G117-B15A

Starbox begin
name= [http://simbad.u-strasbg.fr/simbad/sim-id?Ident=G117-B15A G117-B15A] Starbox observe
epoch=J2000.0 (ICRS)
constell=Leo Minor
ra=RA|09|24|16 [http://adsabs.harvard.edu/abs/1999ApJS..121....1M A Catalog of Spectroscopically Identified White Dwarfs] , George P. McCook and Edward M. Sion, "Astrophysical Journal Supplement" 121, #1 (March 1999), pp. 1–130. CDS ID [http://vizier.u-strasbg.fr/viz-bin/Cat?III/210 III/210] . Astrometric data updated to J2000.0.]
dec=DEC|+35|16.9
appmag_v=15.5Starbox character
u-b=-0.6
b-v=0.2
class=DAV4
variable=DAV Starbox astrometry
prop_mo_dec=22SIMBAD. Accessed June 14, 2007.]
prop_mo_ra=-136 Starbox catalog
names=RY LMi, RY Leonis Minoris, EGGR 65, PG 0921+355, WD 0921+354.

G117-B15Acite journal | last = Kepler| first = S. O. | coauthors = et al.| title = Evolutionary Timescale of the Pulsating White Dwarf G117-B15A: The Most Stable Optical Clock Known | journal = The Astrophysical Journal Letters | volume = 534 | pages = L185-L188 | date = 2000 May 10 | url = http://www.journals.uchicago.edu/doi/full/10.1086/312664 | doi = 10.1086/312664 | bibcode = 2000ApJ...534L.185K | accessdate = 2008-08-20] 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 [ [http://adsabs.harvard.edu/abs/1974ApJ...192..719R 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. [ [http://adsabs.harvard.edu/cgi-bin/bib_query?1976ApJ...205L.155M 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. [cite web | last = Pivetta | first = Marcos| authorlink = | coauthors = | title =The star of the moment | work = | publisher = | date = January 2006| url = http://www.revistapesquisa.fapesp.br/?art=1620&bd=1&pg=1&lg=en| format =| doi = | accessdate = 2007-06-06] 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 =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. [cite web | last = | first = | authorlink = | coauthors = | title =Astronomers Find Most Stable Optical Clock in Heavens; Aids Understanding of Stars' Lives | work = | publisher = McDonald Observatory| date = | url = http://www.spaceref.ca/news/viewpr.html?pid=18424| format =| doi = | accessdate = 2007-06-06] It is also the first pulsating white dwarf to have its main pulsation mode index identified.

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*Ross 548