Alpha Crucis

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This article is about the star. For the Christian college, see Alphacrucis.
Alpha Crucis
Acrux kstars.png
The position of Acrux.
Observation data
Epoch J2000      Equinox J2000
Constellation Crux
Right ascension 12h 26m 35.89522s[1][2]
Declination −63° 05′ 56.7343″[1][2]
Apparent magnitude (V) 0.77 (1.33+1.75+4.86+15.00)[3]
Spectral type B0.5IV + B1V[4]
Radial velocity (Rv) −11.2 / −0.6[5] km/s
Proper motion (μ) RA: −35.83[1][2] mas/yr
Dec.: −14.86[1][2] mas/yr
Parallax (π) 10.13 ± 0.50[1][2] mas
Distance 320 ± 20 ly
(99 ± 5 pc)
Absolute magnitude (MV) −4.14[citation needed]
Mass 14 / 10[citation needed] M
Luminosity 25,000[citation needed] L
Temperature 28,000[citation needed] K
Rotational velocity (v sin i) 124[6] km/s
Companion α Crucis Ab
Period (P) 0.208[citation needed] yr
Semi-major axis (a) 1.0 AU[citation needed]"
Eccentricity (e) 0.0[citation needed]
Inclination (i) 0.0[citation needed]°
Other designations
Alpha Crucis, Acrux, HD 108248
Database references

Alpha Crucis (α Crucis, abbreviated Alpha Cru, α Cru) is a multiple star system located 321 light-years from the Sun[1][2][7] in the constellation of Crux and part of the asterism known as the Southern Cross. With a combined visual magnitude of 0.77,[8] it is the brightest star in Crux and the 13th brightest star[8] in the night sky. It is the southernmost first-magnitude star,[8] just a little more southerly than Alpha Centauri.

Only two components are visually distinguishable; α¹ Crucis (also named Acrux[9]) and α² Crucis.

Stellar properties[edit]

The two components, α¹ and α² Crucis, are separated by 4 arcseconds. α¹ is magnitude 1.40 and α² is magnitude 2.09, both hot class B (almost class O) 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 at least 1,500 years, and may be much longer.[citation needed]

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 of α¹ suggest that the stars will someday explode as supernovae. The fainter component of α¹ may survive to become a massive white dwarf.[citation needed]

Another class-B4 subgiant, Alpha Crucis C or alpha-3 Crucis, lies 90 arcseconds away from triple Alpha 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 with Alpha Crucis.[10][11] It has also been suggested that Alpha Crucis C is under-luminous for its class, meaning that the system would be an optical double star.[12]

Rizzuto and colleagues determined in 2011 that the Alpha 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.[13]

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.[14][15]


α Crucis (Latinised to Alpha Crucis) is the system's Bayer designation.

The historical name Acrux is an 'Americanism' for Alpha Crucis, 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's first bulletin of July 2016[18] included a table of the first two batches of names approved by the WGSN; which included Acrux for α¹ Crucis.

Since Alpha Crucis is at −63° declination, the southernmost first-magnitude star, it is only visible south of latitude 27° North. Therefore, 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, however, the star was visible to ancient Hindu astronomers in India who named it “Tri-shanku”. It was also visible to the ancient Romans and Greeks, who regarded it to be 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.[citation needed]

In culture[edit]

Alpha 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; Alpha Crucis representing the State of São Paulo.[22]


  1. ^ a b c d e f Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; et al. (July 1997), "The Hipparcos Catalogue", Astronomy and Astrophysics, 323: L49–L52, Bibcode:1997A&A...323L..49P 
  2. ^ 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:0708.1752free to read, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 
  3. ^ Tokovinin, A. A. (1997). "MSC - a catalogue of physical multiple stars". Astronomy and Astrophysics Supplement Series. 124 (1): 75–84. Bibcode:1997A&AS..124...75T. doi:10.1051/aas:1997181. ISSN 0365-0138. 
  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. Bibcode:1953GCRV..C......0W. 
  6. ^ Uesugi, Akira; Fukuda, Ichiro (1970), "Catalogue of rotational velocities of the stars", Contributions from the Institute of Astrophysics and Kwasan Observatory, University of Kyoto, 
  7. ^ 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 
  8. ^ a b c David Darling. "Acrux (Alpha Crucis)". Retrieved 2009-09-06. 
  9. ^ "IAU Catalog of Star Names". Retrieved 28 July 2016. 
  10. ^ Shatsky, N.; Tokovinin, A. (2002). "The mass ratio distribution of B-type visual binaries in the Sco OB2 association". Astronomy and Astrophysics. 382: 92. arXiv:astro-ph/0109456free to read. Bibcode:2002A&A...382...92S. doi:10.1051/0004-6361:20011542. 
  11. ^ Eggleton, Peter; Tokovinin, A. (2008). "A catalogue of multiplicity among bright stellar systems". Monthly Notices of the Royal Astronomical Society. 389 (2): 869. arXiv:0806.2878free to read. Bibcode:2008MNRAS.389..869E. doi:10.1111/j.1365-2966.2008.13596.x. 
  12. ^ Kaler, Jim. "ACRUX". Retrieved 5 August 2012. 
  13. ^ 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, arXiv:1106.2857free to read, Bibcode:2011MNRAS.416.3108R, doi:10.1111/j.1365-2966.2011.19256.x. 
  14. ^ NASA/JPL/Space Science Institute. Multimedia - Images - Raw Images. Retrieved 2008-10-21
  15. ^ Cassini "Kodak Moments" - Unmanned Retrieved 2008-10-21
  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. 1" (PDF). Retrieved 28 July 2016. 
  19. ^ Richard Hinckley Allen, Star Names: Their Lore and Meaning, Dover Books, 1963.
  20. ^ (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. ^ (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. ^ "Astronomy of the Brazilian Flag". FOTW Flags Of The World website. 

External links[edit]

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