Tau Sagittarii

Tau Sagittarii (Tau Sgr, τ Sagittarii, τ Sgr) is a star in the southern zodiac constellation of Sagittarius.

τ Sagittarii

Location of τ Sagittarii (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Sagittarius
Right ascension	19h 06m 56.40897s[1]
Declination	–27° 40′ 13.5189″[1]
Apparent magnitude (V)	+3.326[2]
Characteristics
Spectral type	K1 III[3]
U−B color index	+1.185[2]
B−V color index	+1.170[2]
Astrometry
Radial velocity (Rv)	+45.4[4] km/s
Proper motion (μ)	RA: –50.61[1] mas/yr Dec.: -249.80[1] mas/yr
Parallax (π)	28.3195±0.3134 mas[5]
Distance	115 ± 1 ly (35.3 ± 0.4 pc)
Absolute magnitude (MV)	0.48[6]
Details[7]
Mass	1.25 M☉
Radius	15.71 R☉
Luminosity	87.6 L☉
Surface gravity (log g)	2.15 cgs
Temperature	4,459 K
Metallicity [Fe/H]	–0.27 dex
Rotational velocity (v sin i)	1.04[8] km/s
Age	7.91 Gyr
Other designations
Namalsadirah 2, Rabi al Sadira, τ Sagittarii, τ Sgr, Tau Sgr, 40 Sagittarii, CPD−27°6617, FK5 1496, GC 26291, HD 177716, HIP 93864, HR 7234, PPM 269078, SAO 187683
Database references
SIMBAD	data

Description

With an apparent visual magnitude of +3.3,^[2] this is one of the brighter members of the constellation. The distance of this star from Earth is roughly 122 light-years (37 parsecs), based upon parallax measurements.^[1]

This is a spectral type K1 giant star with about 1.25 M_☉. The stellar envelope is slightly cooler than the Sun with an effective temperature of 4,459 K, giving the star a light orange color. The interferometry-measured angular diameter of this star, after correcting for limb darkening, is 3.93 ± 0.04 mas,^[9] which, at its estimated distance, equates to a physical radius of about 16 times the radius of the Sun.^[10]

τ Sagittarii is a suspected double star although no companion has been confirmed yet. A lower metal content (Fe to H ratio is 54% lower than the sun's) and a high peculiar velocity (64 km/s, four times the local average) relative to the Sun suggest the star is a visitor from a different part of the Galaxy. ^[11]

τ  Sagittarii is a red clump giant, a star with a similar mass to the sun which has exhausted its core hydrogen, passed through the red giant branch, and started helium fusion in its core.^[12]

The Wow! signal

τ Sagittarii is the closest constellational star (a star that is part of the traditional outline of a constellation) to the origin of the 1977 Wow! signal.^[13]

Name and etymology

• The star forms part of simple asterisms:
  • γ Sgr, τ Sgr, δ Sgr, ε Sgr, ζ Sgr, λ Sgr, σ Sgr and φ Sgr — the Teapot.^[14]
  • φ Sgr, τ Sgr, ζ Sgr, χ Sgr (double) and σ Sgr — the Returning Ostriches; in Arabic Al Naʽām al Ṣādirah often transliterated as Namalsadirah (النعم السادرة).^[15]
    • Originally four stars in a numerical order: φ Sgr, τ Sgr, χ¹ Sgr and χ² Sgr^[16]
  • ν Sgr, τ Sgr, ψ Sgr, ω Sgr, 60 Sgr and ζ Sgr, Al Udḥiyy — the Ostrich's Nest.^[15]
  • In the entirely separate Chinese tradition — 斗 (Dǒu), meaning Dipper: τ Sgr, φ Sgr, λ Sgr, μ Sgr, σ Sgr and ζ Sgr. The star itself is 斗宿五 (Dǒu Xiù wǔ, English: the Dipper's fifth star).^[17]