Alpha Trianguli Australis

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Alpha Trianguli Australis
Diagram showing star positions and boundaries of the Triangulum Australe constellation and its surroundings
Cercle rouge 100%.svg

Location of α Trianguli Australis (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Triangulum Australe
Right ascension 16h 48m 39.89508s[1]
Declination –69° 01′ 39.7626″[1]
Apparent magnitude (V) 1.91[2]
Spectral type K2 IIb-IIIa[3]
U−B color index +1.56[4]
B−V color index +1.44[4]
Radial velocity (Rv) –3.3[2] km/s
Proper motion (μ) RA: 17.99[1] mas/yr
Dec.: –31.58[1] mas/yr
Parallax (π) 8.35 ± 0.15[1] mas
Distance 391 ± 7 ly
(120 ± 2 pc)
Absolute magnitude (MV) –3.68[5]
Mass 7[3] M
Luminosity 5,500[5] L
Surface gravity (log g) 1.5[3] cgs
Temperature 4,150[3] K
Metallicity [Fe/H] –0.06[3] dex
Age 4.8 × 107[3] years
Other designations
Atria, HR 6217, CD -68°2822, HD 150798, SAO 253700, FK5 625, GC 22558, HIP 82273.[6]
Database references

Alpha Trianguli Australis (α TrA, α Trianguli Australis) is the brightest star in the southern constellation Triangulum Australe, forming an apex of a triangle with Beta and Gamma Trianguli Australis that gives the constellation its name (Latin for southern triangle). This star has the traditional name Atria, which is merely a contraction of its Bayer designation.[7] In traditional Chinese it is called 三角形三 (Mandarin: sān jiǎo xín sān), the Third Star of the Triangle.

Alpha Trianguli Australis is a bright giant star with an apparent magnitude of +1.91. Based upon parallax measurements, this star is located roughly 391 light-years (120 parsecs) distant from the Earth.[1] The estimated age of the star is 48 million years old; sufficiently old for a massive star to evolve away from the main sequence and expand into a giant.[3] It has a mass roughly seven times the mass of the Sun, but is emitting about 5,500 times the Sun's luminosity. The effective temperature of the star's outer envelope is 4,150 K,[3] which gives it the characteristic orange hue of a K-type star.[8] With a diameter 130 times that of our sun, it would almost reach the orbit of Venus if placed at the centre of the Solar System. The proper name Atria is a contraction of its Bayer designation.[9]

There is evidence that Atria may be a binary star. It displays unusual properties for a star of its class, including stellar flares and a higher than normal emission of X-rays. These can be explained by a young, magnetically active companion with a stellar classification of about G0 V. Such a star would have a mass similar to the Sun, with an orbital period of at least 130 years. Young, G-type stars have a high temperature corona and frequently emit flares causing sudden increases in luminosity. The pair may be separated by about 50 Astronomical Units.[5]

In culture[edit]

Atria appears on the flag of Brazil, symbolizing the state of Rio Grande do Sul.[10]


  1. ^ a b c d e f van Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. 
  2. ^ a b Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb (Astronomisches Rechen-Institut Heidelberg) 35 (35), Bibcode:1999VeARI..35....1W 
  3. ^ a b c d e f g h Kovacs, N. (April 1983), "Model-atmosphere analysis of high-dispersion spectra of four red giants and supergiants", Astronomy and Astrophysics 120 (1): 21–35, Bibcode:1983A&A...120...21K 
  4. ^ a b Johnson, H. L.; et al. (1966). "UBVRIJKL photometry of the bright stars". Communications of the Lunar and Planetary Laboratory 4 (99): 99. Bibcode:1966CoLPL...4...99J. 
  5. ^ a b c Ayres, Thomas R.; Brown, Alexander; Harper, Graham M. (April 2007), "α TrA Junior", The Astrophysical Journal 658 (2): L107–L110, Bibcode:2007ApJ...658L.107A, doi:10.1086/514818 
  6. ^ "alf TrA -- Star", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2011-12-26 
  7. ^ Ridpath, Ian (1989), Star tales, James Clarke & Co., p. 125, ISBN 0-7188-2695-7 
  8. ^ "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16 
  9. ^ Kaler, Jim. "Atria (Alpha Trianguli Australis)". Stars. University of Illinois. Retrieved 17 October 2012. 
  10. ^ "Astronomy of the Brazilian Flag". FOTW Flags Of The World website. Retrieved 2011-12-26. 

External links[edit]