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Tau Sagittarii (Tau Sgr, τ Sagittarii, τ Sgr) is a star in the southern zodiac constellation of Sagittarius.
  | |
| 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 | |
| Database references | |
| SIMBAD | data |
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]
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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]
- 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]
- 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]
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