Delta Sagittae

Delta Sagittae (Delta Sge, δ Sagittae, δ Sge) is a binary star in the constellation of Sagitta, with an apparent magnitude of +3.68. The primary component is a red M-type bright giant, and the secondary is a B-type main-sequence star.^[2] It is approximately 430 light years from Earth, based on its Gaia Data Release 2 parallax.^[1]

A visual band light curve for Delta Sagittae, plotted from data published by Tabur et al. (2009)^[13]

δ Sagittae

Location of δ Sagittae (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Sagitta
Right ascension	19h 47m 23.26653s[1]
Declination	+18° 32′ 03.5203″[1]
Apparent magnitude (V)	+3.82[2](3.91[3]/ 6.64)[4]
Characteristics
Spectral type	M2II + B9.5V[4]
U−B color index	+0.98[5]
B−V color index	+1.40[5]
Variable type	LB?[6]
Astrometry
Radial velocity (Rv)	2.5 ± 0.9[7] km/s
Proper motion (μ)	RA: -6.514[1] mas/yr Dec.: 0.849[1] mas/yr
Parallax (π)	5.9674±0.2597 mas[8]
Distance	550 ± 20 ly (168 ± 7 pc)
Absolute magnitude (MV)	−2.58[9]
Orbit[4]
Period (P)	10.15[10] yr
Semi-major axis (a)	0.051″
Eccentricity (e)	0.453[10]
Inclination (i)	140.0°
Longitude of the node (Ω)	170.2°
Periastron epoch (T)	1979.93
Argument of periastron (ω) (secondary)	257.7°
Details
δ Sge A
Mass	4.073[10] M☉
Radius	223 – 267[10] R☉
Surface gravity (log g)	0.74 ± 0.10[11] cgs
Temperature	3,660±170[12] K
δ Sge B
Mass	3.611[10] M☉
Radius	3.3 – 4.7[10] R☉
Luminosity	63[4] L☉
Temperature	10000[4] K
Other designations
δ Sagittae, 7 Sagittae, BD+18 4240, CCDM J19474+1832AB, FK5 743, GC 27391, HD 187076, HIP 97365, HR 7536, IDS 19429+1817 AB, PPM 136976, SAO 105259, WDS J19474+1832AB
Database references
SIMBAD	data

Delta Sagittae is a spectroscopic binary with a composite spectrum, meaning that light from both stars can be detected. It has an orbital period of about 10 years and an eccentricity of about 0.44.^[4] It is also a variable star, with its brightness changing between a maximum of magnitude 3.75 and a minimum of 3.83 in an unpredictable way.^[6]

Delta Sagittae is moving through the Galaxy at a speed of 9.8 km/s relative to the Sun. Its projected Galactic orbit carries it between 23,800 and 35,300 light years from the center of the Galaxy.^{[14][better source needed]}

Naming

In Chinese, 左旗 (Zuǒ Qí), meaning Left Flag, refers to an asterism consisting of δ Sagittae, α Sagittae, β Sagittae, ζ Sagittae, γ Sagittae, 13 Sagittae, 11 Sagittae, 14 Sagittae and ρ Aquilae. Consequently, the Chinese name for δ Sagittae itself is 左旗三 (Zuǒ Qí sān, English: the Third Star of Left Flag.)^[15]