Surface Gravity

The surface gravity, g, of an astronomical or other object is the gravitational acceleration experienced at its surface. 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.

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, 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 is centimeters per second squared, and then taking the base 10 logarithm. Therefore, as gravity affects all things equally, regardless of their mass in grams or kilograms, and because 1 m/s2 = 100 cm/s2, the surface gravity of Earth could be expressed in cgs units as 980.665 cm/s2 and at base 10 logarithm (log g) as 2.992.

The surface gravity of a white dwarf is very high, and of a neutron star even more. The neutron star's compactness gives it a surface gravity of up to 7×1012 m/s² with typical values of a few ×1012 m/s² (that is more than 1011 times that of Earth). One measure of such immense gravity is the fact that neutron stars have an escape velocity of around 100,000 km/s, about a third of the speed of light.