Astronomy:Alpha Crucis

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α Crucis
Acrux kstars.png
The position of Acrux.
Observation data
Equinox J2000.0]] (ICRS)
Constellation Crux
Right ascension  12h 26m 35.89522s[1]
Declination −63° 05′ 56.7343″[1]
Apparent magnitude (V) 0.76[2] (1.33 + 1.75)[3]
Spectral type B0.5IV + B1V[4]
B−V color index −0.26[2]
Radial velocity (Rv)−11.2 / −0.6[5] km/s
Proper motion (μ) RA: −35.83[1] mas/yr
Dec.: −14.86[1] mas/yr
Parallax (π)10.13 ± 0.50[1] mas
Distance320 ± 20 ly
(99 ± 5 pc)
Absolute magnitude (MV)−3.77[6] (−2.2 + −2.7[7])
Companionα Crucis Ab
Period (P)75.7794±0.0037 d
Eccentricity (e)0.46±0.03
Periastron epoch (T)2417642.3±1.6 JD
Argument of periastron (ω)
Semi-amplitude (K1)
41.7±1.2 km/s
Mass17.80 + 6.05[3] M
Luminosity25,000[9] L
Temperature24,000[10] K
Rotational velocity (v sin i)120[10] km/s
Mass15.52[3] M
Luminosity16,000[9] L
Temperature28,000[10] K
Rotational velocity (v sin i)200[10] km/s
Age10.8[11] Myr
Other designations
α Crucis, HIP 60718, CPD−62°2745, WDS J12266-6306, CCDM J12266-6306
α1 Cru: Acrux, HR 4730, HD 108248, FK5 462, GC 16952, 26 G. Crucis
α2 Cru: HR 4731, HD 108249, GC 16953, 2MASS J12263615-6305571, 27 G. Crucis
Database references
α1 Cru
α2 Cru

Alpha Crucis (α Crucis, abbreviated Alpha Cru, α Cru) is a multiple star system 321 light-years from the Sun[1][12] in the constellation of Crux and part of the asterism known as the Southern Cross. With a combined visual magnitude of 0.76, it is the brightest star in Crux and the 13th brightest star in the night sky. It is the southernmost first-magnitude star, 2.3 degrees more southerly than Alpha Centauri.[13]

To the naked eye Alpha Crucis appears as a single star, but it is actually a multiple star system. Two components are visually distinguishable: α1 Crucis and α2 Crucis; alternatively designated α Crucis A and α Crucis B. Both are B-type stars, and are many times more massive and luminous than the Sun. α1 Crucis is itself a spectroscopic binary with components designated α Crucis Aa (also named Acrux[14]) and α Crucis Ab. Its two component stars orbit every 76 days at a separation of about 1 astronomical unit (au).[9]


The constellation Crux

α Crucis (Latinised to Alpha Crucis) is the system's Bayer designation; α1 and α2 Crucis, those of its two constituents. The designations of these two constituents as α Crucis A and B and those of A's components - α Crucis Aa and Ab - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[15]

The historical name Acrux for α1 Crucis is an 'Americanism' coined in the 19th century, but entering common use only by the mid 20th century.[16] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[17] to catalog and standardize proper names for stars. The WGSN states that in the case of multiple stars the name should be understood to be attributed to the brightest component by visual brightness.[18] The WGSN approved the name Acrux for α Crucis Aa on 20 July 2016 and it is now so entered in the IAU Catalog of Star Names.[14]

Since α Crucis is at −63° declination, the southernmost first-magnitude star, it is only visible south of latitude 27° North. It barely rises from cities such as Miami, Florida, or Karachi, Pakistan (both around 25°N) and not at all from New Orleans, Louisiana, or Cairo, Egypt (both about 30°N). Because of Earth's axial precession, the star was visible to ancient Hindu astronomers in India who named it Tri-shanku. It was also visible to the ancient Romans and Ancient Greece , who regarded it as part of the constellation of Centaurus.[19]

In Chinese, 十字架 (Shí Zì Jià, "Cross"), refers to an asterism consisting of Alpha Crucis, Gamma Crucis, Beta Crucis and Delta Crucis.[20] Consequently, α Crucis itself is known as 十字架二 (Shí Zì Jià èr, "the Second Star of Cross".).[21]

This star is known as Estrela de Magalhães ("Star of Magellan") in Portuguese.[22]

Stellar properties

α Crucis with the nearby HD 108250

The two components, α1 and α2 Crucis, are separated by 4 arcseconds. α1 is magnitude 1.40 and α2 is magnitude 2.09, both early class B stars, with surface temperatures of about 28,000 and 26,000 K respectively. Their luminosities are 25,000 and 16,000 times that of the Sun. α1 and α2 orbit over such a long period that motion is only barely seen. From their minimum separation of 430 astronomical units, the period is estimated to be around 1,500 years.[3]

α1 is itself a spectroscopic binary star, with its components thought to be around 14 and 10 times the mass of the Sun and orbiting in only 76 days at a separation of about 1 AU. The masses of α2 and the brighter component α1 suggest that the stars will someday explode as supernovae. The unseen fainter component of α1 may survive to become a massive white dwarf.[9]

The cooler less luminous B class star HR 4729 (HD 108250) lies 90 arcseconds away from triple α Crucis and shares its motion through space, suggesting it may be gravitationally bound to it, and it is therefore generally assumed to be physically associated.[23][24] It is itself a spectroscopic binary system, sometimes catalogued as component C of the α Crucis multiple system, and it has a faint visual companion listed as component D. A further seven faint stars are also listed as companions out to a distance of about two arc-minutes.[25]

Rizzuto and colleagues determined in 2011 that the α Crucis system was 66% likely to be a member of the Lower Centaurus-Crux sub-group of the Scorpius-Centaurus Association. It was not previously seen to be a member of the group.[26]

On 2008 October 2, the Cassini–Huygens spacecraft resolved three of the components (A, B and C) of the multiple star system as Saturn's disk occulted it.[27][28]

In culture

α Crucis is represented in the flags of Australia, New Zealand, Samoa, and Papua New Guinea as one of five stars which comprise the Southern Cross. It is also featured in the flag of Brazil , along with 26 other stars, each of which represents a state; α Crucis representing the State of São Paulo.[29]

The Brazilian oceanographic research vessel Alpha Crucis is named after the star.

The Australian wine company Alpha Crucis[1] is named after the star and is regarded amongst the best wineries in Australia by critic James Halliday.[2]


  1. 1.0 1.1 1.2 1.3 1.4 1.5 van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, doi:10.1051/0004-6361:20078357, Bibcode2007A&A...474..653V 
  2. 2.0 2.1 Corben, P. M. (1966). "Photoelectric magnitudes and colours for bright southern stars". Monthly Notes of the Astron. Soc. Southern Africa 25: 44. Bibcode1966MNSSA..25...44C. 
  3. 3.0 3.1 3.2 3.3 Tokovinin, A. A. (1997). "MSC - a catalogue of physical multiple stars". Astronomy and Astrophysics Supplement Series 124 (1): 75–84. doi:10.1051/aas:1997181. ISSN 0365-0138. Bibcode1997A&AS..124...75T. 
  4. Houk, Nancy (1979), "Michigan catalogue of two-dimensional spectral types for the HD stars", Ann Arbor : Dept. of Astronomy (Ann Arbor, Michigan: Dept. of Astronomy, University of Michigan) 1, 
  5. Wilson, Ralph Elmer (1953). General Catalogue of Stellar Radial Velocities. Washington: Carnegie Institution of Washington. Bibcode1953GCRV..C......0W. 
  6. Kaltcheva, N. T.; Golev, V. K.; Moran, K. (2014). "Massive stellar content of the Galactic supershell GSH 305+01-24". Astronomy & Astrophysics 562: A69. doi:10.1051/0004-6361/201321454. Bibcode2014A&A...562A..69K. 
  7. Van De Kamp, Peter (1953). "The Twenty Brightest Stars". Publications of the Astronomical Society of the Pacific 65: 30. doi:10.1086/126523. Bibcode1953PASP...65...30V. 
  8. Thackeray, A. D.; Wegner, G. (April 1980), "An improved spectroscopic orbit for α1 Crucis", Monthly Notices of the Royal Astronomical Society 191 (2): 217–220, doi:10.1093/mnras/191.2.217, Bibcode1980MNRAS.191..217T 
  9. 9.0 9.1 9.2 9.3 "Acrux". The Hundred Greatest Stars. 2002. pp. 4. doi:10.1007/0-387-21625-1_2. ISBN 0-387-95436-8. 
  10. 10.0 10.1 10.2 10.3 Dravins, Dainis; Jensen, Hannes; Lebohec, Stephan; Nuñez, Paul D. (2010). "Stellar intensity interferometry: Astrophysical targets for sub-milliarcsecond imaging". Proceedings of the SPIE. Optical and Infrared Interferometry II 7734: 77340A. doi:10.1117/12.856394. Bibcode2010SPIE.7734E..0AD. 
  11. Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (2011). "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun". Monthly Notices of the Royal Astronomical Society 410: 190–200. doi:10.1111/j.1365-2966.2010.17434.x. Bibcode2011MNRAS.410..190T. 
  12. Perryman, Michael (2010), "The Making of History's Greatest Star Map", The Making of History's Greatest Star Map:, Astronomers’ Universe (Heidelberg: Springer-Verlag), doi:10.1007/978-3-642-11602-5, ISBN 978-3-642-11601-8, 
  13. Bordeleau, André G. (12 August 2013). "Federative Republic of Brazil: Constellations in the Breeze". Flags of the Night Sky. New York: Springer. pp. 1–72. doi:10.1007/978-1-4614-0929-8_1. ISBN 978-1-4614-0928-1. Retrieved 18 October 2016. 
  14. 14.0 14.1 "IAU Catalog of Star Names". Retrieved 21 November 2016. 
  15. Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
  16. Memoirs of the Rev. Walter M. Lowrie: missionary to China (1849), p. 93. Described as an "Americanism" in The Geographical Journal, vol. 92, Royal Geographical Society, 1938.
  17. "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016. 
  18. "Bulletin of the IAU Working Group on Star Names, No. 2". Retrieved 12 October 2016. 
  19. Richard Hinckley Allen, Star Names: Their Lore and Meaning, Dover Books, 1963.
  20. (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN:978-986-7332-25-7.
  21. (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 , Hong Kong Space Museum. Accessed on line November 23, 2010.
  22. Silva, Guilherme Marques dos Santos; Ribas, Felipe Braga; Freitas, Mário Sérgio Teixeira de (2008). "Transformação de coordenadas aplicada à construção da maquete tridimensional de uma constelação". Revista Brasileira de Ensino de Física 30: 1306.1. doi:10.1590/S1806-11172008000100007. 
  23. Shatsky, N.; Tokovinin, A. (2002). "The mass ratio distribution of B-type visual binaries in the Sco OB2 association". Astronomy and Astrophysics 382: 92–103. doi:10.1051/0004-6361:20011542. Bibcode2002A&A...382...92S. 
  24. Eggleton, Peter; Tokovinin, A. (2008). "A catalogue of multiplicity among bright stellar systems". Monthly Notices of the Royal Astronomical Society 389 (2): 869–879. doi:10.1111/j.1365-2966.2008.13596.x. Bibcode2008MNRAS.389..869E. 
  25. Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (2001). "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog". The Astronomical Journal 122 (6): 3466–3471. doi:10.1086/323920. Bibcode2001AJ....122.3466M. 
  26. Rizzuto, Aaron; Ireland, Michael; Robertson, J. G. (October 2011), "Multidimensional Bayesian membership analysis of the Sco OB2 moving group", Monthly Notices of the Royal Astronomical Society 416 (4): 3108–3117, doi:10.1111/j.1365-2966.2011.19256.x, Bibcode2011MNRAS.416.3108R. 
  27. "Cassini raw image". NASA/JPL/Space Science Institute. Retrieved 2017-10-31. 
  28. Cassini "Kodak Moments" - Unmanned Retrieved 2008-10-21
  29. "Astronomy of the Brazilian Flag". FOTW Flags Of The World website. 

External links

Coordinates: Sky map 12h 26m 35.89522s, −63° 05′ 56.7343″