Surface gravity describes the gravitational acceleration experienced at or very close to the surface of an astronomical body. Conceptually it is the acceleration that a negligible-mass test particle would acquire under the object's gravity without significantly altering the source mass. Surface gravity is a local property of an object and is useful when comparing the apparent strength of gravity at different planets, stars, moons, or other compact objects.
Definition and basic formula
For an approximately spherical, non-rotating body of mass M and radius R, the surface gravity g is commonly estimated by the Newtonian expression g = GM/R^2, where G is the gravitational constant. This formula gives the magnitude of the acceleration a stationary test mass would feel at the reference surface. In real bodies, rotation, oblateness, tidal forces, and internal mass distribution introduce variations: equatorial points on a rapidly rotating planet experience a slightly lower effective gravity because of centrifugal force, and irregularly shaped asteroids have widely varying local accelerations.
Units, notation and log g
Surface gravity is measured as an acceleration. In SI units it is expressed in metres per second squared (m/s^2) and is often compared to Earth's standard gravity (g = 9.80665 m/s^2). In stellar and spectroscopic contexts it is common to use cgs units (cm/s^2) and to report log g, the base‑10 logarithm of g in those cgs units, because g values span many orders of magnitude and the logarithm simplifies classification and comparison. See a concise reference on units and conversions here and an explanation of log g usage here.
How surface gravity is measured
There are several practical approaches to determining surface gravity. For planets and moons visited by spacecraft, g may be calculated from direct measurements of mass and radius, or from tracking Doppler shifts in spacecraft motion caused by the body's gravity. On distant stars, g is inferred from spectral features: pressure-broadened absorption lines and ionization balances depend on atmospheric pressure and hence surface gravity. Terrestrial experiments (for example, time-of-fall or pendulum measurements) determine local g on Earth; see observational techniques here and spacecraft methods here.
Importance and examples
Surface gravity affects many physical and observational properties. On planets it controls surface weight, the scale height of an atmosphere, escape velocity, and the long-term ability to retain volatile gases. In stellar astrophysics, differences in surface gravity distinguish dwarfs, giants and supergiants because pressure and density in the photosphere alter spectral line strengths. For everyday comparison, Earth’s surface gravity provides a convenient reference point; comparative data and typical values are summarized in many planetary tables here.
Special cases and notable distinctions
Not all objects have a single well-defined 'surface.' Gas giants lack a solid surface, so astronomers often adopt a reference level (for example, the 1-bar pressure level) when quoting g. Compact remnants such as white dwarfs and neutron stars have extremely large surface gravities, influencing light propagation and spectral formation near their surfaces. For black holes there is no material surface; however, a concept called the surface gravity of the event horizon is defined in general relativity and enters formulas for black hole temperature and thermodynamics. For technical summaries and theoretical context see advanced treatments here and here.