| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Pegasus |
| Right ascension | 23h 55m 04.05313s[2] |
| Declination | +28° 38′ 01.2422″[2] |
| Apparent magnitude (V) | 7.4[3](7.18–7.78[4]) |
| Characteristics | |
| II Pegasi A | |
| Evolutionary stage | subgiant[5] |
| Spectral type | K2 IV[5] |
| Variable type | RS CVn[4] |
| II Pegasi B | |
| Evolutionary stage | main sequence[5] |
| Spectral type | M0-M3 V[5] |
| Astrometry | |
| II Pegasi A | |
| Radial velocity (Rv) | −20.50[6] km/s |
| Proper motion (μ) | RA: 576.22[2]mas/yr Dec.: 34.69[2]mas/yr |
| Parallax (π) | 25.06±0.51 mas[2] |
| Distance | 130 ± 3 ly (39.9 ± 0.8 pc) |
| Absolute magnitude (MV) | 3.8[5] |
| Details | |
| II Pegasi A | |
| Mass | 0.8[5] M☉ |
| Radius | 3.4[5] R☉ |
| Luminosity | 1.06 L☉ |
| Surface gravity (log g) | 3.2[5] cgs |
| Temperature | 4,600[5] K |
| II Pegasi B | |
| Mass | 0.4[5] M☉ |
| Luminosity | 0.53 L☉ |
| Other designations | |
| II Peg,GJ 4375,HD 224085,BD+27°4642,HIP 117915,LHS 4044,SAO 91578 | |
| Database references | |
| SIMBAD | data |
II Pegasi is abinary star system in theconstellationPegasus with anapparent magnitude of 7.4 and a distance of 130light-years. It is a very activeRS Canum Venaticorum variable (RS CVn), a close binary system with activestarspots.
The primary (II Pegasi A) is a coolsubgiant, an orangeK-type star. It has begun toevolve off themain sequence and expand.[5] Starspots cover about 40% of its surface. The star produces intense flares observable at all wavelengths.[7]
Its smaller companion (II Pegasi B) is too close to be observed directly. It is ared dwarf, anM-type main-sequence star. The stars aretidally locked in a very close orbit with a period of 6.7 days and a separation of a few stellar radii.[5]
X-ray flares from II Pegasi A were observed with theAriel 5 satellite in the 1970s and with laterX-ray observatories. In December 2005, a superflare was detected by theSwift Gamma-Ray Burst Mission.[3] It was the largest stellar flare ever seen and was a hundred million times more energetic than theSun's typicalsolar flare.[8]
