G117-B15A[7] is a small, well-observed variable white dwarf star of the DAV, or ZZ Ceti, type in the constellation of Leo Minor.
| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) | |
|---|---|
| Constellation | Leo Minor |
| Right ascension | 09h 24m 15.25s[2] |
| Declination | +35° 16′ 51.4″[2] |
| Apparent magnitude (V) | 15.5[3] |
| Characteristics | |
| Spectral type | DAV4[3] |
| U−B color index | −0.6[3] |
| B−V color index | +0.2[3] |
| Variable type | DAV[3] |
| Astrometry | |
| Proper motion (μ) | RA: −145.181[2] mas/yr Dec.: −0.053[2] mas/yr |
| Parallax (π) | 17.3947 ± 0.0452 mas[2] |
| Distance | 187.5 ± 0.5 ly (57.5 ± 0.1 pc) |
| Absolute magnitude (MV) | 11.79[4] |
| Details | |
| Mass | 0.69[4] M☉ |
| Surface gravity (log g) | 8.14[4] cgs |
| Temperature | 12,400[5] K |
| Other designations | |
| Database references | |
| SIMBAD | data |
G117-B15A was found to be variable in 1974 by Richer and Ulrych,[8] and this was confirmed in 1976 by McGraw and Robinson.[9] 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.[7] Its age is estimated at 400 million years.[10] Its light curve has a dominant period of 215.2 seconds,[7] which is estimated to increase by approximately one second each 14 million years.[11] G117-B15A has been claimed to be the most stable optical clock ever found, much more stable than the ticks of an atomic clock.[12] It is also the first pulsating white dwarf to have its main pulsation mode index identified.[7]
An X-ray source in the constellation Leo Minor is the white dwarf G117-B15A.[13]