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Pi¹ Pegasi
	Location of π2 Pegasi (circled)
Observation data; Epoch J2000.0 Equinox J2000.0
Constellation	Pegasus
Right ascension	22h 09m 13.61893s
Declination	+33° 10′ 20.4778″
Apparent magnitude (V)	+5.58
Characteristics
Spectral type	G8IIIb
B−V color index	+0.985±0.003
Astrometry
Radial velocity (Rv)	+5.1±0.9 km/s
Proper motion (μ)	RA: −48.117 mas/yr ; Dec.: −73.408 mas/yr
Parallax (π)	10.2111 ± 0.1137 mas
Distance	319 ± 4 ly ; (98 ± 1 pc)
Absolute magnitude (MV)	0.84
Details
Mass	2.48 M☉
Radius	11.00+0.51; −0.83 R☉
Luminosity	62.8±0.8 L☉
Surface gravity (log g)	2.7 cgs
Temperature	4,898+196; −110 K
Metallicity [Fe/H]	−0.22 dex
Rotational velocity (v sin i)	135 km/s
Age	530 Myr
Other designations
	π Peg, 27 Pegasi, BD+32°4349, HD 210459, HIP 109352, HR 8449, SAO 72064
Database references
SIMBAD	data

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]

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]

References

^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
^ ^a ^b ^c Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
^ ^a ^b Abt, H. A. (September 1985), "Visual multiples. VIII. 1000 MK types", Astrophysical Journal Supplement Series, 59: 95–112, Bibcode:1985ApJS...59...95A, doi:10.1086/191064
^ ^a ^b Gontcharov, G. A. (November 2006), "Pulkovo Compilation of Radial Velocities for 35495 Hipparcos stars in a common system", Astronomy Letters, 32 (11): 759–771, arXiv:1606.08053, Bibcode:2006AstL...32..759G, doi:10.1134/S1063773706110065, S2CID 119231169.
^ ^a ^b Takeda, Yoichi; et al. (August 2008), "Stellar Parameters and Elemental Abundances of Late-G Giants", Publications of the Astronomical Society of Japan, 60 (4): 781–802, arXiv:0805.2434, Bibcode:2008PASJ...60..781T, doi:10.1093/pasj/60.4.781.
^ ^a ^b ^c ^d Luck, R. Earle (2015), "Abundances in the Local Region. I. G and K Giants", The Astronomical Journal, 150 (3): 88, arXiv:1507.01466, Bibcode:2015AJ....150...88L, doi:10.1088/0004-6256/150/3/88, S2CID 118505114.
^ ^a ^b ^c Massarotti, Alessandro; et al. (January 2008), "Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity", The Astronomical Journal, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209, S2CID 121883397
^ "pi Peg". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2019-10-13.
^ Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869–879, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x, S2CID 14878976.
^ Schmitt, J. H. M. M.; et al. (March 1990), "X-ray studies of coeval star samples. III. X-ray emission in the UrsaMajor stream.", Astrophysical Journal, 351: 492–499, Bibcode:1990ApJ...351..492S, doi:10.1086/168486.
^ van Belle, Gerard T. (March 2012), "Interferometric observations of rapidly rotating stars", The Astronomy and Astrophysics Review, 20 (1): 51, arXiv:1204.2572, Bibcode:2012A&ARv..20...51V, doi:10.1007/s00159-012-0051-2, S2CID 119273474.
^ Hauck, B.; Jaschek, C. (February 2000), "A-shell stars in the Geneva system", Astronomy and Astrophysics, 354: 157–162, Bibcode:2000A&A...354..157H.
^ Slettebak, A. (September 1986), "H-alpha and near-infrared spectra of late-type Be and A-F-type shell stars.", Publications of the Astronomical Society of the Pacific, 98: 867–871, Bibcode:1986PASP...98..867S, doi:10.1086/131836.