km in diameter) bright granules that are separated by dark intergranular regions.

E. V. KONONOVICH