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G 117-B15A
G117-B15A is a small, well-observed variable white dwarf star of the DAV, or ZZ Ceti, type in the constellation of Leo Minor.
G117-B15A was found to be variable in 1974 by Richer and Ulrych,〔(High-frequency optical variables. II. Luminosity-variable white dwarfs and maximum entropy spectral analysis ), H. B. Richer and T. J. Ulrych, ''Astrophysical Journal'' 192 (September 1974), pp. 719–730.〕 and this was confirmed in 1976 by McGraw and Robinson.〔(High-speed photometry of luminosity-variable DA dwarfs: R808, GD 99, and G 117-B15A ), J. T. McGraw and E. L. Robinson, ''Astrophysical Journal'' 205 (May 1976), pp. L155–L158.〕 In 1984 it was demonstrated that the star's variability is due to nonradial gravity wave 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.〔From Ṗ=2.3·10−15 in Kepler et al.〕 G117-B15A has been claimed to be the most stable optical clock ever found, much more stable than the ticks of an atomic clock. It is also the first pulsating white dwarf to have its main pulsation mode index identified.〔
==X-ray source==
An X-ray source in the constellation Leo Minor is the white dwarf G117-B15A.〔(【引用サイトリンク】title=Astronomers Find Most Stable Optical Clock In Heavens )〕
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