Tau Sagittarii

Tau 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
Radius	16[5] R☉
Surface gravity (log g)	2.75[6] cgs
Temperature	4,860[6] K
Metallicity [Fe/H]	–0.23[6] dex
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.[7]

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 1.5 - 2 Solar masses. The stellar envelope is slightly cooler than the Sun, with an effective temperature of 4,860 K,^[6] 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,^[8] which, at its estimated distance, equates to a physical radius of about 16 times the radius of the Sun.^[5] Tau Sagittarii is a suspected double star although no companion has been confirmed yet. A lower metal content (Fe to H ratio is 70%) and a high peculiar velocity (64 km/s, 4x the local average) relative to the Sun suggest the star is a visitor from a different part of the Galaxy.

The Wow! Signal

Tau Sagittarii is the closest visible star in the night sky to the origin of the 1977 Wow! signal, the only radio signal that has been received that may be a sign of extraterrestrial intelligence. The location of the signal was, in epoch J2000.0 coordinates:

• Right ascension (on the positive horn): 19^h 25^m 31 ± 10^s
• Right ascension (on the negative horn): 19^h 28^m 22 ± 10^s
• Declination (the same for both horns): –26° 57 ± 20′

Name and etymology

• This star, together with :
  • γ Sgr, δ Sgr, ε Sgr, ζ Sgr, λ Sgr, σ Sgr and φ Sgr, consisting the asterism Teapot.^[9][10]
  • φ Sgr, ζ Sgr, χ Sgr and σ Sgr were Al Naʽām al Ṣādirah (النعم السادرة), the Returning Ostriches.^[11] 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).^[12]
  • ν Sgr, ψ Sgr, ω Sgr, 60 Sgr and ζ Sgr were Al Udḥiyy, the Ostrich's Nest.^[11]
• 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, σ Sagittarii itself is known as 斗宿五 (Dǒu Sù wu, English: the Fifth Star of Dipper.)^[13]

References