Epsilon Sagittarii, also known as Kaus Australis, has an apparent visual magnitude of +1.85, making it the most luminous star in the southern sky constellation of Sagittarius. Along with the stars Gamma Sagittarii, Delta Sagittarii, Zeta Sagittarii, Lambda Sagittarii, Sigma Sagittarii, Tau Sagittarii and Phi Sagittarii, Epsilon Sagittarii forms part of the famous Teapot asterism, which is located in the western half of this ancient zodiacal constellation lying in the direction of the Milky Way.
• Constellation: Sagittarius
• Coordinates: RA : 18h 24m 10.31840s|Dec. –34° 23′ 04.6193″
• Distance: 143 light years
• Star Type: B9.5 III
• Mass: (e Sgr A) 3.515 sol (e Sgr B) 0.95 sol
• Radius: (e Sgr A) 6.8 sol
• Apparent Magnitude: +1.85 (Combined)
• Luminosity: (e Sgr A) 363 |( e Sgr B) 0.89 sol
• Surface Temperature: (e Sgr A) 9,960K |( e Sgr B) 5,807K
• Rotational Velocity: (e Sgr A) 236 km/sec
• Age: 232 million years
• Other Designations: Kaus Australis, e Sagittarii, e Sgr, Epsilon Sgr, 20 Sagittarii, CCDM J18242-3423A, FK5 689, GC 25100, HD 169022
Sagittarius is situated between Capricornus to its east and Scorpius to its west, with the constellation visible from latitudes of between +55° and -90°. Being the southern-most of the zodiacal constellations, northern hemisphere residents can observe Sagittarius low on the southern horizon during the months of July, August, and the first days of September, but by the middle of September it starts to set at about the same time as the Sun.
While Epsilon Sagittarii appears to be a single star, it is in fact a binary system whose primary component, designated e Sgr A, is an evolved blue-white giant (B9.5 III) which has exhausted the supply of hydrogen fuel at its core. At a distance of about 143 light years from Earth, the primary component shows an angular diameter of 1.44 milliarcseconds, which translates into a physical radius of roughly 6.8 times that of the Sun. Its mass is also about 3.5 times sol, although the star’s temperature is commensurately higher, which in turn gives it a luminosity of about 363 that of the Sun. Epsilon Sagittarii is also a very fast rotator, with a projected rotational velocity of 236 km/sec, which is a significant percentage of the speed at which it would break apart. Estimates of the stars’ magnetic field vary, although most investigators agree that it falls in a range of between 10.5 Gauss, and 130.5 Gauss. Apart from being a reasonably luminous X-ray source, shining at about 1030 erg s-1, Epsilon Sagittarii also displays an excess of infrared radiation, which suggests that the primary star is surrounded by a disc of dust that is separated from it by about 155 astronomical units, based on the temperature of the dust disc.
The secondary star in the system, designated e Sgr B, is a main-sequence star that is separated from the primary by 2.392 seconds of arc, and a position angle of 142.30 as of 2001. Given that a position angle is measured in an anti-clockwise direction from the north celestial pole, this angle translates into a physical distance of roughly 106 astronomical units, which places the secondary star within the dust disc. Its presence in the disc would explain the higher than expected optical linear polarization of light from the system at this distance, since the secondary star’s light would be scattered by dust in the disc.
The star’s’ traditional name, Kaus Australis, derives from the Arabic word “qaws” (bow) and the Latin word “australis” (southern), with the star therefore marking the southern base of the celestial archer’s bow. The other stars making up the constellation’s bow are Delta Sagittarii (Kaus Media) and Lambda Sagittarii (Kaus Borealis). The star’s name “Kaus Australis” was formally accepted by the International Astronomical Union in 2016.
However, in the old star catalogue known as the Calendarium, which was compiled by Egyptian astronomer Al Achsasi al Mouakket in 1650, Kaus Australis was part of an asterism the Arabs knew as Al Warida (the ostrich), with the star called Thalath al Waridah (third of Warida). One other interesting name for the star comes from China, where the star forms part of an asterism known as “Ji “, which is made up of the stars Gamma Sagittarii, Delta Sagittarii, Eta Sagittarii, and Epsilon Sagittarii. Due to its position in this asterism, Epsilon Sagittarii is known as “Ji Sù san”, which means “the Third Star of Winnowing Basket,” referring to a manuring tray used in Chinese agriculture.