Eta Cassiopeiae

Eta Cassiopeiae (η Cas, η Cassiopeiae) is a binary star system in the constellation of Cassiopeia. It has the traditional name Achird. This system is 19.42 light-years from Terra.

The primary star in the Eta Cassiopeiae system has a stellar classification of G0 V, which makes it a G-type main-sequence star like Sol. The star has 97% of the mass of Sol, 101% of Sol's radius and radiates 129% of Sol's luminosity of  from its outer envelope at an effective temperature of 6,087 K. It rotates at a leisurely rate, with a rotational velocity of 3.15 km s−1.

The cooler and dimmer companion is of stellar classification K7 V; a K-type main sequence star. It has only 57% of the mass of Sol and 66% of Sol's radius. Smaller stars generate energy more slowly, so this component radiates only 6% of the luminosity of the Sol. Its outer atmosphere has an effective temperature of 4,036 K. Compared to Sol, this star, and the primary component, show only half the abundance of elements other than hydrogen and helium—what astronomers term their metallicity.

The pair orbit around each other over a period of 480 Terran years. The two stars are separated by an average distance of 71 AU. However, the large orbital eccentricity of 0.497 means that their closest approach is 36 AU, and a furthest distance of about 106 AU.