21 Aquilae

21 Aquilae is a solitary^[14] variable star in the equatorial constellation of Aquila. It has the variable star designation V1288 Aql; 21 Aquilae is its Flamsteed designation. This object is visible to the naked eye as a dim, blue-white hued star with a baseline apparent visual magnitude of about 5.1. The star is located at a distance of around 680 light-years (210 parsecs) from Earth, give or take a 20 light-year margin of error. It is moving closer to the Earth with a heliocentric radial velocity of –5 km/s.^[6]

21 Aquilae

An ultraviolet band light curve for V1288 Aquilae, adapted from Veto (1980)[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Aquila
Right ascension	19h 13m 42.70120s[2]
Declination	+02° 17′ 37.3103″[2]
Apparent magnitude (V)	5.06 - 5.16[3]
Characteristics
Spectral type	B8II-III(Hg?)[4]
U−B color index	−0.399[5]
B−V color index	−0.065[5]
Variable type	α2 CVn[3]
Astrometry
Radial velocity (Rv)	−5.2[6] km/s
Proper motion (μ)	RA: +10.102[2] mas/yr Dec.: −0.825[2] mas/yr
Parallax (π)	4.7730±0.1052 mas[2]
Distance	680 ± 20 ly (210 ± 5 pc)
Absolute magnitude (MV)	−2.12[7]
Details
Mass	6.7[8] M☉
Radius	6.2[9] R☉
Luminosity	704[8] L☉
Surface gravity (log g)	3.95[8] cgs
Temperature	12,014[8] K
Metallicity [Fe/H]	−0.20[10] dex
Rotation	9.3 d[11]
Rotational velocity (v sin i)	17[10] km/s
Age	165[12] Myr
Other designations
21 Aql, V1288 Aql, BD+02°3824, FK5 3537, HD 179761, HIP 94477, HR 7287, SAO 124408, WDS J19137+0218A[13]
Database references
SIMBAD	data

In 1962, Helmut A. Abt and John C. Gloson published data showing that 21 Aquilae was a variable star.^[15] Based on that publication, the star was given its variable star designation in 1972.^[16]

The stellar classification of this star is B8 II-III, with the luminosity class of II-III suggesting that the spectrum displays elements of both a giant star and a bright giant. It is a chemically peculiar star of the Mercury-Manganese type (CP3),^[10] although some catalogues consider that status to be doubtful.^[17] This is a probable Alpha² Canum Venaticorum variable that ranges in visual magnitude from 5.06 down to 5.16.^[3] The star is radiating 704 times the luminosity of the Sun from its photosphere at an effective temperature of 12,014 K; this gives it the blue-white glow of a B-type star.^[18]

21 Aquilae is catalogued as an optical double star, having a 12th magnitude companion 37″ away as of 2010. It was first identified as a double star by John Herschel.^[19] The companion is a distant background object.^[20]