II Pegasi

II Pegasi

File:IIPegLightCurve.png A light curve for II Pegasi, plotted from Hipparcos data
Observation data Epoch J2000      Equinox J2000
Constellation	Pegasus
Right ascension	Template:RA
Declination	Template:DEC
Apparent magnitude (V)	7.4 (7.18–7.78)
Characteristics
II Pegasi A
Evolutionary stage	subgiant
Spectral type	K2 IV
Variable type	RS CVn
II Pegasi B
Evolutionary stage	main sequence
Spectral type	M0-M3 V
Astrometry
II Pegasi A
Radial velocity (Rv)	−20.50 km/s
Proper motion (μ)	RA: 576.22 mas/yr Dec.: 34.69 mas/yr
Parallax (π)	25.06±0.51 mas
Distance	Template:Rnd ± Template:Rnd ly (Template:Rnd ± Template:Rnd pc)
Absolute magnitude (MV)	3.8
Details
II Pegasi A
Mass	0.8 Template:Solar mass
Radius	3.4 Template:Solar radius
Luminosity	1.06 Template:Solar luminosity
Surface gravity (log g)	3.2 cgs
Temperature	4,600 K
II Pegasi B
Mass	0.4 Template:Solar mass
Luminosity	0.53 Template:Solar luminosity
Metallicity [Fe/H]	{{{metal_fe2}}} dex
Other designations
II Peg, GJ 4375, HD 224085, BD+27°4642, HIP 117915, LHS 4044, SAO 91578
Database references
SIMBAD	data

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.

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

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.

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. It was the largest stellar flare ever seen and was a hundred million times more energetic than the Sun's typical solar flare.