W Sagittarii

W Sagittarii

Template:Location mark Location of W Sgr (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Sagittarius
Right ascension	Template:RA
Declination	Template:DEC
Apparent magnitude (V)	4.29 - 5.14
Characteristics
Aa1
Spectral type	F4 - G2Ib
U−B Template:Engvar	+0.52
B−V Template:Engvar	+0.78
Variable type	δ Cep
Aa2
Spectral type	A5V - F5V
Ab
Spectral type	A0 V
Astrometry
Radial velocity (Rv)	−28.04 km/s
Proper motion (μ)	RA: +4.372 mas/yr Dec.: −5.588 mas/yr
Parallax (π)	2.365±0.1765 mas
Distance	Template:Rnd ± Template:Rnd ly (Template:Rnd ± Template:Rnd pc)
Absolute magnitude (MV)	−3.76
Orbit
Primary	Aa1
Companion	Aa2
Period (P)	yr
Semi-major axis (a)	12.9 ± 0.3" (5.67 ± 0.13 AU)
Eccentricity (e)	0.41 ± 0.02
Inclination (i)	7.0 ± 0.8°
Longitude of the node (Ω)	68.4 ± 4.0°
Periastron epoch (T)	2004.16 ± 0.01
Argument of periastron (ω) (secondary)	328.0 ± 1.3°
Orbit
Primary	Aa
Companion	Ab
Period (P)	172.9 yr
Semi-major axis (a)	63 AU
Details
Aa
Mass	5.8 Template:Solar mass
Radius	61.0 Template:Solar radius
Luminosity	2,690 Template:Solar luminosity
Surface gravity (log g)	1.50 - 2.15 cgs
Temperature	5,380 - 6,474 K
Metallicity [Fe/H]	+0.02 dex
Aa2
Mass	1.4 - 2.0 Template:Solar mass
Metallicity [Fe/H]	dex
Ab
Mass	2.2 Template:Solar mass
Metallicity [Fe/H]	{{{metal_fe2}}} dex
Other designations
γ1 Sgr, W Sagittarii, HR 6742, HD 164975, SAO 186237, HIP 88567, ADS 11029, CCDM 18050-2935
Database references
SIMBAD	data

W Sagittarii (W Sgr, Gamma¹ Sagittarii (γ¹ Sgr)) is a multiple star system star in the constellation Sagittarius, and a Cepheid variable star.

W Sagittarii is an optical line-of-sight companion nearly a degree from the much brighter γ² Sgr (Al Nasl) which marks the nozzle or spout of the teapot asterism.

System

W Sgr is listed as component A of a multiple star system catalogued as ADS 11029 and WDS J18050-2935. Components B and C are at 33" and 46" respectively and both are 13th magnitude. They are purely optical companions, not physically associated with W Sgr.

Component A, W Sgr, is itself a triple star system, with the components referred to as W Sgr Aa1, Aa2, and Ab. These have also been referred to as components Aa, Ab, and B respectively. The outer companion Ab has been resolved at a separation of 0.14" and is over 5 magnitudes fainter than the primary supergiant. The inner components can only be identified spectroscopically by their radial velocity variations. The primary is a Template:Solar mass yellow supergiant, while the secondary is an early F main sequence star with a mass less than Template:Solar mass.

Variability

File:WSgrLightCurve.png

A light curve for W Sagittarii, plotted from TESS data

The supergiant component W Sgr Aa1 is a variable star which pulsates regularly between magnitudes 4.3 and 5.1 every 7.59 days. During the pulsations, that temperature and spectral type also vary. It is classified as a Classical Cepheid (δ Cephei) variable.