luminosity


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luminosity,

in astronomy, the rate at which energy of all types is radiated by an object in all directions. A star's luminosity depends on its size and its temperature, varying as the square of the radius and the fourth power of the absolute surface temperature. The sun is a medium-sized star with a luminosity of 3.8×1033 ergs per sec. The luminosities of other stars are commonly expressed in terms of the sun's luminosity. The known luminosities of stable stars range from about 1/1,000,000 that of the sun for a relatively cool white dwarfwhite dwarf,
in astronomy, a type of star that is abnormally faint for its white-hot temperature (see mass-luminosity relation). Typically, a white dwarf star has the mass of the sun and the radius of the earth but does not emit enough light or other radiation to be easily
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 to about 1,000,000 times that of the sun for the hottest known supergiant star. See magnitudemagnitude,
in astronomy, measure of the brightness of a star or other celestial object. The stars cataloged by Ptolemy (2d cent. A.D.), all visible with the unaided eye, were ranked on a brightness scale such that the brightest stars were of 1st magnitude and the dimmest stars
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; mass-luminosity relationmass-luminosity relation,
in astronomy, law stating that the luminosity of a star is proportional to some power of the mass of the star. More massive stars are in general more luminous.
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; stellar evolutionstellar evolution,
life history of a star, beginning with its condensation out of the interstellar gas (see interstellar matter) and ending, sometimes catastrophically, when the star has exhausted its nuclear fuel or can no longer adjust itself to a stable configuration.
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.

luminosity

(loo-mă-noss -ă-tee) Symbol: L . The intrinsic or absolute brightness of a star or other celestial body, equal to the total energy radiated per second from the body, i.e. the total outflow of radiant flux. The luminosity of a body may be calculated over all wavelengths – the bolometric luminosity – or at particular wavebands. Bolometric luminosity is related to the body's surface area and effective temperature, T eff, by a form of Stefan's law:
L = 4πR 2σT eff 4
where σ is Stefan's constant and R is the radius. Thus two stars with similar T eff (i.e. of the same spectral type) but greatly different luminosities must differ in size: they belong to different luminosity classes within that spectral type, as determined from their spectra. In the luminosity class of main-sequence stars, luminosity decreases as temperature decreases; the luminosity of giant stars however increases with decreasing temperature: red giants are much brighter than yellow giants; the luminosity of supergiants drops and then rises with decreasing temperature. The luminosity of stars is in theory dependent on mass if their chemical composition is similar. It has been found that with the notable exception of highly evolved stars the mass-luminosity relation is obeyed approximately.

The luminosity of a star or other body can be expressed as a multiple or fraction of the Sun's luminosity, L O, which is equal to 3.83 × 1026 watts. The ratio L/L O is given by

2.5 log10 (L/L O) = M OM
where M O and M are the absolute bolometric magnitudes of Sun and star; M O is equal to 4.76. There is a very great range in stellar luminosity from about a million times to less than one ten thousandth that of the Sun's luminosity.

luminosity

[‚lü·mə′näs·əd·ē]
(nucleonics)
A measure of the performance of a colliding-beam system, equal to the reaction rate or number or interactions per second divided by the interaction cross section.

luminosity

The ratio of luminous flux to the corresponding radiant flux at a particular wavelength; expressed in lumens per watt.
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