Eta Cassiopeiae is a star system 19.4 light years away from Earth, in the constellation Cassiopeia. The proper name Achird seems to have been applied in recent times and has no clear meaning, one unsupported source suggesting "girdle." It is known as the Third Star of Wang Liang in Chinese.
Binary Orbit
Eta Cassiopeia is a well known binary first discovered by Sir William Herschel in 1779. The period is about 500 years, with the separation varying between 5 and 16". The stars provide a lovely contrast; observers report gold, yellow, or topaz for the primary; and orange, red, purple, or garnet for the secondary.
The two stars are separated by an average distance of 71 AU. However, the large orbital eccentricity (e=0.497) means that their closest approach is as small as 36 AU. For comparison, the semi-major axis of Neptune's orbit is 30 AU. Except for the much-longer period, η Cas is something of a northern-hemisphere version of Alpha Centauri.
Components
The primary star in the η Cas system is of apparent magnitude 3.45. It is a yellow main sequence star of spectral class G3 V, nearly identical to our Sun. It contains 90% - 110% of the Sun's mass, has almost the same diameter (98% - 101%), and is 1.2 times its luminosity. It therefore resembles what our Sun might look like if we were to observe it from η Cas.
Its companion is a cooler and dimmer (magnitude 7.51) orange dwarf of spectral class K7 V. This star has 56% - 60% of the Sun's mass, 66% of its diameter, and around 3% of its luminosity. The companion is an RS Canum Venaticorum-type variable, and its brightness varies by 0.05 magnitudes.
There are six dimmer optical components listed in the Washington Double Star Catalog. However, none of them are related to the η Cas system and are in reality more distant stars.
Planets
If the brighter G-type star were to have an Earthlike planet, the dimmer K star would shine its sky with an orangey light of five full Moons. Stars with planets, however, seem to have metal contents similar to or greater the Sun's. η Cas A (and presumably η Cas B) is metal-poor, its iron content relative to hydrogen only half the Sun's, so planets seem unlikely. An attempt to find large planets from December 1986 to February 1987 failed to detect any large periodic variations in radial velocities.