Eta Cassiopeiae

Eta Cassiopeiae (η Cassiopeiae, abbreviated Eta Cas, η Cas) is a binary star system in the northern constellation of Cassiopeia. Its binary nature was first discovered by William Herschel in August 1779. Based upon parallax measurements, the distance to this system is Template:Convert.<ref name=aaa474_2_653/> The two components are designated Eta Cassiopeiae A (officially named Achird Template:IPAc-en)<ref name="IAU-LSN">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and B.

Nomenclature

η Cassiopeiae (Latinised to Eta Cassiopeiae) is the system's Bayer designation. The designations of the two constituents as Eta Cassiopeiae A and B derive from the convention used by the Washington Multiplicity Catalog (WMC) for star systems, and adopted by the International Astronomical Union (IAU).<ref name="planetnaming">Template:Cite arXiv</ref>

The proper name Achird was apparently first applied to Eta Cassiopeiae in the Skalnate Pleso Atlas of the Heavens published in 1950, but is not known prior to that.<ref name=hr/> Richard Hinckley Allen gives no historical names for the star in his book Star Names: Their Lore and Meaning.<ref name=allen/> In 2016, the IAU organized a Working Group on Star Names (WGSN)<ref name="WGSN">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.<ref name="TriRpt18">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It approved the name Achird for the component Eta Cassiopeiae A on 5 September 2017 and it is now so included in the List of IAU-approved Star Names.<ref name="IAU-LSN"/>

In Chinese astronomy, Eta Cassiopeiae is within the Legs mansion, and is part of the {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}) asterism named for a famous charioteer during the Spring and Autumn period. The other components are Beta Cassiopeiae (Caph), Kappa Cassiopeiae, Alpha Cassiopeiae (Schedar) and Lambda Cassiopeiae.<ref>Template:In lang 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, Template:ISBN.</ref> Consequently, the Chinese name for Eta Cassiopeiae itself is {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}, Template:Langx).<ref>Template:In lang 香港太空館 - 研究資源 - 亮星中英對照表 Template:Webarchive, Hong Kong Space Museum. Accessed online November 23, 2010.</ref>

Properties

File:Eta Cassiopeiae.jpg

η Cas in optical light

Eta Cassiopeiae's two components are orbiting around each other over a period of 472 years.<ref name=NEID2025/> The two stars are separated by an average distance of Template:Val, where an AU is the average distance between the Sun and the Earth. However, the large orbital eccentricity of 0.4916<ref name=NEID2025/> means that their periapsis, or closest approach, is as small as 35 AU, with an apoapsis of about 105 AU.<ref>From a(1+e) and a(1−e) for apoapsis and periapsis respectively.</ref> 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. The primary has been reported to be a spectroscopic binary, but this has never been confirmed.<ref name=hr/>

Eta Cassiopeiae A has a stellar classification of G0 V,<ref name=aaa520_A79/> which makes it a G-type main-sequence star like the Sun. It therefore resembles what the Sun might look like were humans to observe it from Eta Cassiopeiae. The star has 103% of the mass of the Sun<ref name=NEID2025/> and 100% of the Sun's radius.<ref name=gray2019/> It is of apparent magnitude 3.44,<ref name=clpl4_99/> radiating 129%<ref name=aaa338/> of the luminosity of the Sun from its outer envelope at an effective temperature of Template:Val.<ref name=gray2019/> It appears to be rotating at a leisurely rate, with a projected rotational velocity of Template:Nowrap.<ref name=aaa520_A79/>

The cooler and dimmer (magnitude 7.51<ref name=aaa338/>) Eta Cassiopeiae B is of stellar classification K7 V;<ref name=aaa338/> a K-type main-sequence star. It has only 55%<ref name=NEID2025/> of the mass of the Sun and 66%<ref name=apjss53/> of the Sun's radius. Smaller stars generate energy more slowly, so this component radiates only 6%<ref name=aaa338/> of the luminosity of the Sun. Its outer atmosphere has an effective temperature of 4,036 K.<ref name=aaa338/>

Compared to the Sun, both components show only half the abundance of elements other than hydrogen and helium—what astronomers term their metallicity.<ref name=aaa338/>

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.<ref name=mnras427_4_2723/>

• Cassiopeia.JPG

  Cassiopeia constellation

• Achird binary system.gif

  Eta Cassiopeiae

Possible planetary system

Eta Cassiopeiae A shows a peridiocity of Template:Convert on its radial velocity, which is not correlated to any known stellar activity cycles. This periodicity could be explained by an exoplanet with a mass of at least Template:Earth mass. Future research is needed to confirm this putative planet, eliminating the possibility of a long-term activity cycle.<ref name=NEID2025/>

See also

• Lists of stars
• List of stars in Cassiopeia

External links

• {{#invoke:citation/CS1|citation

|CitationClass=web }}

• {{#invoke:citation/CS1|citation

|CitationClass=web }}