Kaus Australis (ε Sagittarii) Facts

Kaus Australis is an evolved giant star of spectral type B9.5 III, located in the southern part of the Sagittarius constellation at around 143 light-years / 44 parsecs away from the Sun.

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Kaus Australis, known as Epsilon Sagittarii, is the brightest star in the zodiacal constellation of Sagittarius and the 37th brightest star in the night sky. It is a binary star system.

Key Facts & Summary

  • Kaus Australis is an evolved giant star of spectral type B9.5 III, located in the southern part of the Sagittarius constellation at around 143 light-years / 44 parsecs away from the Sun.
  • It has an apparent magnitude of +1.85 and an absolute magnitude of -1.41.
  • Kaus Australis is a binary star system, consisting of Epsilon Sagittarii A and Epsilon Sagittarii B.
  • Epsilon Sagittarii A is the primary star, having around 3.515 solar masses and 6.8 solar radii, much bigger, and massive than our Sun.
  • Epsilon Sagittarii A is 363 times brighter than our Sun, and it is also hotter, with average surface temperatures at around 9.960 K.
  • Epsilon Sagittarii A is also a fast spinner, having a rotational velocity estimated at around 236 km / 146.6 mi per second.
  • Epsilon Sagittarii B is a main-sequence star much smaller than its companion. 
  • Epsilon Sagitarrii B has around 95% of the sun’s mass and 89% its luminosity.
  • Epsilon Sagitarri B is also slightly hotter than our sun, with surface average temperatures at around 5.807 K.
  • The two stars are separated from each other at a distance of around 106 AU.
  • The primary star, Epsilon Sagitarri A, is surrounded by a circumstellar disk of dust.
  • The circumstellar disk of dust is located at around 155 AU from Epsilon Sagitarri A.
  • This means that the secondary star, Epsilon Sagitarri B, is actually engulfed in this disk of dust.
  • Another candidate star for this stellar system is speculated to exist but it is very uncertain.

Though the brightest stars are usually designated alpha, it is not necessarily a rule of thumb. The traditional name, Kaus Australis, actually means “the southern bow.”

It is derived from the Arabic word qaws, meaning “bow” and the Latin “australis”, meaning southern. It was named as such since it is the southernmost star marking the bow of the celestial Archer.

Formation

Kaus Australia formed at around 232 million years ago. It is quite a young star, much younger than our sun. It is unknown if the star is associated with any other group of moving stars, or if both stars in the system formed at the same time. 

Distance, Size, and Mass

Kaus Australis is located at around 143 light-years / 44 parsecs away from the Sun. It is the 37th brightest star in the night sky. It is also a binary star system.

The primary star, designated Epsilon Sagittarii A, is much bigger than our Sun. Epsilon Sagittarri A has 3.515 solar masses and 6.8 solar radii. It has an angular diameter of around 1.44 milliarcseconds.

The secondary star, Epsilon Sagittarii B, is smaller than both the primary star and the sun. Epsilon Sagittarri B has only 0.95 solar masses, or 95% of the sun’s mass though its radius is currently unknown.

Other Characteristics

Epsilon Sagittarii B is an evolved giant star of spectral type B9.5 III, appearing blue-white in color. It has an apparent magnitude of +1.85 and an absolute magnitude of -1.41.

The primary star is 363 times brighter than our Sun, and it is also hotter, with average surface temperatures at around 9.960 K. This is around 1.7 times hotter than our sun.

The star is surrounded by a circumstellar disk of dust evidenced by excessive emissions of infrared radiation Epsilon Sagittarii A is also a fast spinner, having a rotational velocity estimated at around 236 km / 146.6 mi per second.

Less is known about the secondary star, Epsilon Sagittarii B, other than the fact that it is a main-sequence star. It shines only with around 89% the sun’s luminosity and it is also engulfed by the circumstellar disk of dust. The average surface temperatures on Epsilon Sagittarri B, are almost exactly the same as on the sun’s surface, 5.807 K. For comparison, the sun has surface temperatures of 5.778 K.

Stellar System

Epsilon Sagittarii A and B are separated from one another by a distance of 2.392 arcseconds or around 106 AU. The circumstellar disk surrounding them extends at a distance of around 155 AU.

Another star separated at around 32.3 arcseconds from Epsilon Sagittarii A, is also believed to be part of the stellar system. It is unclear though if it is gravitationally bound to the primary star or it is simply in the same line of sight.

Location

Kaus Australis is located in the zodiacal constellation of Saggitarius, the celestial archer. This constellation is situated exactly towards the center of the Milky Way.

Together with Kaus Media and Kaus Borealis, Kaus Australis marks the celestial Archer’s bow. The same stars together with Alnasl, Ascella, Nunki, Tau Sagittarii, and Phi Sagittarii form the Teapot asterism. This asterism is very bright and dominates the western half of Sagittarius.

Kaus Australis marks the celestial archer’s bow tip, being the southernmost star, and it is also the lower right star of the Teapot marking the teapot’s body with Ascella, Kaus Media, and Phi Sagittarii.

Since the constellation of Saggitarius lies in the direction of the Milky Way’s center, it is located in the densest region of the galaxy. There are many interesting stars and deep-sky objects in Saggitarius, such as globular clusters, Messier objects, nebulae, and much more.

The month of August is the best time to try and observe the celestial objects in Sagittarius. Most objects are not visible year-round to northern observers and even farther north, they are completely invisible. When it comes to the mid-northern latitudes, they can only be seen above the southern horizon during the summer months.

The Future

Epsilon Sagittarii B is classified as a zero-age main-sequence star. This means it has only joined the main sequence by burning hydrogen into helium in its core. Epsilon Sagittarii A on the other hand, has already exhausted its hydrogen supplies and has started to expand into a giant. It will continue to expand several times its current size.

Did you know?

  • Among the stars of the Teapot asterism, only Kaus Australis and Nunki were included in the list of the 58 navigational stars. Only the brightest and the most easily recognizable stars are included in this group.
  • The Egyptian astronomer Al Achsasi al Mouakket listed Kaus Australis as “the third of Warida” in his 17th-century Calendarium.
  • Warida is an asterism that consists of Kaus Australis, Kaus Media, Eta Sagitarrii, and Gamma Sagitarri. Warida translates to “the ostrich going down the water”, a representation of the two celestial groups of ostriches in Sagittarius, the other group is marked by Sigma, Phi, Zeta, Chi, and Tau Sagittarii. Kaus Australis and Polis represented the ostriches’ keepers.
  • Kaus Australis, Kaus Media, and Alnasl formed the ancient Akkadian asterism known as Sin-nun-tu, meaning shadow, while in the Babylonian compendium MUL.APIN, Kaus Australis was listed as the Bark.
  • The Chinese named the star the Third Star of Winnowing Basket – an asterism consisting of Gamma, Delta, and Eta Sagittarii.
  • The Kalapalo people of Mato Grosso state in Brazil called this star, along with many others from the neighboring Scorpius constellation, Taugi kusugu, “Taugi’s fishing basket.”

Sources:

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

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