Epoch J2000 Equinox J2000
|Right ascension||00h 49m 06.29070s|
|Declination||+57° 48′ 54.6758″|
|Apparent magnitude (V)||3.44/7.51|
|Spectral type||G0 V + K7 V|
|U−B color index||+0.02/1.03|
|B−V color index||+0.58/1.39|
|Variable type||RS CVn|
|Radial velocity (Rv)||+10.0 ± 0.1 km/s|
|Proper motion (μ)||RA: 1086.59 mas/yr
Dec.: –559.43 mas/yr
|Parallax (π)||167.98 ± 0.48 mas|
|Distance||19.42 ± 0.06 ly
(5.95 ± 0.02 pc)
|Companion||Eta Cassiopeiae B|
|Period (P)||480 yr|
|Semi-major axis (a)||11.9939"|
|Longitude of the node (Ω)||98.42°|
|Periastron epoch (T)||1889.6|
|Argument of periastron (ω)
|η Cas A|
|Mass||0.972 ± 0.012 M☉|
|Radius||1.0386 ± 0.0038 R☉|
|Luminosity||1.2321 ± 0.0074 L☉|
|Temperature||5973 ± 8 K|
|Metallicity [Fe/H]||–0.31 dex|
|Rotational velocity (v sin i)||3.15 km/s|
|Age||5.4 ± 0.9 Gyr|
|η Cas B|
|Mass||0.57 ± 0.07 M☉|
|Temperature||4036 ± 150 K|
Eta Cassiopeiae (η Cas, η Cassiopeiae) is a star system in the northern circumpolar constellation of Cassiopeia. It has the traditional name Achird. Based upon parallax measurements, the distance to this system is 19.42 light-years (5.95 parsecs) from Earth.
In Chinese, 王良 (Wáng Liáng), meaning Wang Liang, refers to an asterism consisting of η Cassiopeiae, β Cassiopeiae, κ Cassiopeiae, α Cassiopeiae and λ Cassiopeiae. Consequently, η Cassiopeiae itself is known as 王良三 (Wáng Liáng sān, English: the Third Star of Wang Liang.)
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 the Sun. It therefore resembles what our Sun might look like if we were to observe it from Eta Cassiopeiae. The star has 97% of the mass of the Sun and 101% of the Sun's radius. It is of apparent magnitude 3.44, radiating 129% of the luminosity of the Sun from its outer envelope at an effective temperature of 6,087 K. It appears to be rotating at a leisurely rate, with a projected rotational velocity of 3.15 km s−1.
The cooler and dimmer magnitude 7.51 companion is of stellar classification K7 V; a K-type main sequence star. It has only 57% of the mass of the Sun and 66% of the Sun's radius. Smaller stars generate energy more slowly, so this component radiates only 6% of the luminosity of the Sun. Its outer atmosphere has an effective temperature of 4,036 K. Compared to the Sun, this star, and the primary component, show only half the abundance of elements other than hydrogen and helium—what astronomers term their metallicity.
This is a binary star first discovered by William Herschel in August 1779. The pair are orbiting around each other over a period of 480 years. Based on an estimated semimajor axis of 12″ and a parallax of 0.168″, the two stars are separated by an average distance of 71 AU, where an AU is the average distance between the Sun and the Earth. However, the large orbital eccentricity of 0.497 means that their periapsis, or closest approach, is as small as 36 AU, with an apoapsis of about 106 AUs. For comparison, the semi-major axis of Neptune is 30 AU. There are six dimmer optical components listed in the Washington Double Star Catalog. However, none of them are related to the Eta Cassiopeiae system and are in reality more distant stars.
A necessary condition for the existence of a planet in this system are stable zones where the object can remain in orbit for long intervals. For hypothetical planets in a circular orbit around the individual members of this star system, this maximum orbital radius is computed to be 9.5 AU for the primary and 7.1 AU for the secondary. (Note that the orbit of Mars is 1.5 AU from the Sun.) A planet orbiting outside of both stars would need to be at least 235 AU distant.
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