Pi1 Cygni

Pi¹ Cygni, also named Azelfafage /əˈzɛlfəfeɪdʒ/, is a star in the northern constellation of Cygnus. It is visible to the naked eye, having a combined apparent visual magnitude of 4.66.^[2] The distance to this system can be roughly gauged by its annual parallax shift of 1.89 mas,^[1] which yields a separation of around 1,700 light years from the Sun, give or take a hundred light years.

For other stars with this Bayer designation, see π Cygni.

Pi¹ Cygni

Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Cygnus
Right ascension	21h 42m 05.66458s[1]
Declination	+51° 11′ 22.6415″[1]
Apparent magnitude (V)	4.66[2]
Characteristics
Spectral type	B3 IV[3]
B−V color index	−0.11[2]
Astrometry
Proper motion (μ)	RA: +5.29[1] mas/yr Dec.: −1.78[1] mas/yr
Parallax (π)	1.89±0.15 mas[1]
Distance	1,700 ± 100 ly (530 ± 40 pc)
Absolute magnitude (MV)	−3.91[4]
Orbit[5]
Period (P)	26.33 d
Eccentricity (e)	0.00
Periastron epoch (T)	2431306.5±10.0 JD
Argument of periastron (ω) (secondary)	0.00°
Semi-amplitude (K1) (primary)	16.5 km/s
Details
π1 Cyg A
Mass	10.1±0.1[3] M☉
Radius	5.6[6] R☉
Luminosity	16,538[7] L☉
Temperature	18,360±1,100[2] K
Rotational velocity (v sin i)	55[8] km/s
Age	25.1±0.8[3] Myr
Other designations
Azelfafage, π1 Cyg, 80 Cyg, BD+50°3410, FK5 3733, HD 206672, HIP 107136, HR 8301, SAO 33665[9]
Database references
SIMBAD	data

Nomenclature

π¹ Cygni (Latinised to Pi¹ Cygni, abbreviated Pi¹ Cyg, π¹ Cyg) is the star's Bayer designation. The designations of the two components as Pi¹ Cygni A and B derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).^[10]

It bore the traditional name Azelfafage, derived from the Arabic al-sulaḥfāh, the tortoise, originally a name for Lyra that was misattributed to a star in Cygnus.^[11][12] R. H. Allen's 1899 book Star Names incorrectly^[11] states that the name is derived from ظلف الفرس Dhilf al-faras meaning "the horse track" or (probably) ذيل الدجاجة Dhail al-dajāja^h meaning "the tail of the hen".^[13] In 2016, the IAU organized a Working Group on Star Names (WGSN)^[14] to catalogue and standardize proper names for stars. The WGSN approved the name Azelfafage for Pi¹ Cygni on 12 September 2016 and it is now so included in the List of IAU-approved Star Names.^[15]

In Chinese, 螣蛇 (Téng Shé), meaning Flying Serpent, refers to an asterism consisting of Pi¹ Cygni, Alpha Lacertae, 4 Lacertae, Pi² Cygni, HD 206267, Epsilon Cephei, Beta Lacertae, Sigma Cassiopeiae, Rho Cassiopeiae, Tau Cassiopeiae, AR Cassiopeiae, 9 Lacertae, 3 Andromedae, 7 Andromedae, 8 Andromedae, Lambda Andromedae, Kappa Andromedae, Psi Andromedae and Iota Andromedae. Consequently, the Chinese name for Pi¹ Cygni itself is 螣蛇四 (Téng Shé sì, English: the Fourth Star of Flying Serpent).^[16]

Properties

Pi¹ Cygni has been described as a single-lined spectroscopic binary with a close, circular orbit, having a period of just 26.33 days.^[5] However, a 2020 paper finds no radial velocity variations on the timescale of the published orbit.^[17]

The primary, visible, star is a somewhat evolved B-type subgiant star with a stellar classification of B3 IV.^[3] It has an estimated 10 times the mass of the Sun and around 5.6^[6] times the Sun's radius. The star radiates 16,538^[7] times the solar luminosity from its outer atmosphere at an effective temperature of roughly 18,360 K.^[2] It is about 25^[3] million years old and is spinning with a projected rotational velocity of 55 km/s.^[8]