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 Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline ...

in August 1779. Based upon parallax measurements, the distance to this system is from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

. The two components are designated Eta Cassiopeiae A (officially named Achird , the traditional name for the system) 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 The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

(IAU). The proper name ''Achird'' was apparently first applied to Eta Cassiopeiae in the

Skalnate Pleso Atlas of the Heavens The ''Skalnaté Pleso Atlas of the Heavens'' (''Atlas Coeli Skalnaté Pleso 1950.0'') is a set of 16 celestial charts covering the entire sky. It is named after the Skalnaté Pleso Observatory in Slovakia where it was produced. The first versi ...

published in 1950, but is not known prior to that.

Richard Hinckley Allen In astronomy, stars have a variety of different stellar designations and names, including catalogue designations, current and historical proper names, and foreign language names. Only a tiny minority of known stars have proper names; all others ha ...

gives no historical names for the star in his book '' Star Names: Their Lore and Meaning''. In 2016, the IAU organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(WGSN) to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems. 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. In

Chinese astronomy Astronomy in China has a long history stretching from the Shang dynasty, being refined over a period of more than 3,000 years. The ancient Chinese people have identified stars from 1300 BCE, as Chinese star names later categorized in the twe ...

, Eta Cassiopeiae is within the Legs mansion, and is part of the () 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. Consequently, the

Chinese name Chinese names or Chinese personal names are names used by individuals from Greater China and other parts of the Chinese-speaking world throughout East and Southeast Asia (ESEA). In addition, many names used in Japan, Korea and Vietnam are ofte ...

for Eta Cassiopeiae itself is (, en, the Third Star of Wang Liang).香港太空館 - 研究資源 - 亮星中英對照表
, Hong Kong Space Museum. Accessed online November 23, 2010.

Properties

Eta Cassiopeiae's two components are orbiting around each other over a

period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

of 480 years. Based on an estimated semi-major axis of 12″ and a parallax of 0.168″, the two stars are separated by an average distance of , where an AU is the average distance between the

and the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

. However, the large orbital eccentricity of 0.497 means that their

periapsis An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any elli ...

, or closest approach, is as small as 36 AU, with an

apoapsis An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any elli ...

of about 106 AU. For comparison, the semi-major axis of Neptune is 30 AU. There are six dimmer

optical components Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...

listed in the

Washington Double Star Catalog The Washington Double Star Catalog, or WDS, is a catalog of double stars, maintained at the United States Naval Observatory. The catalog contains positions, magnitudes, proper motions and spectral types and has entries for (as of June 2017) 141, ...

. 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. Eta Cassiopeiae A has a stellar classification of G0 V, 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 97% of the mass of the Sun and 101% of the Sun's radius. It is of

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...

3.44, radiating 129% of the

luminosity of the Sun The solar luminosity (), is a unit of radiant flux ( power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun. One nominal ...

from its outer envelope at an effective temperature of . It appears to be rotating at a leisurely rate, with a

projected rotational velocity Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface. The rotation of a star produces an equatorial bulg ...

of . The cooler and dimmer (magnitude 7.51) Eta Cassiopeiae B is of stellar classification K7 V; a

K-type main-sequence star A K-type main-sequence star, also referred to as a K-type dwarf or an orange dwarf, is a main-sequence (hydrogen-burning) star of spectral type K and luminosity class V. These stars are intermediate in size between red M-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, both components show only half the abundance of elements other than

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

and

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

—what astronomers term their metallicity. 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 Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

is 1.5 AU from the Sun.) A planet orbiting outside of both stars would need to be at least 235 AU distant. File:Cassiopeia.JPG, Cassiopeia constellation File:Achird binary system.gif, Eta Cassiopeiae

References

External links

* * {{Stars of Cassiopeia Cassiopeiae, Eta Binary stars Cassiopeia (constellation) G-type main-sequence stars K-type main-sequence stars Cassiopeiae, Eta Cassiopeiae, 24 003821 RS Canum Venaticorum variables 0219 004614 Durchmusterung objects Gliese and GJ objects

Nomenclature

Properties

See also

References

External links