Sigma Draconis

Primary of Apocatequil.

Main sequence dwarf of spectral type G9. Formerly it was classified as K0, with the spectrum of this star serving as one of the stable anchor points by which other stars are classified. 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.