Eddington luminosity

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Eddington luminosity (sometimes also called the Eddington limit) is the largest luminosity that can pass through a layer of gas in hydrostatic equilibrium, supposing spherical symmetry. Using the mass-luminosity relation, it can be used to set limits on the maximum mass of a star.

Derivation

If the luminosity of a star exceeds the Eddington luminosity of a layer on the stellar surface, the gas layer is ejected from the star. This limit is obtained by setting the outward radiation pressure equal to the inward gravitational force. Both forces decrease by inverse square laws, so once equality is reached, the hydrodynamic flow is different throughout the star.

If the Thomson scattering cross-section is used and the gas is assumed to be purely made of ionized hydrogen, the Eddington Luminosity is given by

<math>L_{\rm Edd}=4\pi G M m_{\rm p} c/\sigma_{\rm T}\cong 1.3\times10^{38}\left(\frac{M}{M_\bigodot}\right){\rm erg\ s}^{-1}</math>

where <math>M</math> is the mass of the central object, <math>m_{\rm p}</math> is the mass of a proton and <math>\sigma_{\rm T}</math> is the Thomson cross-section.

The exact value of Eddington luminosity depends on the chemical composition of the gas layer and the spectral energy distribution of the emission. Gas with cosmological abundances of hydrogen and helium is much more transparent than gas with solar abundance ratios. Atomic line transitions can greatly increase the effects of radiation pressure, and line driven winds exist in some bright stars.

Examples

Gamma ray bursts, novae and supernovae are examples of systems exceeding their Eddington luminosity by a large factor for very short times. In those cases, the result is a radical change in physical structure (namely, the ejection of a fraction of the star's mass).

Some X-ray binaries and active galaxies are able to maintain luminosities close to the Eddington limit for very long times.

References

• {{cite book

| author=Juhan Frank, Andrew King, Derek Raine
| title=Accretion Power in Astrophysics
| publisher=Cambridge University Press
| edition=Third Edition
| year=2002
| id =ISBN 0521629578}}de:Eddington-Grenze

it:Limite di Eddington pl:Jasność Eddingtona pt:Limite de Eddington sl:Eddingtonova meja fi:Eddingtonin raja