Epsilon Sagittarii can best be viewed in the month of August.[12] The star is at least 10 degrees away from the ecliptic, and the Sun passes it overhead around December 25-26th. The star is visible from late January to late November from mid-northern latitudes. From mid-southern latitudes, the star is visible from early January until the middle of December. The star is not visible in areas above 55°N latitude. From latitudes below 55°S, Epsilon Sagittarii is a circumpolar star.
The primary star, ε Sagittarii A, of this binary star system has a stellar classification of B9.5 III,[3] with the luminosity class of III suggesting this is an evolvedgiant star that has exhausted the supply of hydrogen at its core. The interferometry-measured angular diameter of this star, after correcting for limb darkening, is 1.44 ± 0.06 mas,[13] which, at its estimated distance, equates to a physical radius of about 6.8 times the radius of the Sun.[6] This is a close match to the empirically-determined value of 6.9 solar radii.[14] It has about 3.5 times the mass of the Sun and is radiating around 363 times the Sun's luminosity from its outer atmosphere at an effective temperature of 9,960 K.[5] At this heat, the star glows with a blue-white hue.[15]
This star is spinning rapidly with a projected rotational velocity of 236 km s−1.[9] It has a magnetic field with a strength in the range 10.5–130.5 Gauss[16] and it is an X-ray source with a luminosity of about 1030 erg s−1.[5] The system displays an excess emission of infrared radiation, which suggests the presence of a circumstellar disk of dust. Based upon the temperature of this disk, it is orbiting at a mean separation of 155 AU from the primary.[17]
As of 2001, the secondary star, ε Sagittarii B, is located at an angular separation of 2.392 arcseconds from the primary along a position angle of 142.3°. At the distance of this system, this angle is equivalent to a physical separation of about 106 AU, which places it inside the debris disk. It is a main sequence star with about 95% of the mass of the Sun. The system has a higher optical linear polarisation than expected for its distance from the Sun; this has been attributed to light scattered off the disk from the secondary.[18] Prior to its 1993 identification using an adaptive opticscoronagraph, this companion may have been responsible for the spectral anomalies that were attributed to the primary star.[19] There is a candidate stellar companion at an angular separation of 32.3 arcseconds.[5]
It bore the traditional name Kaus Australis, which derived from the Arabic قوس qaws 'bow' and Latinaustrālis 'southern'. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[20] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[21] included a table of the first two batches of names approved by the WGSN; which included Kaus Australis for the star ε Sagittarii A.
In the catalogue of stars in the CalendariumofAl Achsasi al Mouakket, this star was designated Thalath al Waridah, or Thalith al Waridah, meaning 'third of Warida'.[22]
^ abcdJohnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4: 99–100, Bibcode:1966CoLPL...4...99J
^ abHouk, Nancy; Smith-Moore, M. (1979), "Michigan catalogue of two-dimensional spectral types for the HD stars", Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars. Volume_3. Declinations -40.0° to -26.0°, 3, Ann Arbor, Michigan: Dept. of Astronomy, University of Michigan, Bibcode:1982mcts.book.....H
^Evans, D. S. (June 20–24, 1966). "The Revision of the General Catalogue of Radial Velocities". In Batten, Alan Henry; Heard, John Frederick (eds.). Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30. Determination of Radial Velocities and Their Applications. Vol. 30. University of Toronto: International Astronomical Union. p. 57. Bibcode:1967IAUS...30...57E.
^Adelman, Saul J. (December 2004). "The physical properties of normal A stars". In Zverko, J.; Ziznovsky, J.; Adelman, S. J.; Weiss, W. W. (eds.). The A-Star Puzzle, held in Poprad, Slovakia, July 8-13, 2004. IAU Symposium. Vol. 2004, no. 224. Cambridge, UK: Cambridge University Press. pp. 1–11. Bibcode:2004IAUS..224....1A. doi:10.1017/S1743921304004314.
^Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN978-1-931559-44-7.
^Richichi, A.; Percheron, I.; Khristoforova, M. (February 2005), "CHARM2: An updated Catalog of High Angular Resolution Measurements", Astronomy and Astrophysics, 431 (2): 773–777, Bibcode:2005A&A...431..773R, doi:10.1051/0004-6361:20042039
^Jerzykiewicz, M.; Molenda-Zakowicz, J. (September 2000), "Empirical Luminosities and Radii of Early-Type Stars after Hipparcos", Acta Astronomica, 50: 369–380, Bibcode:2000AcA....50..369J
^Golimowski, David A.; Durrance, Samuel T.; Clampin, Mark (March 1993), "Detection of an apparent star 2.1 arcsec from the circumstellar disk candidate Epsilon Sagittarii", Astronomical Journal, 105 (3): 1108–1113, Bibcode:1993AJ....105.1108G, doi:10.1086/116498
^Rogers, J. H. (February 1998), "Origins of the ancient constellations: I. The Mesopotamian traditions", Journal of the British Astronomical Association, 108 (1): 9–28, Bibcode:1998JBAA..108....9R
