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α Trianguli Australis

Location of α Trianguli Australis (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Triangulum Australe
Right ascension	16 48 39.89508
Declination	−69° 01′ 39.7626″
Apparent magnitude (V)	1.91
Characteristics
Spectral type	K2Ib-IIa
U−B color index	+1.56
B−V color index	+1.44
Astrometry
Radial velocity (Rv)	−3.3 km/s
Proper motion (μ)	RA: +17.99 mas/yr Dec.: −31.58 mas/yr
Parallax (π)	8.35 ± 0.15 mas
Distance	391 ± 7 ly (120 ± 2 pc)
Absolute magnitude (MV)	−3.68
Details
Mass	7 M☉
Radius	143 – 167 R☉
Luminosity	5,500 L☉
Surface gravity (log g)	1.5 cgs
Temperature	4,150 K
Metallicity	−0.06 dex
Age	48 Myr
Other designations
Atria, HR 6217, CD −68°2822, HD 150798, SAO 253700, FK5 625, GC 22558, HIP 82273.
Database references
SIMBAD	data

Alpha Trianguli Australis (Latinised from α Trianguli Australis, abbreviated Alpha TrA, α TrA), officially named Atria /ˈeɪtriə/, is the brightest star in the southern constellation of Triangulum Australe, forming an apex of a triangle with Beta Trianguli Australis and Gamma Trianguli Australis that gives the constellation its name (Latin for southern triangle).

Nomenclature

α Trianguli Australis (Latinised to Alpha Trianguli Australis) is the star's Bayer designation. The historical name Atria is a contraction. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Atria for this star.

In Chinese caused by adaptation of the European southern hemisphere constellations into the Chinese system, 三角形 (Sān Jiǎo Xíng), meaning Triangle, refers to an asterism consisting of α Trianguli Australis, β Trianguli Australis and γ Trianguli Australis. Consequently, α Trianguli Australis itself is known as 三角形三 (Sān Jiǎo Xíng sān, English: the Third Star of Triangle.)

Physical characteristics

Alpha Trianguli Australis is a supergiant or 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. The estimated age of the star is 48 million years old; sufficiently old for a massive star to evolve away from the main sequence. N. Covacs give Alpha Trianguli Australis a stellar classification of K2IIb-IIa, with the luminosity class indicating an evolutionary stage between a bright giant and a regular giant. However, a study led by Thomas R. Ayres found that its absolute magnitude is similar to that of supergiants such as Beta Aquarii or Beta Camelopardalis, so its stellar classification should be at least K2Ib-IIa.

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, which gives it the characteristic orange hue of a K-type star. With a diameter 143 or 167 times that of the Sun, it would reach or almost reach the orbit of Venus if placed at the centre of the Solar System.

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.

In culture

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

Notes

1. Applying the Stefan–Boltzmann law with a nominal solar effective temperature of 5,772 K:

   ${\displaystyle {\sqrt {(5772/4,150)^{4}*5,500}}=143.46\ R\odot }$

2. Applying the Stefan–Boltzmann law with a nominal solar effective temperature of 5,772 K:

   ${\displaystyle {\sqrt {(5772/4,179)^{4}*7,623}}=166.56\ R\odot }$

References

1. ^ 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. S2CID 18759600.
2. ^ Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb, 35 (35), Astronomisches Rechen-Institut Heidelberg: 1, Bibcode:1999VeARI..35....1W
3. ^ 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, S2CID 119375854
4. ^ 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. ^ 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
6. ^ Escorza, A.; Boffin, H. M. J.; Jorissen, A.; Van Eck, S.; Siess, L.; Van Winckel, H.; Karinkuzhi, D.; Shetye, S.; Pourbaix, D. (2017-12-01). "Hertzsprung-Russell diagram and mass distribution of barium stars". Astronomy and Astrophysics. 608: A100. arXiv:1710.02029. doi:10.1051/0004-6361/201731832. ISSN 0004-6361.
7. "alf TrA -- Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2011-12-26
8. 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. ISBN 978-1-931559-44-7.
9. "IAU Catalog of Star Names". Retrieved 28 July 2016.
10. Ridpath, Ian (1989), Star tales, James Clarke & Co., p. 125, ISBN 0-7188-2695-7
11. Kaler, Jim. "Atria (Alpha Trianguli Australis)". Stars. University of Illinois. Retrieved 17 October 2012.
12. "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
13. "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
14. (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 29 日 Archived 2011-05-22 at the Wayback Machine
15. "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on February 22, 2012, retrieved 2012-01-16
16. "Astronomy of the Brazilian Flag". FOTW Flags Of The World website. Retrieved 2011-12-26.

External links

• Kaler, James B., "ATRIA (Alpha Trianguli Australis)", Stars, University of Illinois, archived from the original on 2008-12-05, retrieved 2011-12-26

v t e Constellation of Triangulum Australe
List of stars in Triangulum Australe Triangulum Australe in Chinese astronomy
Stars	Bayer α (Atria) β γ δ ε ζ η θ ι κ Variable R S U X RT EK KX LP LT LX MX HR 5611 5621 5684 5713 5792 5851 5884 5898 5905 5920 5951 6037 6089 6125 6182 6231 6233 HD 141969 144481 147018 150097 153389 Other 4U 1626-67 Triangulum Australe X-1 WISE 1639−6847 WR 69 X1556-605
Exoplanets HD 147018 b c
Star clusters NGC 6025
Nebulae NGC 5979
Galaxies ESO 69-6 ESO 137-001
Category

• Bayer objects
• Henry Draper Catalogue objects
• Hipparcos objects
• K-type bright giants
• Triangulum Australe
• Bright Star Catalogue objects
• Durchmusterung objects
• Stars with proper names
• K-type supergiants
• K-type giants