V636 Scorpii

V636 Scorpii is a multiple star system in the constellation Scorpius, 3,000 light years away. The primary is a Classical Cepheid (δ Cephei) variable and its visual magnitude varies from 6.4 to 6.9.

V636 Sco

A light curve for V636 Scorpii, plotted from TESS data[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Scorpius
Right ascension	17h 22m 46.47796s[2]
Declination	−45° 36′ 51.3868″[2]
Apparent magnitude (V)	6.40 - 6.92[3]
Characteristics
Cepheid
Spectral type	F7/8Ib/II-G5[3]
Variable type	δ Cep[3]
companion
Spectral type	B9.5V[4]
Astrometry
Radial velocity (Rv)	9.09 ± 0.17[5] km/s
Proper motion (μ)	RA: -3.05 ± 1.03[2] mas/yr Dec.: -2.40 ± 0.38[2] mas/yr
Parallax (π)	1.15±0.76 mas[2]
Distance	approx. 3,000 ly (approx. 900 pc)
Absolute magnitude (MV)	-3.64[4]
Orbit
Period (P)	1,318 days[4]
Semi-major axis (a)	6.7[4]
Eccentricity (e)	0.26[6]
Details
Cepheid
Mass	5.6[4] M☉
Radius	~50[7] R☉
Luminosity	2,500[4] L☉
Metallicity	+0.07[8]
companion
Mass	2.4 M☉
Other designations
V636 Sco, HIP 85035, CD−45°11441, HD 156979, SAO 227880
Database references
SIMBAD	data

V636 Scorpii is a spectroscopic binary, and the fainter companion is thought to itself consist of two stars. The primary is a luminous yellow star and a δ Cephei variable. The less massive companion orbits every 3.6 years and is apparently a B9.5 main sequence star, but the dynamics of the system suggest that it may actually be a pair of stars is a close orbit.^[9]

The Cepheid primary pulsates regularly with a period of 6.79671 days. It is a yellow-white supergiant or bright giant that is 5.6 times as massive as the Sun and 2,500 times as luminous.^[4]