Eta Cassiopeiae (η Cassiopeiae, abbreviated Eta Cas, η Cas) is a

binary star A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...

system in the

northern Northern may refer to the following: Geography * North North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating ...

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Cassiopeia Cassiopeia or Cassiopea may refer to: Greek mythology * Cassiopeia (mother of Andromeda), queen of Aethiopia and mother of Andromeda * Cassiopeia (wife of Phoenix), wife of Phoenix, king of Phoenicia * Cassiopeia, wife of Epaphus, king of Egy ...

. Its binary nature was first discovered by

William Herschel Frederick William Herschel ( ; ; 15 November 1738 – 25 August 1822) was a German-British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline Herschel. Born in the Electorate of Hanover ...

in August 1779. Based upon

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

measurements, the distance to this system is . The two components are designated Eta Cassiopeiae A (officially named Achird ) and B.

Nomenclature

''η Cassiopeiae'' ( Latinised to ''Eta Cassiopeiae'') is the system's

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek alphabet, Greek or Latin letter followed by the genitive case, genitive form of its parent constellation's Latin name. The original list of Bayer design ...

. 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 system A star system or stellar system is a small number of stars that orbit each other, bound by gravity, gravitational attraction. It may sometimes be used to refer to a single star. A large group of stars bound by gravitation is generally calle ...

s, and adopted by the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

(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 ''Star Names: Their Lore and Meaning'' is an 1899 book by Richard Hinckley Allen, that discusses the names of stars, constellations, and their histories. Background and authorship Richard Hinckley Allen (1838, near Buffalo, New York – 1908 ...

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 China, ancient Chinese people have identified stars from 1300 BCE, as Chinese star names later categori ...

, Eta Cassiopeiae is within the Legs mansion, and is part of the () asterism named for a famous

chariot A chariot is a type of vehicle similar to a cart, driven by a charioteer, usually using horses to provide rapid Propulsion, motive power. The oldest known chariots have been found in burials of the Sintashta culture in modern-day Chelyabinsk O ...

eer during the

Spring and Autumn period The Spring and Autumn period () was a period in History of China, Chinese history corresponding roughly to the first half of the Eastern Zhou (256 BCE), characterized by the gradual erosion of royal power as local lords nominally subject t ...

. The other components are

Beta Cassiopeiae Beta Cassiopeiae (β Cassiopeiae, abbreviated Beta Cas or β Cas), officially named Caph , is a Delta Scuti variable star in the constellation of Cassiopeia. It is a giant star belonging to the spectral class F2. The white star of seco ...

(Caph), Kappa Cassiopeiae,

Alpha Cassiopeiae Alpha Cassiopeiae or α Cassiopeiae, also named Schedar (), is a Magnitude (astronomy), second-magnitude star in the northern constellation of Cassiopeia (constellation), Cassiopeia. Though listed as the "Bayer designation, alpha star" b ...

(Schedar) and Lambda Cassiopeiae. Consequently, the

Chinese name Chinese may refer to: * Something related to China * Chinese people, people identified with China, through nationality, citizenship, and/or ethnicity **Han Chinese, East Asian ethnic group native to China. **'' Zhonghua minzu'', the supra-ethni ...

for Eta Cassiopeiae itself is (, ).

Properties

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

period Period may refer to: Common uses * Period (punctuation) * Era, a length or span of time *Menstruation, commonly referred to as a "period" Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (o ...

of 472 years. The two stars are separated by an average distance of , where an AU is the average distance between the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

and the

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

. However, the large

orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values be ...

of 0.4916 means that their

periapsis An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

, or closest approach, is as small as 35 AU, with an

apoapsis An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

of about 105 AU.From '' a''(1+''e'') and '' a''(1−''e'') for apoapsis and periapsis respectively. For comparison, the semi-major axis of

Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...

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 optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...

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 January 2024) 1 ...

. 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 A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...

, but this has never been confirmed. Eta Cassiopeiae A has an

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

of 3.44. It has a

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...

of G0 V, which makes it a

G-type main-sequence star A G-type main-sequence star (spectral type: G-V), also often, and imprecisely, called a yellow dwarf, or G star, is a main sequence, main-sequence star (luminosity class V) of stellar classification, spectral type G. Such a star has about 0.9 to ...

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 Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

and

radius of the Sun Solar radius is a unit of distance used to express the size of objects in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3: 1\,R_ = 6.957\ti ...

. It radiates 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 s ...

from its

photosphere The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...

at an

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface 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 bu ...

of . The cooler and dimmer (magnitude 7.51) Eta Cassiopeiae B is of stellar classification K7 V; a K-type main-sequence star. It has only 55% of the mass of the Sun and 66% of the Sun's radius. This star radiates only 6% of the luminosity of the Sun from its photosphere at an

of 4,036 K. Compared to the Sun, both components show only half the abundance of elements other than

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

and

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

—what astronomers term their

metallicity In astronomy, metallicity is the Abundance of the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...

. 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. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

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

Possible planetary system

Eta Cassiopeiae A shows a peridiocity of on its

radial velocity The radial velocity or line-of-sight velocity of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity ...

, which is not correlated to any known stellar activity cycles. This periodicity could be explained by an

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

with a mass of at least . Future research is needed to confirm this putative planet, eliminating the possibility of a long-term activity cycle.

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 BD+57 0150 0034 Achird

Nomenclature

Properties

Possible planetary system

See also

References

External links