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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.

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] (7.18 - 7.78[3])
II Pegasi A
Evolutionary stage subgiant[4]
Spectral type K2 IV[4]
Variable type RS CVn[3]
II Pegasi B
Evolutionary stage main sequence[4]
Spectral type M0-M3 V[4]
II Pegasi A
Radial velocity (Rv)-20.50[5] 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
Distance130 ± 3 ly
(39.9 ± 0.8 pc)
Absolute magnitude (MV)3.8[4]
II Pegasi A
Mass0.8[4] M
Radius3.4[4] R
Luminosity1.06 L
Surface gravity (log g)3.2[4] cgs
Temperature4600[4] K
II Pegasi B
Mass0.4[4] M
Luminosity0.53 L
Other designations
II Peg, GJ 4375, HD 224085, BD+27°4642, HIP 117915, LHS 4044, SAO 91578
Database references

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.[4] Starspots cover about 40% of its surface. The star produces intense flares observable at all wavelengths.[6]

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.[4]

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.[7]


  1. ^ a b c d e 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. ^ 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 Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
  4. ^ 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.
  5. ^ Karataş, Y.; Bilir, S.; Eker, Z.; Demircan, O. (2004). "Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution". Monthly Notices of the Royal Astronomical Society. 349 (3): 1069. arXiv:astro-ph/0404219. Bibcode:2004MNRAS.349.1069K. doi:10.1111/j.1365-2966.2004.07588.x.
  6. ^ 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.
  7. ^ Wanjek, Christopher (6 November 2006). "Monster Stellar Flare Seen by NASA Scientists Dwarfs All Others". National Aeronautics and Space Administration. Retrieved 31 March 2014.