FG Sagittae

FG Sagittae, is a supergiant star in the constellation Sagitta at a distance of 8000 light-years. When first noted in 1943, it was identified to be a variable star, and its spectrum was registered as of B4Ieq ("blue") in 1955. Since then it has expanded and reddened to G ("yellow") in 1991,[5] and then further to about K ("orange"). FG started to pulsate when becoming an A-type star (soon after being registered as B4Ieq) with a period of 15 days, now the period has increased to over 100 days.

FG Sagittae
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Sagitta
Right ascension  20h 11m 56.05947s[1]
Declination +20° 20′ 04.3672″[1]
Apparent magnitude (V) 9.45[2]
Astrometry
Parallax (π)0.7630 ± 0.2302[3] mas
Distanceapprox. 4,000 ly
(approx. 1,300 pc)
Characteristics
Spectral type B4Ieq-K2Ib[4]
U−B color index +0.75[2]
B−V color index +1.21[2]
Other designations
FG Sge, HIP 99527, PN ARO 169, ALS 10924, IRAS 20097+2010, PN G060.3-07.3, AN 377.1943, Hen 3-1844, JP11 5474, CSI+20-20097, Hen 2-457, LS II +20 19, TYC 1626-619-1, CSV 5066, Hen 1-5, 2MASS J20115606+2020044, UBV M 50884, PK 060-07 1, AAVSO 2007+20
Database references
SIMBADdata

FG Sagittae is the central star of the planetary nebula Henize 1-5.

Since 1992 the star has exhibited fadings and recoveries similar to that of a R Coronae Borealis variable star; this behavior is emphasized by a hydrogen deficiency typical for this class of stars.[6]

It has been proposed that this star has undergone a "late thermal pulse" (LTP) of helium fusion after having left the asymptotic giant branch (AGB) to move towards the hottest end of the "white dwarf cooling track". This thermal pulse is believed to have revived this aged star to once again, for a short time, behave as an AGB star.[6]

See also

References
  1. Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
  2. Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237. Bibcode:2002yCat.2237....0D.
  3. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  4. Shenavrin, V. I.; Taranova, O. G.; Nadzhip, A. E. (2011). "Search for and study of hot circumstellar dust envelopes". Astronomy Reports. 55: 31. Bibcode:2011ARep...55...31S. doi:10.1134/S1063772911010070.
  5. "Life and times of a chameleon star". Newscientist. Retrieved 2017-09-27.
  6. Lawlor, T. M; MacDonald, J (2003). "Sakurai's Object, V605 Aquilae, and FG Sagittae: An Evolutionary Sequence Revealed". The Astrophysical Journal. 583 (2): 913. Bibcode:2003ApJ...583..913L. doi:10.1086/345411.
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