The surface gravity, g, of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration due to gravity experienced by a hypothetical test particle which is very close to the object's surface and which, in order not to disturb the system, has negligible mass. For objects where the surface is deep in the atmosphere and the radius not known, the surface gravity is given at the 1 bar pressure level in the atmosphere.
Surface gravity is measured in units of acceleration, which, in the SI system, are meters per second squared. It may also be expressed as a multiple of the Earth's standard surface gravity, which is equal to[1]
g = 9.80665 m/s2
In astrophysics, the surface gravity may be expressed as log g, which is obtained by first expressing the gravity in cgs units, where the unit of acceleration and surface gravity is centimeters per second squared (cm/s2), and then taking the base-10 logarithm of the cgs value of the surface gravity.[2] Therefore, the surface gravity of Earth could be expressed in cgs units as 980.665 cm/s2, and then taking the base-10 logarithm ("log g") of 980.665, and we get 2.992 as "log g".
The surface gravity of a white dwarf is very high, and of a neutron star even higher. A white dwarf's surface gravity is around 100,000 g (106 m/s2) whilst the neutron star's compactness gives it a surface gravity of up to 7×1012 m/s2 with typical values of order 1012 m/s2 (that is more than 1011 times that of Earth). One measure of such immense gravity is that neutron stars have an escape velocity of around 100,000 km/s, about a third of the speed of light. For black holes, the surface gravity must be calculated relativistically.
^Taylor, Barry N., ed. (2001). The International System of Units (SI)(PDF). United States Department of Commerce: National Institute of Standards and Technology. p. 29. Retrieved 2012-03-08. {{cite book}}: |work= ignored (help)
^Smalley, B. (13 July 2006). "The Determination of Teff and log g for B to G stars". Keele University. Retrieved 31 May 2007.
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