photosphere

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photosphere,

luminous, apparently opaque layer of gases that forms the visible surface of the sunsun,
intensely hot, self-luminous body of gases at the center of the solar system. Its gravitational attraction maintains the planets, comets, and other bodies of the solar system in their orbits.
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 or any other star. The photosphere lies between the dense interior gases and the more attenuated gases of the chromospherechromosphere
[Gr.,=color sphere], layer of rarefied, transparent gases in the solar atmosphere; it measures 6,000 mi (9,700 km) in thickness and lies between the photosphere (the sun's visible surface) and the corona (its outer atmosphere).
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. 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

(foh-tŏ-sfeer) The ‘visible’ surface of the Sun and source of the absorption spectrum that is characteristic of most stars. The photosphere of a star is considerably more dense than the atmospheric layers that lie above it, i.e. the chromosphere and corona.

The solar photosphere is a stratum several hundred kilometers thick, from which almost all the energy emitted by the Sun is radiated into space. Within the photosphere the temperature falls from about 6000 K just above the convective zone to about 4000 K at the temperature minimum, where the photosphere merges with the chromosphere.

The intensity of the solar photosphere, which decreases at visible wavelengths from the center to the limb of the disk (see limb darkening), is due to the radiation emitted, principally by negative hydrogen ions (H), at depths of up to a few hundred kilometers. At higher levels, where the density of H ions is too low for appreciable opacity, the lower temperature gives rise to the absorption of radiation at discrete wavelengths. The Fraunhofer lines of the resulting absorption spectrum have provided the key to determining the chemical composition of the photosphere, because a direct comparison can be made with the laboratory spectra of known elements under various conditions.

Regions of the solar photosphere (and lower chromosphere) several thousand kilometers in diameter rhythmically rise and fall with a period of about 5 minutes over a time span of less than half an hour, attaining a maximum velocity of about 0.5 km s–1. These vertical oscillations are thought to be produced by the outward propagation of low-frequency sound waves, generated by turbulence in the convective zone. The internal vibrations of the Sun, the subject area of helioseismology, can reveal information on the solar interior.

See also faculae; granulation; supergranulation; sunspots.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
As a result, despite the realization that the spectrum of the K-corona implies that the corona is self-luminous and displays an apparent temperature no higher than that of the photosphere [2], advocates of the gaseous models of the Sun have no choice but to postulate that coronal apparent temperatures far exceed those of the solar surface.
The rigid rotation of the corona is highly suggestive that it possesses condensed matter whose associated magnetic field lines are anchored at the level of the photosphere. Such a structure, if endowed with a elevated electron affinity [2], would provide an elegant network for channeling electrons from the outer reaches of the solar atmosphere onto the photospheric surface.
Having in mind this proportion, the radiation temperatures at the radii of core and photosphere (and an arbitrary radius as well) can be expressed as
In this case, to determine temperature of the photosphere, one can use the well-known formula for thermal radiation power, considering core as a radiation source:
At the same time, Langley had previously measured the solar spectrum and was setting the temperature of the photosphere at ~6,000 K.
A temperature within the corona (>1.0x[10.sup.6] K) which exceeded that of the photosphere (~6,000 K) indicated a violation of the 2nd law of thermodynamics.
The photospheres give a 360o view of Welsh landmarks that include Raglan Castle, Tintern Abbey and the prehistoric burial chamber of Pentre Ifan.
The collection of work displayed through the newly-developed photographic medium known as the "photosphere" was officially launched yesterday at the Wales Millennium Centre, reflecting sites of Welsh heritage and landscape.
The horizontal mean free path--in other words, the average distance traveled without interacting --of a photon in the solar photosphere is about 50 km, and so this might be expected to be the smallest observable length scale, because of the smoothing effect of radiative energy transfer.
The problem for the gas models rests in their prediction that the photosphere has a density (~[10.sup.-7] g/[cm.sup.3] [148]) which is 10,000 times lower than that of the Earth's atmosphere at sea level--surpassing some of the best vacuums on Earth.
Herbert Spencer was the first to advance that the body of the Sun was gaseous [1], but he believed, much like Gustav Kirchhoff [2], that the photosphere was liquid [3,4].
The exterior surface of the photosphere being the limit that will attain the ascending currents which carry the phenomenon of incandescence in the superior layers, a very-admissible symmetry suffices in a globe where the most complete homogeneity must have freely established itself, to give to this limit surface the shape of a sphere, but a sphere that is incredibly uneven" [9].