G117-B15A is a small, well-observed

variable Variable may refer to: * Variable (computer science), a symbolic name associated with a value and whose associated value may be changed * Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many ...

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

of the DAV, or ZZ Ceti, type in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

of Leo Minor. G117-B15A was found to be variable in 1974 by Richer and Ulrych, and this was confirmed in 1976 by McGraw and Robinson. In 1984 it was demonstrated that the star's variability is due to nonradial

gravity wave In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media when the force of gravity or buoyancy tries to restore equilibrium. An example of such an interface is that between the atmosphere ...

pulsations. As a consequence, its timescale for period change is directly proportional to its cooling timescale, allowing its cooling rate to be measured using astroseismological techniques. Its age is estimated at 400 million years. Its light curve has a dominant period of 215.2 seconds, which is estimated to increase by approximately one second each 14 million years. G117-B15A has been claimed to be the most stable optical clock ever found, much more stable than the ticks of an

atomic clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...

. It is also the first pulsating white dwarf to have its main pulsation mode index identified. An X-ray source in the

Leo Minor is the white dwarf G117-B15A.

Notes

See also