limb darkeningastronomy
Main
in astrophysics, gradual decrease in brightness of the disk of the Sun or of another star as observed from its centre to its edge, or limb. This phenomenon is readily apparent in photographs of the Sun. The darkening is greatest for blue light, amounting to a drop of as much as 90 percent from the Sun’s photosphere to its outer atmospheric regions. Such limb darkening occurs because the solar atmosphere increases in temperature with depth. At the centre of the solar disk, an observer sees the deepest and warmest layers that emit the most light. At the limb, only the upper, cooler layers that produce less light can be seen. Observations of solar limb darkening are used to determine the temperature structure of the Sun’s atmosphere. Information derived from such observations is applied in studying other stars.
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Aspects of this topic are discussed in the following places at Britannica.
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eclipse of binary stars
( in eclipse: Eclipsing binary stars )
...Sun. Other stars flare in brightness as mass is exchanged from one component to the other. Rapid rotation of some stars flattens their shapes into ellipsoids. Even the long-known solar phenomenon of limb darkening, the gradual decrease in brightness from the centre to the edge of the Sun’s disk, has been detected in the component stars of eclipsing binaries.
- solar surface ( in Sun: History of observation )
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stellar processes
( in star: Stellar atmospheres )
...in brightness depends on the wavelength of the radiation detected; it is large in violet light, is small in red light, and nearly vanishes when the Sun is imaged in infrared radiation. This limb darkening arises because the Sun becomes hotter toward its core. At the centre of the disk, radiation is received from deeper and hotter layers (on average) instead of from the limb, and the...
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in astrophysics, gradual decrease in brightness of the disk of the Sun or of another star as observed from its centre to its edge, or limb. This phenomenon is readily apparent in photographs of the Sun. The darkening is greatest for blue light, amounting to a drop of as much as 90 percent from the Sun’s photosphere to its outer atmospheric regions. Such limb darkening occurs because the solar atmosphere increases in temperature with depth. At the centre of the solar disk, an observer sees the deepest and warmest layers that emit the most light. At the limb, only the upper, cooler layers that produce less light can be seen. Observations of solar limb darkening are used to determine the temperature structure of the Sun’s atmosphere. Information derived from such observations is applied in studying other stars.
Aspects of this topic are discussed in the following places at Britannica.
-
eclipse of binary stars eclipse
...Sun. Other stars flare in brightness as mass is exchanged from one component to the other. Rapid rotation of some stars flattens their shapes into ellipsoids. Even the long-known solar phenomenon of limb darkening, the gradual decrease in brightness from the centre to the edge of the Sun’s disk, has been detected in the component stars of eclipsing binaries.
- solar surface Sun
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stellar processes star
...in brightness depends on the wavelength of the radiation detected; it is large in violet light, is small in red light, and nearly vanishes when the Sun is imaged in infrared radiation. This limb darkening arises because the Sun becomes hotter toward its core. At the centre of the disk, radiation is received from deeper and hotter layers (on average) instead of from the limb, and...
visible surface of the Sun, from which is emitted most of the Sun’s light that reaches Earth directly. Since the Sun is so far away, the edge of the photosphere appears sharp to the naked eye, but in reality the Sun has no surface, since it is too hot for matter to exist in anything but a plasma state—that is, as a gas composed of ionized atoms. Scientists consider the “surface” of the Sun to be the region above which most photons (the quantum carriers of light energy) escape. The photosphere is thus a layer some 400 km (250 miles) thick. The temperatures in this layer range from 4,400 kelvins (K; 4,100 °C, or 7,400 °F) at the top to 10,000 K (9,700 °C, or 17,500 °F) at the bottom. Photons generated deeper than this cannot get out without absorption and reemission. The density of the ionized gas is about 1/1,000 that of air at Earth’s surface, but it is much more opaque, because of strong absorption of light by the hydrogen ions.
A low-resolution image of the photosphere shows little structure except for a darkening toward the outermost regions, called limb darkening. Near the edge, light comes from higher up in the photosphere, where the temperature is lower and the radiation weaker. This allows measurement of the temperature gradient.
Large-scale images of the photosphere show a granular structure. Each granule, or cell, is a mass of hot gas 1,000 km (600 miles) in diameter; the granules rise because of convection inside the Sun, radiate energy, and sink back within a few minutes to be replaced by other granules in a constantly changing pattern.
From measurement of magnetic fields and motions, a coarse pattern of supergranules, each some 30,000...