Alkaid (η Ursae Majoris) Facts

Alkaid is a blue main-sequence star of spectral type B3 V, located at around 103.9 light-years / 31.9 parsecs away from the Sun.

Alkaid, also known as Eta Ursae Majoris, is the third brightest star in the constellation of Ursa Major, the celestial Great Bear. It is also the 38th brightest star in the night sky sharing the title with Sargas – Theta Scorpii.

Key Facts & Summary

  • Alkaid is a blue main-sequence star of spectral type B3 V, located at around 103.9 light-years / 31.9 parsecs away from the Sun.
  • It is the easternmost star of the famous Big Dipper / Plough asterism and unlike most stars of this asterism, it isn’t part of the Ursa Major moving group of stars.
  • Alkaid is also one of the 15 Behenian Fixed Stars. A group of stars used in medieval times in magic rituals.
  • The star’s spectrum has served since 1943 as one of the stable anchor points by which other stars are classified.
  • Alkaid is an X-ray emitting star with broadened absorption lines in its spectrum due to its rapid rotation which may be responsible for the presence of a weak disk of material shrugged off of the star.
  • The star has a rotational velocity of around 150 km / 93.2 mi per second.
  • Alkaid has an apparent magnitude of +1.86 and an absolute magnitude of -0.67.
  • It is a very young star, being only around 10 million years old or so. Much younger than our sun.
  • Alkaid is much more massive than our sun, having 610% of the sun’s mass.
  • It is also bigger, with an estimated 3.4 solar radii.
  • Alkaid is a very energetic star, being at around 594 times brighter than our Sun.
  • The star has a surface gravity of around 3.8 cgs and it is also hotter than our sun, having surface temperatures of around 15.540 K.
  • The radial velocity of Alkaid has been estimated to be at around -10.9 km / -6.7 mi per second.
  • Currently, no planets or other companions have been detected.

Throughout the ages, Eta Ursae Majoris bore many names such as Alkaid, Elkaid, or Benetnasch. The most traditional name, Alkaid, is of Arabic origin and translates to “leader.”

The reason for this is because the name is actually a derivation from an Arabic phrase that translates to “the leader of the daughters of the bier.” It is referencing the star’s symbolic meaning since to the ancient Arabs, the Big Dipper asterism symbolized a funeral procession.

Consequently, the stars forming the Dipper’s handle – Alkaid, Mizar, and Alioth represented the mourners while the stars which formed the Dipper’s bowl, Megrez, Phecda, Dubhe, and Merak, represented the bier or coffin.

Alkaid was also, in the medieval times, one of the 15 Behenian Fixed Stars. These stars were believed to have magical properties and were associated with gemstones, plants, or planets. 

The plants or gemstones were used in magic rituals since they represented one of the specific stars and their magical traits,  while the planets had other astrological characteristics that were influenced by those stars. In the case of Alkaid, it was associated with the Moon and Venus. Its gemstone was magnet while the plant that represented it was succory.

Formation

Most estimates point out that Alkaid likely formed around 10 million years ago, give or take 2.8 million years. It is a very young star and in regards to its neighbors in the Big Dipper asterism, it isn’t part of the Ursa Major moving group of stars. Thus, its origin is unknown. Alkaid likely formed in a regular molecular cloud of dust and gas. Gravity pulled the swirling gas and dust together and resulted in the third brightest star of the Ursa Major constellation, Alkaid.

Distance, Size, and Mass

Alkaid is located at around 103.9 light-years / 31.9 parsecs away from the Sun. It is relatively close to us, and since it is the 38th brightest star in the night sky, it can be seen with the naked eye. It shares its title as the 38th brightest with another star, Sargas – Theta Scorpii.

Alkaid is much more massive and bigger than our sun. It has 610% of the sun’s mass or 6.1 solar masses. Alkaid’s radius has been estimated to be at around 3.4 solar radii, or 340% the sun’s radius. Since a star’s diameter is usually twice its radius, Alkaid is several times bigger than our sun.

Other Characteristics

Alkaid is a blue main-sequence star of spectral type B3 V, and though it is young, it has begun burning hydrogen into helium in its core. It has an apparent magnitude of +1.86 and an absolute magnitude of -0.67.

Alkaid’s spectrum has served since 1943 as one of the stable anchor points by which other stars are classified. It is an X-ray emitting star with broadened absorption lines in its spectrum due to its rapid rotation which may be responsible for the presence of a weak disk of material shrugged off of the star. 

Usually, it is speculated that planets may form in these discs of dust, however, Alkaid has been greatly studied and observed and currently, no planets have been confirmed, or any other companions. The star has a rotational velocity of around 150 km / 93.2 mi per second.

This high velocity does not only contribute to the star’s material expulsion but also distorts the star giving it an oblate spheroidal shape. This usually means that its equatorial radius is bigger than its radius at the poles and it may be possible that there are also temperature variations because of this. Some other stars which appear to share this trait, are Vega and Achernar.

Alkaid is a very energetic star, being at around 594 times brighter than our Sun. The star has a surface gravity of around 3.8 cgs and it is also hotter than our sun, having surface temperatures of around 15.540 K. This is around 2.6 times hotter than our Sun. The radial velocity of Alkaid has been estimated to be at around -10.9 km / -6.7 mi per second.

Location

Alkaid is located in the constellation of Ursa Major, the celestial Great Bear. It is the leftmost star of the Big Dipper’s handle. Alkaid also marks the Great Bear’s celestial tail.

Alkaid and thus the Ursa Major constellation is circumpolar and can be seen throughout the year to most observers in the northern hemisphere. All the months of spring are favorable in observing this constellation. 

Ursa Major is the third largest constellation and the largest of all the northern constellations. It hosts many interesting stars and deep sky objects such as galaxies, nebulae, Messier objects, and clusters.

The Future

Most B-type stars such as Alkaid, have short life spans. The star is expected to burn all of its fuel in less than 100 million years after which it will start to evolve into a giant star. It will later shed its outer layers and become a white dwarf with a mass of around 85% that of the sun’s mass.

Did you know?

  • The Chinese knew Alkaid as the Seventh Star of the Northern Dipper – an asterism which corresponds to the western Big Dipper asterism. It was also known as the Star of Twinkling Brilliance.
  • The other Asian countries such as Japan and Korean knew Alkaid as Hagusei – the military breaking star – and Mukokseong – most corner star – This suggests that both names originated in ancient China.
  • In Hindu mythology, Alkaid was associated with one of the Seven Sages, one of the sons of Brahma and the grandfather of the deities Asuras and Devas.
  • In Chinese fortune-telling, the star is associated with the north direction – was believed to be unlucky and thus both hunters and soldiers traditionally avoided pointing their guns in Alkaid’s direction.

Sources:

  1. Wikipedia
  2. Star-facts
  3. Universeguide
  4. Astronomytrek

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