Acrux

File:Deep Crux wide field with fog.jpg]]

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,{{dubious|date=April 2019}} and adopted by the International Astronomical Union (IAU).{{cite arXiv |title=On the naming convention used for multiple star systems and extrasolar planets |date=2010 |eprint=1012.0707 |class=astro-ph.SR |last1= Hessman |first1=F. V. |last2= Dhillon |first2=V. S. |last3= Winget |first3=D. E. |last4= Schreiber |first4=M. R. |last5= Horne |first5=K. |last6= Marsh |first6=T. R. |last7= Guenther |first7=E. |last8= Schwope |first8=A. |last9= Heber |first9=U. }}{{Unreliable source?|date=April 2019|reason=This is only a proposal, but wasn't ratified. WDS is the adopted usage here.}}

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.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.{{Better source needed|date=April 2019|reason=Acrux was named by American Elijah H. Burritt in the 1850s}} In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN){{cite web | url=https://www.iau.org/science/scientific_bodies/working_groups/280/ | title=IAU Working Group on Star Names (WGSN)|access-date=22 May 2016}} 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.{{cite web | url=http://www.pas.rochester.edu/~emamajek/WGSN/WGSN_bulletin2.pdf | title=Bulletin of the IAU Working Group on Star Names, No. 2 |access-date=12 October 2016}} 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.

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.Richard Hinckley Allen, Star Names: Their Lore and Meaning, Dover Books, 1963.

In Chinese, {{lang|zh|十字架}} ({{lang|zh-Latn|Shí Zì Jià}}, "Cross"), refers to an asterism consisting of Acrux, Mimosa, Gamma Crucis and Delta Crucis.{{in lang|zh}} 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}}. Consequently, Acrux itself is known as {{lang|zh|十字架二}} ({{lang|zh-Latn|Shí Zì Jià èr}}, "the Second Star of Cross").{{in lang|zh}} [http://www.lcsd.gov.hk/CE/Museum/Space/Research/StarName/c_research_chinengstars_a_al.htm 香港太空館 - 研究資源 - 亮星中英對照表] {{webarchive|url=https://web.archive.org/web/20100903162121/http://www.lcsd.gov.hk/CE/Museum/Space/Research/StarName/c_research_chinengstars_a_al.htm |date=2010-09-03 }}, Hong Kong Space Museum. Accessed on line November 23, 2010.

This star is known as Estrela de Magalhães ("Star of Magellan") in Portuguese.{{cite journal|doi=10.1590/S1806-11172008000100007 |title=Transformação de coordenadas aplicada à construção da maquete tridimensional de uma constelação |journal=Revista Brasileira de Ensino de Física |volume=30 |pages=1306.1–1306.7 |year=2008 |last1=Silva |first1=Guilherme Marques dos Santos |last2=Ribas |first2=Felipe Braga |last3=Freitas |first3=Mário Sérgio Teixeira de |doi-access=free }}

File:Acrux.png (the 2nd-brightest star)]]

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 {{Val|26000|fmt=commas|ul=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.

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 {{Val|1|ul=AU}}. The masses of α² and the brighter component of α¹ suggest that the stars will someday expand into blue and red supergiants (similar to Betelgeuse and Antares) before exploding as supernovae.{{cite book|doi=10.1007/0-387-21625-1_2|chapter=Acrux|title=The Hundred Greatest Stars|pages=4–5|year=2002|isbn=978-0-387-95436-3|last1=Kaler|first1=James B.}} Component Ab may perform electron capture in the degenerate O+Ne+Mg core and trigger a supernova explosion,{{cite journal |last1=S. E. Woosley, Alexander Heger |title=The Remarkable Deaths of 9 - 11 Solar Mass Stars |journal=Astrophysics |date=May 25, 2015 |volume=810 |issue=1 |page=34 |doi=10.1088/0004-637X/810/1/34 |arxiv=1505.06712 |bibcode=2015ApJ...810...34W |s2cid=119163256 }} otherwise it will become a massive white dwarf.{{cite book|doi=10.1007/0-387-21625-1_2|chapter=Acrux|title=The Hundred Greatest Stars|pages=4–5|year=2002|isbn=978-0-387-95436-3|last1=Kaler|first1=James B.}}

Photometry with the TESS satellite has shown that one of the stars in the α Crucis system is a β Cephei variable, although α¹ and α² Crucis are too close for TESS to resolve and determine which one is the pulsator.

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.{{citation | last1=Rizzuto | first1=Aaron | last2=Ireland| first2=Michael | last3=Robertson | first3=J. G. | title=Multidimensional Bayesian membership analysis of the Sco OB2 moving group | journal=Monthly Notices of the Royal Astronomical Society |date=October 2011 | volume=416 | issue=4 | pages=3108–3117 | doi=10.1111/j.1365-2966.2011.19256.x | doi-access=free | bibcode=2011MNRAS.416.3108R | arxiv=1106.2857 | s2cid=54510608 | postscript=. }} A bow shock is present around α Crucis, and is visible in the infrared spectrum, but is not aligned with α Crucis; the bow shock likely formed from large-scale motions in the interstellar matter.{{cite journal | doi=10.52526/25792776-24.71.1-42 | title=The runaway nature and origin of α Crucis system | date=2024 | last1=Torosyan | first1=M. | last2=Azatyan | first2=N. | last3=Nikoghosyan | first3=E. | last4=Samsonyan | first4=A. | last5=Andreasyan | first5=D. | journal=Communications of the Byurakan Astrophysical Observatory | volume=71 | pages=42–47 | arxiv=2407.09934 | bibcode=2024CoBAO..71...42T }}

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.{{cite journal|last1=Shatsky|first1=N. |last2=Tokovinin|first2=A. |title=The mass ratio distribution of B-type visual binaries in the Sco OB2 association|journal=Astronomy and Astrophysics|date=2002|volume=382|pages=92–103|arxiv = astro-ph/0109456 |bibcode = 2002A&A...382...92S |doi = 10.1051/0004-6361:20011542 |s2cid=16697655 }}{{cite journal|last1=Eggleton|first1=Peter|last2=Tokovinin|first2=A. |title=A catalogue of multiplicity among bright stellar systems|journal=Monthly Notices of the Royal Astronomical Society|date=2008|volume=389|issue=2|pages=869–879|arxiv = 0806.2878 |bibcode = 2008MNRAS.389..869E |doi = 10.1111/j.1365-2966.2008.13596.x |doi-access=free |s2cid=14878976}} 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.{{cite journal|bibcode= 2001AJ....122.3466M|title= The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog|journal= The Astronomical Journal|volume= 122|issue= 6|pages= 3466–3471|last1= Mason|first1= Brian D.|last2= Wycoff|first2= Gary L.|last3= Hartkopf|first3= William I.|last4= Douglass|first4= Geoffrey G.|last5= Worley|first5= Charles E.|year= 2001|doi= 10.1086/323920|doi-access= free}}

On 2 October 2008, the Cassini–Huygens spacecraft resolved three of the components (A, B and C) of the multiple star system as Saturn's disk occulted it.{{cite web|publisher=NASA/JPL/Space Science Institute |url=https://saturn.jpl.nasa.gov/raw_images/197551/ |title=Cassini raw image|work=Cassini: The Grand Finale |access-date=2017-10-31 }}[http://www.unmannedspaceflight.com/index.php?showtopic=1809&view=findpost&p=127561 Cassini "Kodak Moments" - Unmanned Spaceflight.com]. Retrieved 2008-10-21

{{reflist|group="orbit note"}}

Acrux is represented in the flags of Australia, New Zealand, Samoa, and Papua New Guinea as one of five stars that compose 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.{{cite web

|title=Astronomy of the Brazilian Flag

|url=https://flagspot.net/flags/br_astro.html

|publisher=FOTW Flags Of The World website

}} As of 2015, it is also represented on the cover of the Brazilian passport.

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

• Lists of stars

{{Notelist}}

{{Reflist|30em|refs=

{{citation | first=F. | last=van Leeuwen |date=November 2007 | title=Validation of the new Hipparcos reduction | journal=Astronomy and Astrophysics | volume=474 | issue=2 | pages=653–664 | bibcode=2007A&A...474..653V | doi=10.1051/0004-6361:20078357 |arxiv = 0708.1752 | s2cid=18759600 }}

{{citation | last=Perryman | first=Michael | title=The Making of History's Greatest Star Map | location=Heidelberg | publisher=Springer-Verlag | date=2010 | doi=10.1007/978-3-642-11602-5| series=Astronomers' Universe | isbn=978-3-642-11601-8 | bibcode=2010mhgs.book.....P | url=https://cds.cern.ch/record/1338896 | url-access=subscription }}

{{citation | title=Michigan catalogue of two-dimensional spectral types for the HD stars | journal=Ann Arbor: Dept. Of Astronomy | volume=1 | last1=Houk | first1=Nancy | date=1979 | bibcode=1978mcts.book.....H }}

{{cite journal | last=Wilson | first=Ralph Elmer | date=1953 | title=General Catalogue of Stellar Radial Velocities | journal=Carnegie Institute Washington D.C. Publication | bibcode=1953GCRV..C......0W }}

{{citation | first1=Kenneth R. | last1=Lang | title=Astrophysical formulae | volume=1 | series=Astronomy and astrophysics library | edition=3 | publisher=Birkhäuser | year=2006 | isbn=3-540-29692-1 | url=https://books.google.com/books?id=OvTjLcQ4MCQC&pg=PA41 }}. The radius (R_*) is given by:

:$\backslash begin\{align\}\; 2\backslash cdot\; R\_*$

& = \frac{(10^{-3}\cdot 99\cdot 0.52)\ \text{AU}}{0.0046491\ \text{AU}/R_{\bigodot}} \\

& 11.073\cdot R_{\bigodot}

\end{align}
The angular diameter used (0.52{{nbsp}}milliarcseconds) is from [https://vizier.cds.unistra.fr/viz-bin/VizieR?-source=II/224&ID1=108249 CADARS]. Distance (99{{nbsp}}parsecs) is from Hipparcos.

{{cite journal|bibcode=1984ApJ...277..791N|title=Evolution of 8-10 solar mass stars toward electron capture supernovae. I - Formation of electron-degenerate O + NE + MG cores|journal=Astrophysical Journal|volume=277|pages=791|last1=Nomoto|first1=K.|year=1984|doi=10.1086/161749|doi-access=free}}

}}

• http://jumk.de/astronomie/big-stars/acrux.shtml
• http://www.daviddarling.info/encyclopedia/A/Acrux.html

{{Stars of Crux}}

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{{DEFAULTSORT:Acrux}}

Category:Double stars

Category:B-type main-sequence stars

Category:B-type subgiants

Category:Spectroscopic binaries

Category:Triple star systems

6

Category:Lower Centaurus Crux

Category:Crux

Crucis, Alpha

4730 1

Category:Durchmusterung objects

060718

108248 9

Crucis, 26 7

Acrux