Encyclopedia of Astrobiology

Living Edition
| Editors: Muriel Gargaud, William M. Irvine, Ricardo Amils, Henderson James Cleaves, Daniele Pinti, José Cernicharo Quintanilla, Michel Viso

Hydrostatic Equilibrium

  • Daniel Rouan
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-27833-4_758-2



Hydrostatic equilibrium designates a stable state of a fluid where there is a condition of balance between gravitational force and pressure gradient. In a planetary atmosphere, under simplifying assumptions (perfect gas, isothermal), the hydrostatic equilibrium condition leads to a law of pressure (p) variation with altitude (z) that is exponential: p = p0 exp(−z/z0), where z0 is the scale height (8 km on Earth). In a star, the radiation pressure creates an additional outward force that participates in the hydrostatic equilibrium equation. Note that hydrostatic equilibrium implies that the star’s or planet’s shape is a sphere in the simplest case and an oblate spheroid for a rotating body.

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© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.Observatoire Paris-Site de MeudonLESIAMeudonFrance