Eta Cassiopeiae
Encyclopedia
Eta Cassiopeiae (η Cas, η Cassiopeiae) is a star system
19.4 light years away from Earth
, in the constellation Cassiopeia
. Sometimes the traditional name Achird is used.
In Chinese
, , 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
(main sequence) star of spectral type
G0V, putting it in the same spectral class as our Sun
, which is of spectral type G2V. It therefore resembles what our Sun might look like if we were to observe it from Eta Cassiopeiae. The star is of apparent magnitude
3.45. The star has a cooler and dimmer (magnitude 7.51) orange dwarf companion of spectral type K7V. The system is an RS Canum Venaticorum type
variable star
and its brightness varies by 0.05 magnitudes.
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. 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.
Star system
A star system or stellar system is a small number of stars which orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems.-Binary star systems:A stellar...
19.4 light years away from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, in the constellation Cassiopeia
Cassiopeia (constellation)
Cassiopeia is a constellation in the northern sky, named after the vain queen Cassiopeia in Greek mythology, who boasted about her unrivalled beauty. Cassiopea was one of the 48 constellations listed by the 2nd century Greek astronomer Ptolemy, and it remains one of the 88 modern constellations today...
. Sometimes the traditional name Achird is used.
In Chinese
Chinese language
The Chinese language is a language or language family consisting of varieties which are mutually intelligible to varying degrees. Originally the indigenous languages spoken by the Han Chinese in China, it forms one of the branches of Sino-Tibetan family of languages...
, , meaning Wang Liang
Legs (Chinese constellation)
The Legs mansion is one of the Twenty-eight mansions of the Chinese constellations. It is one of the western mansions of the White Tiger.-Asterisms:...
, refers to an asterism consisting of η Cassiopeiae, β Cassiopeiae
Beta Cassiopeiae
Beta Cassiopeiae is a Delta Scuti variable star in the constellation Cassiopeia. It is a subgiant or giant star belonging to the spectral class F2, with a mean apparent magnitude of +2.27 and absolute magnitude of +1.16...
, κ Cassiopeiae
Kappa Cassiopeiae
Kappa Cassiopeiae is a star in the constellation Cassiopeia.κ Cassiopeiae is a blue-white B-type supergiant with a mean apparent magnitude of +4.17. It is approximately 4100 light years from Earth. It is classified as an Alpha Cygni type variable star and its brightness varies from magnitude...
, α Cassiopeiae
Alpha Cassiopeiae
Alpha Cassiopeiae is a second magnitude star in the constellation Cassiopeia, with the traditional name Schedar, sometimes spelt Shedir. Though listed as the alpha star by Johann Bayer, Schedar's visual brightness closely rivals the beta star in the constellation, Caph...
and λ Cassiopeiae
Lambda Cassiopeiae
Lambda Cassiopeiae is a binary star in the constellation Cassiopeia. The system has a combined apparent magnitude of +4.74 and is approximately 355 light years from Earth....
. Consequently, η Cassiopeiae itself is known as
Characteristics
The primary star in the Eta Cassiopeiae system is a yellow dwarfMain sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
(main sequence) star of spectral type
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
G0V, putting it in the same spectral class as our Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, which is of spectral type G2V. It therefore resembles what our Sun might look like if we were to observe it from Eta Cassiopeiae. The star is of apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
3.45. The star has a cooler and dimmer (magnitude 7.51) orange dwarf companion of spectral type K7V. The system is an RS Canum Venaticorum type
RS Canum Venaticorum variable
RS Canum Venaticorum variables are a type of variable star. They are close binary stars having active chromospheres which can cause large stellar spots. These spots are believed to cause variations in their observed luminosity...
variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
and its brightness varies by 0.05 magnitudes.
Based on an estimated semimajor axis of 12″ and a parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
of 0.168″, the two stars are separated by an average distance of 71 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
, where an AU is the average distance between the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
and the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. However, the large orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
of 0.497 means that their periapsis, or closest approach, is as small as 36 AU. For comparison, the semi-major axis of Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
is 30 AU. There are six dimmer optical components
Double star
In observational astronomy, a double star is a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope. This can happen either because the pair forms a binary star, i.e...
listed in the Washington Double Star Catalog
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 102,387 pairs of double stars. The catalog also includes multiple stars...
. However, none of them are related to the Eta Cassiopeiae system and are in reality more distant stars.
See also
- Lists of stars in the constellation Cassiopeia
- Class G Stars
- RS Canum Venaticorum variables
- Eta Cassiopeiae in fiction