II Pegasi
Star in the constellation Pegasus
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II Pegasi is a binary star system in the constellation Pegasus with an apparent magnitude of between 7.2 and 7.8 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.
| Observation data Epoch J2000 Equinox ICRS | |
|---|---|
| Constellation | Pegasus |
| Right ascension | 23h 55m 04.05313s[2] |
| Declination | +28° 38′ 01.2422″[2] |
| Apparent magnitude (V) | 7.18–7.78[3] |
| Characteristics | |
| 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] |
| Astrometry | |
| II Pegasi A | |
| Radial velocity (Rv) | −20.50[5] 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[4] |
| Details | |
| II Pegasi A | |
| Mass | 0.8[4] M☉ |
| Radius | 3.4[4] R☉ |
| Luminosity | 1.06 L☉ |
| Surface gravity (log g) | 3.2[4] cgs |
| Temperature | 4,600[4] K |
| II Pegasi B | |
| Mass | 0.4[4] 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 |
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 be 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.[7] It was the largest stellar flare ever seen and was a hundred million times more energetic than the Sun's typical solar flare.[8]