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Achird (η Cassiopeiae) Worksheets

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Key Facts & Information

  • Achird is a binary star system that can be found in the Northern circumpolar constellation of Cassiopeiae.
  • It is also known as Eta Cassiopeiae. Its being binary in nature was discovered in 1779 by William Herschel.
  • It is located at around 19.42 light years away from the Sun.
  • The star system of Achird is composed of the primary star Eta Cassiopeiae A, together with its companion, Eta Cassiopeiae B.
  • Eta Cassiopeiae A is a G-type main-sequence type star of spectral type G0V which is similar to our Sun, while Eta Cassiopeiae B is a K-type main-sequence star of spectral type K7V.
  • Eta Cassiopeiae A is older than our Sun. It has an estimated age of 5.4 million years. Its rotational velocity is 3.15 km / 1.9 mi per second.
  • It is slightly lighter and bigger than our Sun with 0.9 solar masses and 1.03 solar radii. It is radiating at around 129% of the Sun’s brightness. Its temperature is 5,793 K.
  • The companion star is cooler than the primary star, having a temperature of 4,036 K. It generates energy slowly because it is small, with its brightness at 6% of our Sun’s luminosity.
  • The name Achird was approved by the IAU in 2017 for the Eta Cassiopeiae A.


  • It is older than our Sun. It may have been formed in the same way as the Sun, which was through swirling dust and gas being pulled together due to gravity.

Distance, Size & Mass

  • Eta Cassiopeiae A is one of the closest stars to us at 19 light years away from the Sun. It is special because it is the same with our Sun.
  • The primary star is slightly lighter than our Sun with 0.972 solar masses and 1.03 solar radii.
  • The secondary star, on the other hand, is smaller with 0.57 solar masses and 0.66 solar radii.

Other Characteristics

  • The primary star has a magnitude of 3.44, while the secondary star has a magnitude of 7.51.
  • Both of these stars only have half of the elements found in the Sun except for hydrogen and helium.

Stellar System

  • The star system has an orbital period of around 480 years. The stars are 71 AU away from each other. Their orbital eccentricity is 0.497. This shows that they have 36 AU as their smallest distance and 106 AU is their greatest distance.


  • The star system is located in Cassiopeiae constellation. This constellation is one of the 43 constellations listed by Ptolemy, a 2nd century Greek astronomer.
  • This constellation is located in the northern sky. It was named after the queen, Cassiopeiae, who was vain and boasted about her matchless beauty.
  • The constellation is easy to recognise because of its “W” shape, formed by its five brightest stars. Others stars are Caph, Cih, Rucha  and Segin.
  • Achird is the faintest star in the constellation. The brightest star in the constellation is called Alpha Cassiopeiae or Schedar.
  • There around fourteen star systems that can be found in this constellation. Some of which are exoplanets, planets outside our Solar System and Milky Ways, containing several open clusters, young bright galactic disc stars and nebulae.
  • An irregular galaxy, IC 10, can also be found here. It is the closest starburst galaxy to us, and the only known one in the local group.
  • It can be seen throughout the year but in the subtropics it can be most clearly seen from September to early November.
  • The constellation of Cassiopeiae remains one of the 88 modern constellations.

Life Habitability

  • Currently, it is still unknown if there are any planets around the star system. If there is an existing planet, this should be located in a stable zone for life to develop.
  • A planet that will orbit around the two stars needs to be at least 235 AU since the maximal orbital radius of Eta Cassiopeiae A is at 9.5 AU, while for Eta Cassiopeiae this is 7.1 AU.

The Future

  • Eta Cassiopeiae A has already lived half of its life. It will one day become a red giant.
  • It is unknown what will happen to Eta Cassiopeiae B when this happens.

Did You Know?

  • In the Cassiopeiae constellation where Achird is located, there is also a supernova remnant called Cassiopeiae A. It is the brightest extrasolar radio source located in the sky, with frequencies above 1 GHz.
  • In Chinese astronomy, Achird is located within the Legs mansion. It is also a part of the Wang Liang asterism. It is named after a famous charioteer.
  • It is known by Chinese as Wang Liang san, which means “The Third Star of Wang Liang”. Wang Lian is composed of Beta Cassiopeiae, Kappa Cassiopeiae, Alpha Cassiopeiae, and Lambda Cassiopeiae.