Achird, otherwise known as Eta Cassiopeiae, is a binary star system located in the northern circumpolar constellation of Cassiopeia. Its binary nature was discovered in 1779, by William Herschel.
Key Facts & Summary
- The Achird star system is situated at around 19.42 light-years away from the Sun.
- The Achird star system is composed out of the primary star Eta Cassiopeiae A, and its companion Eta Cassiopeiae B.
- The apparent magnitude of the stars is 3.44 / 7.51, and the absolute magnitude is 4.57.
- The primary component, Eta Cassiopeiae A, is a G-type main-sequence star of spectral type G0V, similar to our Sun.
- Eta Cassiopeiae A is slightly lighter than our Sun, having 0.9 solar masses, and a radius of 1.03 solar radii, slightly bigger than our Sun.
- The average surface temperatures on Eta Cassiopeiae A have been estimated at around 5,973 K, a bit hotter than on our Sun.
- Eta Cassiopeiae A is older than our Sun, having an estimated age of 5.4 billion years.
- The rotational velocity of the primary star is 3.15 km / 1.9 mi per second.
- Eta Cassiopeiae A is radiating around 129% of the Sun’s luminosity.
- The secondary star, Eta Cassiopeiae B, is a K-type main-sequence star of spectral type K7V.
- Eta Cassiopeiae B has only 57% of our Sun’s mass, and 66% of its radius.
- The secondary star is also cooler, having an effective temperature of around 4,036 K.
- Eta Cassiopeiae B generates energy slowly since it is small, radiating only 6% of our Sun’s luminosity.
- Both components, compared to our Sun, have shown only half the abundance of elements other than hydrogen and helium.
- The two components of the Achird star system orbit around one another once every 480 years.
The name Achird was supposedly first applied to Eta Cassiopeiae in the Skalnate Pleso Atlas of the Heavens that was published in 1950 but is not known before that.
Richard Hinckley Allen gives no historical names for the star in his book Star Names: Their Lore and Meaning. In 2017, the IAU approved the name Achird for the primary star, Eta Cassiopeiae A.
Achird / Eta Cassiopeiae – formed around 5.4 ± 0.9 billion years ago. It is similar in age to our Sun, and probably formed in the same way, from swirling dust and gas pulled together by gravity.
Distance, Size, and Mass
Achird / Eta Cassiopeiae – is located at around 19 light-years away from the Sun. It is among the closest stars to us, and what makes the primary star special, is that is is similar to our own Sun.
The primary component, Eta Cassiopeiae A, has only 0.972 solar masses, or 97.2% of our Sun’s mass, thus it is slightly lighter. However, Eta Cassiopeiae A has 1.03 solar radii, which is 103% of our Sun’s radius.
The secondary star, Eta Cassiopeiae B, is smaller, having only 0.57 solar masses, or 57% of our Sun’s mass, and 0.66 solar radii or 66% of our Sun’s radius.
The primary star, Eta Cassiopeiae A, is a G-type main-sequence star of spectral type G0V, similar to our Sun, and has an apparent magnitude of 3.44.
Eta Cassiopeiae A has an average surface temperature estimated at around 5,973 K, a bit hotter than on our Sun which has temperatures of around 5,778 K.
The rotational velocity of the primary star is 3.15 km / 1.9 mi per second. Eta Cassiopeiae A is radiating around 129% of the Sun’s luminosity.
The secondary star, Eta Cassiopeiae B, is a K-type main-sequence star of spectral type K7V, having an apparent magnitude of 7.51. Eta Cassiopeiae B is also cooler, having an effective temperature of around 4,036 K, it is also cooler than our Sun.
Eta Cassiopeiae B generates energy slowly since it is small, radiating only 6% of our Sun’s luminosity. Both components, compared to our Sun, have shown only half the abundance of elements other than hydrogen and helium.
The two components of the Achird / Eta Cassiopeiae star system have an orbital period of around 480 years. Eta Cassiopeiae A and B are separated from one another by about 71 AU.
However, they have a large orbital eccentricity of 0.497, this means that their closest approach to one another is as small as 36 AU, and their greatest distancing is at 106 AU. For comparison, the semi-major axis of Neptune is 30 AU.
The Achird / Eta Cassiopeiae star system is located in the constellation of Cassiopeia, which is one of the 48 constellations listed by the 2nd-century Greek astronomer Ptolemy, and it remains one of the 88 modern constellations today.
This constellation which is situated in the northern sky is named after the vain queen Cassiopeia in Greek mythology, who boasted about her unrivaled beauty.
Achird is among the fainter stars in the constellation, yet what makes it special is its similarity to our Sun and its closeness to us. The brightest star in the constellation of Cassiopeia, is Alpha Cassiopeia otherwise known as Schedar.
The constellation of Cassiopeia is easily recognizable due to its “w” shape in the night sky, formed by its five brightest stars. This constellation hosts some of the most luminous stars known, including the yellow hypergiants Rho Cassiopeiae, and V509 Cassiopeiae, and the white hypergiant 6 Cassiopeiae.
Around fourteen star systems, some of which have exoplanets, are located in Cassiopeia. A rich section of the Milky Way runs through Cassiopeia, containing several open clusters, young luminous galactic disc stars, and nebulae.
IC 10 is also situated here, it is an irregular galaxy that is the closest known starburst galaxy to us, and the only known one in the Local Group of galaxies.
The constellation of Cassiopeia is usually visible throughout the year, in the sub(tropics) it is clearest from September to early November.
It is currently unknown if the Achird / Eta Cassiopeiae star system has any planets. However, if there would be a planet here, it would have to be situated in the stable zones for life to develop.
If hypothetical stars would have a circular orbit among the individual members of this star system, then the maximum orbital radius is estimated to be at 9.5 AU for Eta Cassiopeiae A, and 7.1 AU for Eta Cassiopeiae B. A planet orbiting outside of both stars would need to be at least 235 AU.
Since Eta Cassiopeiae A is so similar to our Sun, it has already lived approximately half of its life. Eta Cassiopeiae A will one day become a red giant, and the consequences of this upon its companion star, Eta Cassiopeiae B, are hard to predict.
Did you know?
- In the same constellation where Achird is situated, there is also Cassiopeia A, which is a supernova remnant and the brightest extrasolar radio source in the sky at frequencies above 1 GHz.
- In Chinese astronomy, Eta Cassiopeiae is within the Legs mansion, and it is part of the Wang Liang asterism, which is named for a famous charioteer during the Spring and Autumn period.
- The Chinese know Eta Cassiopeiae as Wang Liang san – the Third Star of Wang Liang – Wang Liang is an asterism formed by Beta Cassiopeiae (Caph), Kappa Cassiopeiae, Alpha Cassiopeiae (Schedar), and Lambda Cassiopeiae.