Acrux

Acrux /ˈeɪkrʌks/, designated α Crucis (Latinised to Alpha Crucis, abbreviated Alpha Cru, α Cru) is a multiple star system 321 light-years from the Sun[1][13] 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 object 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.[14]

This article is about the star. For the Christian college, see Alphacrucis. For research ship, see Alpha Crucis (research vessel).
Acrux

The position of Acrux
Observation data
Epoch J2000      Equinox J2000
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]
Characteristics
Spectral type B0.5IV + B1V[4]
B−V color index −0.26[2]
Astrometry
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])
Orbit[8]
Primaryα Crucis Aa
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)
(secondary)		21±6°
Semi-amplitude (K1)
(primary)		41.7±1.2 km/s
Details
α1
Mass17.80 + 6.05[3] M☉
Radius7.8[9] R☉
Luminosity25,000[10] L☉
Temperature24,000[11] K
Rotational velocity (v sin i)120[11] km/s
α2
Mass15.52[3] M☉
Radius5.4 R☉
Luminosity16,000[10] L☉
Temperature28,000[11] K
Rotational velocity (v sin i)200[11] km/s
Age10.8[12] 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
SIMBADα Cru
α1 Cru
α2 Cru

To the naked eye Acrux appears as a single star, but it is actually a multiple star system containing six components. Through optical telescopes, Acrux appears as a triple star, whose two brightest components are visually separated by about 4 arcseconds and are known as Acrux A and Acrux B, α1 Crucis and α2 Crucis, or α Crucis A and α Crucis B. Both components 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 (officially named Acrux, historically the name of the entire system)[15][16] and α Crucis Ab. Its two component stars orbit every 76 days at a separation of about 1 astronomical unit (AU).[10] HR 4729, also known as Acrux C, is a more distant companion, forming a triple star through small telescopes. C is also a spectroscopic binary, which brings the total number of stars in the system to at least five.

Nomenclature
The constellation Crux

α Crucis (Latinised to Alpha Crucis) is the system's Bayer designation; α1 and α2 Crucis, those of its two main components stars. The designations of these two constituents as Acrux A and Acrux B and those of A's components - Acrux Aa and Acrux Ab - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[17]

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.[18] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[19] 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.[20] The WGSN approved the name Acrux for the star Acrux Aa on 20 July 2016 and it is now so entered in the IAU Catalog of Star Names.[16]

Since Acrux is at −63° declination, making it the southernmost first-magnitude star, it is only visible south of latitude 27° North. It barely rises from cities such as Miami, United States, or Karachi, Pakistan (both around 25°N) and not at all from New Orleans, United States, 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 Greeks, who regarded it as part of the constellation of Centaurus.[21]

In Chinese, 十字架 (Shí Zì Jià, "Cross"), refers to an asterism consisting of Acrux, Mimosa, Gamma Crucis and Delta Crucis.[22] Consequently, Acrux itself is known as 十字架二 (Shí Zì Jià èr, "the Second Star of Cross".).[23]

This star is known as Estrela de Magalhães ("Star of Magellan") in Portuguese.[24]

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. Component Ab may survive to become a massive white dwarf.[10]

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.[25]

The cooler less luminous B class star HR 4729 (HD 108250) lies 90 arcseconds away from triple star system 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.[26][27] It is itself a spectroscopic binary system, sometimes catalogued as component C (Acrux C) of the Acrux multiple system. Another fainter visual companion listed as component D or Acrux D. A further seven faint stars are also listed as companions out to a distance of about two arc-minutes.[28]

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.[29][30]

In culture

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 representing the State of São Paulo.[31] It is also represented (as part of the Southern Cross) on the cover of the current (from 2015) Brazilian passport.

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

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. Corben, P. M. (1966). "Photoelectric magnitudes and colours for bright southern stars". Monthly Notes of the Astron. Soc. Southern Africa. 25: 44. Bibcode:1966MNSSA..25...44C.
  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, 1, Bibcode:1978mcts.book.....H
  5. Wilson, Ralph Elmer (1953). "General Catalogue of Stellar Radial Velocities". Carnegie Institute Washington D.C. Publication. Bibcode:1953GCRV..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. arXiv:1312.5592. Bibcode:2014A&A...562A..69K. doi:10.1051/0004-6361/201321454. S2CID 54222753.
  7. Van De Kamp, Peter (1953). "The Twenty Brightest Stars". Publications of the Astronomical Society of the Pacific. 65 (382): 30. Bibcode:1953PASP...65...30V. doi:10.1086/126523.
  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, Bibcode:1980MNRAS.191..217T, doi:10.1093/mnras/191.2.217
  9. https://www.wolframalpha.com/input/?i=acrux+luminosity
  10. Kaler, James B. (2002). "Acrux". The Hundred Greatest Stars. pp. 4–5. doi:10.1007/0-387-21625-1_2. ISBN 978-0-387-95436-3.
  11. 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. arXiv:1009.5815. Bibcode:2010SPIE.7734E..0AD. doi:10.1117/12.856394. S2CID 55641060.
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  14. 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.
  15. 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.
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  17. 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].
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  23. (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived 2010-09-03 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
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