Editing Acrux (section)

==Stellar properties==
[[File:Acrux.png|thumb|left|α Crucis with the nearby [[HR 4729|HD 108250]] (the 2nd-brightest star)]]
The two components, α<sup>1</sup> and α<sup>2</sup> Crucis, are separated by 4 [[arcseconds]]. α<sup>1</sup> is magnitude 1.40 and α<sup>2</sup> is magnitude 2.09, both early [[stellar classification|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 [[Solar luminosity|that of the Sun]]. α<sup>1</sup> and α<sup>2</sup> 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.<ref name="Tokovinin1997"/>

α<sup>1</sup> 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 α<sup>2</sup> and the brighter component of α<sup>1</sup> suggest that the stars will someday expand into [[Blue supergiant|blue]] and [[red supergiants]] (similar to [[Betelgeuse]] and [[Antares]]) before exploding as [[supernova]]e.<ref name=kaler>{{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.}}</ref> Component Ab may perform electron capture in the degenerate O+Ne+Mg core and trigger a supernova explosion,<ref name=nomoto/><ref>{{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 }}</ref> otherwise it will become a massive white dwarf.<ref name=kaler>{{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.}}</ref>

Photometry with the [[TESS]] satellite has shown that one of the stars in the α Crucis system is a [[Beta Cephei variable|β Cephei variable]], although α<sup>1</sup> and α<sup>2</sup> Crucis are too close for TESS to resolve and determine which one is the pulsator.<ref name=Sharma/>

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.<ref name="mnras416_3108">{{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=. }}</ref> 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.<ref>{{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 }}</ref>

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.<ref>{{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 }}</ref><ref>{{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}}</ref> 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.<ref name="wds">{{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}}</ref>

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.<ref>{{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 }}</ref><ref>[http://www.unmannedspaceflight.com/index.php?showtopic=1809&view=findpost&p=127561 Cassini "Kodak Moments" - Unmanned Spaceflight.com]. Retrieved 2008-10-21</ref>

{|class="wikitable"
|+Acrux system
!colspan=4|
!Separation<br />([[minute and second of arc|arcsec]])
!Projected<br />separation<br />([[astronomical unit|AU]])
!Orbital<br />period
![[stellar classification|Spectral<br />type]]
!Mass<br />([[solar mass|M<sub>☉</sub>]])
![[apparent magnitude|App. mag.]]<br />(V)
|-
|align=center rowspan=6| Acrux ABC ||align=center rowspan=3 style="background-color: #EEEEEE"| [[HR 4729]] ABC<br />(Acrux C & CP)<br /><ref group="orbit note" name="orbit">HR 4729 and Acrux A are separated by 90 arcseconds, resulting in a projected separation of 9400 AU/0.15 light years. This combined binary system has an estimated orbital period of 120,000 years.</ref> ||align=center colspan=2 style="background-color: #EEEEEE"| α<sup>1</sup> Crucis CP ||align=center style="background-color: #EEEEEE"| 2.1 ||align=center style="background-color: #EEEEEE"| 220 ||align=center style="background-color: #EEEEEE"| 930 years ||align=center style="background: {{star-color|M}}; | M0V ||align=center| 0.47 ||align=center| 15.0
|-
|align=center rowspan=2 style="background-color: #CCCCCC"| HR 4729 AB ||align=center style="background-color: #CCCCCC"| HR 4729 A ||rowspan=2 align=center style="background-color: #CCCCCC"| 0.00046 ||rowspan=2 align=center style="background-color: #CCCCCC"| 0.048 ||rowspan=2 align=center style="background-color: #CCCCCC"| 1.225 days ||align=center style="background: {{star-color|B}}; | B4V ||align=center| 8.68 ||align=center rowspan=2| 4.9<br />(combined)
|-
|align=center style="background-color: #CCCCCC"| HR 4729 B ||align=center style="background: {{star-color|G}}; | G?V ||align=center| 0.97
|-
|align=center rowspan=3 style="background-color: #DDDDDD"| Acrux AB<br />(α<sup>1</sup> and α<sup>2</sup>)<br /><ref group="orbit note" name="orbit"/> ||align=center colspan=2 style="background-color: #DDDDDD"| α<sup>2</sup> Crucis ||align=center style="background-color: #DDDDDD"| 4.4 ||align=center style="background-color: #DDDDDD"| 460 ||align=center style="background-color: #DDDDDD"| 1470 years ||align=center style="background: {{star-color|B}}; | B1Vn ||align=center| 15.52 ||align=center| 1.8
|-
|align=center rowspan=2 style="background-color: #BBBBBB"| α<sup>1</sup> Crucis ||align=center style="background-color: #BBBBBB"| Acrux Aa ||align=center rowspan=2 style="background-color: #BBBBBB"| 0.0094 ||align=center rowspan=2 style="background-color: #BBBBBB"| 0.99 ||align=center rowspan=2 style="background-color: #BBBBBB"| 75.8 days ||align=center style="background: {{star-color|B}}; | B0.5IV ||align=center| 17.80 ||align=center rowspan=2|1.3<br />(combined)
|-
|align=center style="background-color: #BBBBBB"| Acrux ab ||align=center style="background: {{star-color|B}}; | B7?V ||align=center| 4.49
|}
{{reflist|group="orbit note"}}