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 _O – M

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.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

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.

(optics)

luminosity factor

luminosity

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

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References in periodicals archive

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The procedures that the above four models commonly follow in the explanation of the SNeIa measurements include the following four steps: (1) Modifying the FE with an appropriate input of [LAMBDA], scalar field, perturbation, or external energy; (2) Determining the expansion rates (Hubble parameter) of the universe according to their modified FEs; (3) Submitting their expansion rates into the [D.sub.L] - Z relation; (4) Comparing the obtained redshift dependence of their luminosity distances with the SNeIa measurements.

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