Tau Sagittarii

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)	26.82 ± 0.86[1] mas
Distance	122 ± 4 ly (37 ± 1 pc)
Details
Mass	1.25[5] M☉
Radius	15.71[5] R☉
Luminosity	87.6[5] L☉
Surface gravity (log g)	2.15[5] cgs
Temperature	4,459[5] K
Metallicity [Fe/H]	–0.27[5] dex
Rotational velocity (v sin i)	1.04[6] km/s
Age	7.91[5] Gyr
Other designations
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

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

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 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,^[7] which, at its estimated distance, equates to a physical radius of about 16 times the radius of the Sun.^[8]

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.

τ Sagittarii is a red clump giant, a star with similar mass to the sun which has exhausted its core hydrogen, passed through the red giant branch, and started helium fusion in its core.^[9]

The Wow! signal

τ Sagittarii is the closest visible star in the night sky to the origin of the 1977 Wow! signal.^[10]

Name and etymology

• This star, together with :
  • γ Sgr, δ Sgr, ε Sgr, ζ Sgr, λ Sgr, σ Sgr and φ Sgr, comprising the asterism Teapot.^[11][12]
  • φ Sgr, ζ Sgr, χ Sgr and σ Sgr were Al Naʽām al Ṣādirah (النعم السادرة), the Returning Ostriches.^[13] According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Al Naʽām al Ṣādirah or Namalsadirah was originally the title for four stars: φ Sgr as Namalsadirah I, τ Sgr as Namalsadirah II, χ¹ Sgr as Namalsadirah III and χ² Sgr as Namalsadirah IV (except σ Sgr and ζ Sgr).^[14]
  • ν Sgr, ψ Sgr, ω Sgr, 60 Sgr and ζ Sgr were Al Udḥiyy, the Ostrich's Nest.^[13]
• In Chinese, 斗 (Dǒu), meaning Dipper, refers to an asterism consisting of τ Sagittarii, φ Sagittarii, λ Sagittarii, μ Sagittarii, σ Sagittarii and ζ Sagittarii. Consequently, τ Sagittarii itself is known as 斗宿五 (Dǒu Sù wu, English: the Fifth Star of Dipper.)^[15]

References