Pi1 Pegasi

For other stars with this Bayer designation see π Pegasi

Pi¹ Pegasi, Latinized from π¹ Pegasi, is a star in the constellation Pegasus. Based upon changes to the proper motion of the visible component, this is a probable astrometric binary.^[9] It has a yellow hue and is dimply visible to the naked eye with a combined apparent visual magnitude of +5.58.^[2] The system is located approximately 319 light years distant from the Sun based on parallax,^[1] and is drifting further away with a radial velocity of +5 km/s.^[4] It is a member of the Ursa Major Moving Group of co-moving stars.^[10]

π¹ Pegasi (right) and π² Pegasi (left) in optical light

Pi¹ Pegasi

Location of π2 Pegasi (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Pegasus
Right ascension	22h 09m 13.61893s[1]
Declination	+33° 10′ 20.4778″[1]
Apparent magnitude (V)	+5.58[2]
Characteristics
Spectral type	G8IIIb[3]
B−V color index	+0.985±0.003[2]
Astrometry
Radial velocity (Rv)	+5.1±0.9[4] km/s
Proper motion (μ)	RA: −48.117[1] mas/yr Dec.: −73.408[1] mas/yr
Parallax (π)	10.2111±0.1137 mas[1]
Distance	319 ± 4 ly (98 ± 1 pc)
Absolute magnitude (MV)	0.84[5]
Details
Mass	2.48[6] M☉
Radius	11.00+0.51 −0.83[1] R☉
Luminosity	62.8±0.8[1] L☉
Surface gravity (log g)	2.7[7] cgs
Temperature	4,898+196 −110[1] K
Metallicity [Fe/H]	−0.22[5] dex
Rotational velocity (v sin i)	135[7] km/s
Age	530[6] Myr
Other designations
π Peg, 27 Pegasi, BD+32°4349, HD 210459, HIP 109352, HR 8449, SAO 72064[8]
Database references
SIMBAD	data

The visible component is an aging giant star with a stellar classification of G8IIIb.^[3] It has a high rate of spin, with a projected rotational velocity of 135 km/s.^[7] This is giving it an equatorial bulge that is 17% larger than the polar radius.^[11] It is a shell star, being orbited by a circumstellar shell of cooler gas.^[12][13] This star is 530^[6] million years old with 2.5^[6] times the mass of the Sun. With the supply of hydrogen exhausted at its core, the star has cooled and expanded to 11^[1] times the Sun's radius. It is radiating 63^[1] times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 4,898 K.^[1]