Alpha Crucis

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This article is about the star. For the Christian college, see Alphacrucis.
Acrux AB
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
Mass 14 / 10 M
Luminosity 25,000 L
Temperature 28,000 K
Rotational velocity (v sin i) 124[6] km/s
Companion α Crucis Ab
Period (P) 0.208 yr
Semi-major axis (a) 1.0 AU"
Eccentricity (e) 0.0
Inclination (i) 0.0°
Database references

Alpha Crucis (α Cru, α Crucis, also Acrux, HD 108248) is the brightest star in the constellation Crux, the Southern Cross, and, at a combined visual magnitude 0.77,[7] is the 13th brightest star[7] in the night sky. Acrux is the southernmost first-magnitude star,[7] just a little more southerly than Alpha Centauri.

Physical properties[edit]

Acrux is a multiple star system located 321 light-years from the Earth.[1][2][8] Only two components are visually distinguishable, α1 and α2, 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.

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.

Another class-B4 subgiant, Alpha Crucis C or alpha-3 Crucis, lies 90 arcseconds away from triple Acrux and shares Acrux's motion through space, suggesting it may be gravitationally bound to Acrux. It is therefore generally assumed to be physically associated with Acrux.[9][10] 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.[11]

Rizzuto and colleagues determined in 2011 that the Acrux 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.[12]


"Acrux" is an "Americanism" for the full Bayer designation "Alpha Crucis", coined in the 19th century, but entering common use only by the mid 20th century.[13]

Since Acrux is at −63° declination, the southernmost first-magnitude star, it is only visible south of latitude 27°N. 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 Centaurus.[14]

In Chinese, 十字架 (Shí Zì Jià, "Cross"), refers to an asterism consisting of α Crucis, γ Crucis, β Crucis and δ Crucis.[15] Consequently, α Crucis itself is known as 十字架二 (Shí Zì Jià èr, "the Second Star of Cross".).[16]

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

Acrux 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. Acrux represents the State of São Paulo.[17]

Notable observations[edit]

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.[18][19]


  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. ^ a b c David Darling. "Acrux (Alpha Crucis)". Retrieved 2009-09-06. 
  8. ^ 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 
  9. ^ 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. 
  10. ^ 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. 
  11. ^ Kaler, Jim. "ACRUX". Retrieved 5 August 2012. 
  12. ^ 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. 
  13. ^ 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.
  14. ^ Richard Hinckley Allen, Star Names: Their Lore and Meaning, Dover Books, 1963.
  15. ^ (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.
  16. ^ (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.
  17. ^ "Astronomy of the Brazilian Flag". FOTW Flags Of The World website. 
  18. ^ NASA/JPL/Space Science Institute. Multimedia - Images - Raw Images. Retrieved 2008-10-21
  19. ^ Cassini "Kodak Moments" - Unmanned Retrieved 2008-10-21

External links[edit]

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