W Sagittarii
Star in the constellation Sagittarius
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W Sagittarii (W Sgr, Gamma1 Sagittarii (γ1 Sgr)) is a multiple star system star in the constellation Sagittarius, and a Cepheid variable star.
 | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Sagittarius |
| Right ascension | 18h 05m 01.22643s[1] |
| Declination | −29° 34′ 48.3222″[1] |
| Apparent magnitude (V) | 4.29 - 5.14[2] |
| Characteristics | |
| Aa1 | |
| Spectral type | F4 - G2Ib[2] |
| U−B color index | +0.52[3] |
| B−V color index | +0.78[3] |
| Variable type | δ Cep[2] |
| Aa2 | |
| Spectral type | A5V - F5V[4] |
| Ab | |
| Spectral type | A0 V[5] |
| Astrometry | |
| Radial velocity (Rv) | −28.04[6] km/s |
| Proper motion (μ) | RA: +4.372[1] mas/yr Dec.: −5.588[1] mas/yr |
| Parallax (π) | 2.365±0.1765 mas[1] |
| Distance | 1,400 ± 100 ly (420 ± 30 pc) |
| Absolute magnitude (MV) | −3.76[5] |
| Orbit[4] | |
| Primary | Aa1 |
| Name | Aa2 |
| Period (P) | 4.33±0.01 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[5] | |
| Primary | Aa |
| Name | Ab |
| Period (P) | 172.9 yr |
| Semi-major axis (a) | 63 AU |
| Details | |
| Aa | |
| Mass | 5.8[5] M☉ |
| Radius | 61.0[7] R☉ |
| Luminosity | 2,690[5] L☉ |
| Surface gravity (log g) | 1.50 - 2.15[8] cgs |
| Temperature | 5,380 - 6,474[8] K |
| Metallicity [Fe/H] | +0.02[9] dex |
| Aa2 | |
| Mass | 1.4 - 2.0[4] M☉ |
| Ab | |
| Mass | 2.2[5] M☉ |
| 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 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.[10]
Component A, W Sgr, is itself a triple star system, with the components referred to as W Sgr Aa1, Aa2, and Ab.[11] These have also been referred to as components Aa, Ab, and B respectively.[10] 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 6 M☉ yellow supergiant, while the secondary is an early F main sequence star with a mass less than 1.4 M☉.[10]
Variability
In early June of 1866, Johann Friedrich Julius Schmidt discovered that the star is a variable star.[13] The supergiant component W Sgr Aa1 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.[5]