Xi Geminorum

Xi Geminorum (ξ Gem) is a star in the zodiac constellation Gemini with the traditional name Alzirr. It forms one of the four feet of the outline demarcating the Gemini twins. The star has an apparent visual magnitude of 3.35, which is bright enough for it to be seen with the naked eye. From parallax measurements, its distance from the Earth can be estimated as 58.7 light-years (18.0 parsecs).

Alzirr has a stellar classification of F5 IV, which is subgiant star that is in the process of evolving away from the main sequence of stars like the Sun. It has about 162% of the Sun's mass and is radiating more than 11 times the luminosity of the Sun. This energy is being emitted from the outer envelope of the star at an effective temperature of 6,464 K. This causes the star to take on the yellow-white hue common to F-type stars.

X-ray emission has been detected from this star, which has an estimated X-ray luminosity of 1.06 × 10²⁹erg s⁻¹. It has the spectroscopic signature of a rapidly rotating star, with a projected rotational velocity of about 66 km s⁻¹. Although generally considered a single star, there is some evidence that it may instead be a spectroscopic binary system consisting of two component stars of equal mass.