Tau Sagittarii

Tau Sagittarii

Location of τ Sagittarii (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Sagittarius
Right ascension	19^h 06^m 56.40897^s[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.^[9]

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.^[10][11]
  • φ Sgr, ζ Sgr, χ Sgr and σ Sgr were Al Naʽām al Ṣādirah (النعم السادرة), the Returning Ostriches.^[12] 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).^[13]
  • ν Sgr, ψ Sgr, ω Sgr, 60 Sgr and ζ Sgr were Al Udḥiyy, the Ostrich's Nest.^[12]
• 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.)^[14]

References

1. van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357
2. Celis S., L. (October 1975), "Photoelectric photometry of late-type variable stars", Astronomy and Astrophysics Supplement Series 22: 9–17, Bibcode:1975A&AS...22....9C
3. Gray, R. O.; et al. (July 2006), "Contributions to the Nearby Stars (NStars) Project: spectroscopy of stars earlier than M0 within 40 pc-The Southern Sample", The Astronomical Journal 132 (1): 161–170, arXiv:astro-ph/0603770, Bibcode:2006AJ....132..161G, doi:10.1086/504637
4. Wilson, R. E. (1953). General Catalogue of Stellar Radial Velocities. Carnegie Institute of Washington D.C. Bibcode:1953GCRV..C......0W.
5. Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library 1 (3rd ed.), Birkhäuser, ISBN 3-540-29692-1. The radius (R_*) is given by:

   $\begin{align} 2\cdot R_* & = \frac{(10^{-3}\cdot 37\cdot 3.93)\ \text{AU}}{0.0046491\ \text{AU}/R_{\bigodot}} \\ & \approx 31.3\cdot R_{\bigodot} \end{align}$

6. Jones, K. L.; et al. (June 1992), "Spectroscopic investigation of cool giants and the authenticity of their reported microwave emission", Monthly Notices of the Royal Astronomical Society 256 (3): 535–544, Bibcode:1992MNRAS.256..535J, doi:10.1093/mnras/256.3.535
7. "HD 177716 -- High proper-motion Star", SIMBAD Astronomical Object Database (Centre de Données astronomiques de Strasbourg), retrieved 2012-02-12
8. Richichi, A.; Percheron, I.; Khristoforova, M. (February 2005), "CHARM2: An updated Catalog of High Angular Resolution Measurements", Astronomy and Astrophysics 431: 773–777, Bibcode:2005A&A...431..773R, doi:10.1051/0004-6361:20042039
9. http://www.news.com.au/technology/science/the-worlds-biggest-mysteries-scientists-still-cant-solve/story-fnjwl1aw-1227045377722
10. "Sagittarius". deepsky.astroinfo.org. Retrieved 30 August 2008.
11. skywatchers Archived May 17, 2007, at the Wayback Machine.
12. Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p. 355. ISBN 0-486-21079-0. Retrieved 2012-09-04.
13. Jack W. Rhoads - Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars, Jet Propulsion Laboratory, California Institute of Technology; November 15, 1971
14. (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日