Pi Cephei

Pi Cephei (π Cephei) is a trinary star located in the constellation Cepheus.^[6] With a combined apparent magnitude of about 4.4,^[2] the system is faintly visible to the naked eye. The inner pair of stars orbits in 1.5 years while the outer companion completes one orbit in about 160 years.^[6]

π Cephei

Observation data Epoch J2000      Equinox J2000
Constellation	Cepheus
Right ascension	23h 07m 53.854s[1]
Declination	+75° 23′ 15.00″[1]
Apparent magnitude (V)	4.419[2] 4.61[3] + 6.75[4])
Characteristics
Spectral type	G7III[5] / F5V[5] / A7V-A9V[6]
U−B color index	−0.46[citation needed]
B−V color index	+0.8[citation needed]
Astrometry
Radial velocity (Rv)	−18.6±0.9 km/s
Proper motion (μ)	RA: +6.81±1.05[1] mas/yr Dec.: −34.06±0.88[1] mas/yr
Parallax (π)	13.8±0.41 mas[6]
Distance	236 ± 7 ly (72 ± 2 pc)
Absolute magnitude (MV)	0.24
Orbit[5]
Primary	π Cep Aa
Companion	π Cep Ab
Period (P)	556.72±0.05 d
Semi-major axis (a)	39.0±3.9 mas[6]
Eccentricity (e)	0.297±0.006
Inclination (i)	99.0±2.5[6]°
Longitude of the node (Ω)	109.2±3.5[6]°
Periastron epoch (T)	2,439,172.9±1.6
Argument of periastron (ω) (secondary)	7.6±1.2°
Semi-amplitude (K1) (primary)	24.18±0.15 km/s
Orbit[7]
Primary	π Cephei A (Aa + Ab)
Companion	π Cephei B
Period (P)	162.8±2.8 yr
Semi-major axis (a)	0.810±0.050″
Eccentricity (e)	0.5968±0.0067
Inclination (i)	30.0±3.0°
Longitude of the node (Ω)	90.3±4.9°
Periastron epoch (T)	B 1934.573±0.35
Argument of periastron (ω) (secondary)	90.0±4.4°
Details[6]
π Cep Aa
Mass	3.63±0.53 M☉
Surface gravity (log g)	3.05±0.11[8] cgs
Temperature	5,226±92[8] K
Metallicity [Fe/H]	0.29±0.05[8] dex
Age	100[8] Myr
π Cep Ab
Mass	3.27±0.48 M☉
π Cep B
Mass	1.93±0.23 M☉
Other designations
HD 218658, HR 8819, 33 Cephei, BD+74°1006, SAO 10629, HIP 114222[9]
Database references
SIMBAD	data

Pi Cephei was found to have a visual companion star by Otto Wilhelm von Struve in 1843.^[6] That the primary is itself a spectroscopic binary was first noticed by William Wallace Campbell in 1901 using photographic plates taken at Lick Observatory.^[10]