II Pegasi

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II Pegasi
Observation data
Epoch J2000      Equinox J2000
Constellation Pegasus
Right ascension 23h 55m 04.05313s[1]
Declination +28° 38' 01.2422"'[1]
Apparent magnitude (V) 7.4[2]
II Pegasi A
Evolutionary stage subgiant[3]
Spectral type K2 IV[3]
Variable type RS CVn[1]
II Pegasi B
Evolutionary stage main sequence[3]
Spectral type M0-M3 V[3]
II Pegasi A
Radial velocity (Rv) -20.50[1] km/s
Proper motion (μ) RA: 576.22[1] mas/yr
Dec.: 34.69[1] mas/yr
Parallax (π) 25.06 ± 0.51[1] mas
Distance 130 ± 3 ly
(39.9 ± 0.8 pc)
Absolute magnitude (MV) 3.8[3]
II Pegasi A
Mass 0.8[3] M
Radius 3.4[3] R
Luminosity 1.06 L
Surface gravity (log g) 3.2[3] cgs
Temperature 4600[3] K
II Pegasi B
Mass 0.4[3] M
Luminosity 0.53 L
Other designations
V* II Peg, GJ 4375, HD 224085, BD+27° 4642, HIP 117915, LHS 4044, SAO 91578[1]
Database references

II Pegasi is a binary star system in the constellation of Pegasus with an apparent magnitude of 7.4 and a distance of 130 light years. It is a very active RS Canum Venaticorum variable (RS CVn), a close binary system with active starspots.[1]

The primary (II Pegasi A) is a cool subgiant, an orange K-type star. It has begun to evolve off the main sequence and expand.[3] Starspots cover about 40% of its surface. The star produces intense flares observable at all wavelengths.[4]

Its smaller companion (II Pegasi B) is too close to have been observed directly. It is a red dwarf, an M-type main sequence star. The stars are tidally locked in a very close orbit with a period of 6.7 days and a separation of a few stellar radii.[3]

X-ray flares from II Pegasi A were observed with the Ariel 5 satellite in the 1970s and with later x-ray observatories. In December 2005 a superflare was detected by the Swift Gamma-Ray Burst Mission.[2] It was the largest stellar flare ever seen and was a hundred million times more energetic than The Sun's typical solar flare.[5]


  1. ^ a b c d e f g h i "Basic data: V* II Peg — Variable of RS CVn type". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 4 April 2014. 
  2. ^ a b Osten, Rachel A.; Drake, Stephen; Tueller, Jack; Cummings, Jay; Perri, Matteo; Moretti, Alberto; Covino, Stefano (2007). "Nonthermal Hard X-Ray Emission and Iron Ka Emission from a Superflare on II Pegasi". The Astrophysical Journal 654 (2): 1052–1067. arXiv:astro-ph/0609205. Bibcode:2007ApJ...654.1052O. doi:10.1086/509252. 
  3. ^ a b c d e f g h i j k l Berdyugina, Svetlana V.; Jankov, S.; Ilyin, I.; Tuominen, I.; Fekel, F. C. (1998). "The active RS Canum Venaticorum binary II Pegasi. I. Stellar and orbital parameters". Astronomy and Astrophysics 334: 863–872. Bibcode:1998A&A...334..863B. 
  4. ^ Covino, S.; Tagliaferri, G.; Pallavicini, R.; Mewe, R.; Poretti, E. (2000). "The active binary star II Pegasi with it BeppoSAX". Astronomy and Astrophysics 355 (2): 681–687. arXiv:astro-ph/9911352. Bibcode:2000A&A...355..681C. 
  5. ^ Wanjek, Christopher (6 November 2006). "Monster Stellar Flare Seen by NASA Scientists Dwarfs All Others". National Aeronautics and Space Administration. Retrieved 31 March 2014.