Epoch J2000 Equinox J2000
|Right ascension||19h 32m 21.59026s|
|Declination||+69° 39′ 40.2354″|
|Apparent magnitude (V)||4.674|
|Spectral type||G9 V|
|U−B color index||+0.386|
|B−V color index||+0.791|
|Radial velocity (Rv)||26.7 km/s|
|Proper motion (μ)||RA: 598.07 mas/yr
Dec.: −1738.40 mas/yr
|Parallax (π)||173.77 ± 0.18 mas|
|Distance||18.77 ± 0.02 ly
(5.755 ± 0.006 pc)
|Absolute magnitude (MV)||5.87|
|Radius||0.778 ± 0.008 R☉|
|Luminosity||0.428 ± 0.013 L☉|
|Surface gravity (log g)||4.60 cgs|
|Temperature||5,299 ± 32 K|
|Metallicity [Fe/H]||−0.20 dex|
|Rotational velocity (v sin i)||1.5 km/s|
Sigma Draconis, the Bayer designation for this star, was established in 1603 as part of the Uranometria, a star catalogue produced by German celestial cartographer Johann Bayer. The proper name Alsafi was derived from Arabic Athāfi, erroneously transcribed from the Arabic plural Athāfiyy, by which the nomads designated the tripods of their open-air kitchens. It was the name of an association of this star, τ Dra and υ Dra. According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Athāfi or Alsafi were the title for three stars: σ Dra as Alsafi, τ Dra as Athāfi I and υ Dra as Athāfi II.
In Chinese, 天廚 (Tiān Chú), meaning Celestial Kitchen, refers to an asterism consisting of σ Draconis, δ Draconis, ε Draconis, ρ Draconis, 64 Draconis and π Draconis. Consequently, σ Draconis itself is known as 天廚二 (Tiān Chú èr, English: the Second Star of Celestial Kitchen.)
The star is a main sequence dwarf of spectral type K0 V, which has long served as a K0 V spectral standard star   . Its classification as K0 V defines one of the rare anchor points of the MK system that have remained unchanged since the original 1943 MKK Atlas. The radius has been directly measured using interferometery with the CHARA Array, which yields a result of 77.8% of the Sun's radius. It has 87% of the Sun's mass, but the luminosity of this star is only 43% that of the Sun. The projected rotation rate (v sin i) is relatively low at 1.5 km/s. It is considered a slightly metal-poor star, meaning that it has a lower proportion of isotopes with masses more than helium when compared to the Sun.
The temperature, luminosity and surface activity appear to vary slightly in a manner very similar to the sunspot cycle, although the full length of the cycle has not yet been determined. The total variability is among the lowest of all stars that have been measured by the Hipparcos spacecraft.
The components of Sigma Draconis's space velocity are U=+36, V=+40 and W=-10 km/s. This gives the star an unusually large orbital eccentricity about the Milky Way galaxy of 0.30 (compared to 0.06 for the Sun.) The mean galactocentric distance for the orbit is 10.3 kiloparsecs (about 34,000 light-years).
As of 2007, no Jupiter-size or larger companion had been detected about the star and there was no indication of excess infrared radiation that would be evidence of circumstellar matter (such as a debris disk).
Between 2004 and 2013, extensive radial velocity measurements were gathered on Sigma Draconis using the High Resolution Echelle Spectrometer on the Keck Observatory. Towards the end of that period, a significant radial velocity trend peak at around 310 days and another possible trend near 2800 days were emerging from the accumulated data. The trend around 310 days indicated a planet orbiting inside the habitable zone with a minimum of 11.2 Earth masses. However, the radial velocity peak near 2800 days was suspected to be caused by a bias in the data set collected by the telescope and not an indication of a second planet. Combining the Keck/HIRES data with further measurements taken in 2013 from the new Automated Planet Finder telescope indeed did show a significantly strengthened trend at 308 days consistent with a Uranus sized planet, of greater than around 12 Earth masses, but it also showed a much reduced radial velocity trend near 2800 days.
(in order from star)
|b (unconfirmed)||≳12 M⊕||0.814[note 1]||308||~0||—||—|
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