Editing Crux (section)

==Features==

===Stars===
{{further|List of stars in Crux}}
[[File:Constellation Crux.jpg|thumb|right|upright=1|The constellation Crux as it can be seen by the naked eye]]
[[File:Southern Cross with clouds.jpg|thumb|Crux with clouds, from Cape Town]]

Within the constellation's borders, there are 49 stars brighter than or equal to [[apparent magnitude]] 6.5.{{efn|1=Objects of magnitude 6.5 are among the faintest visible to the unaided eye in suburban-rural transition night skies.<ref>{{cite web|url=http://www.skyandtelescope.com/resources/darksky/3304011.html?page=1&c=y|title=The Bortle Dark-Sky Scale|last=Bortle|first=John E.|date=February 2001|work=[[Sky & Telescope]]|publisher=Sky Publishing Corporation|access-date=29 November 2014|archive-date=31 March 2014|archive-url=https://web.archive.org/web/20140331202746/http://www.skyandtelescope.com/resources/darksky/3304011.html?page=1&c=y|url-status=dead}}</ref>}}<ref name=tirionconst/> The four main stars that form the asterism are Alpha, Beta, Gamma, and Delta Crucis.
* [[α Crucis]] or Acrux is a triple star 321 light-years from Earth. A rich blue in colour, with a visual [[apparent magnitude|magnitude]] 0.8 to the unaided eye, it has two close components of a similar magnitude, 1.3 and 1.8 respectively, plus another much wider component of the 5th magnitude. The two close components are resolved in a small amateur telescope and the wide component is readily visible in a pair of binoculars.
* [[β Crucis]] or Mimosa is a blue-hued giant star of magnitude 1.3, and lies 353 light-years from Earth. It is a [[Beta Cephei]]-type [[variable star]] with a variation of less than 0.1 magnitudes.{{sfn|Ridpath|Tirion|2017|pp=134–135}}
* [[γ Crucis]] or Gacrux is an optical [[double star]]. The primary is a red-hued giant star of magnitude 1.6, 88 light-years from Earth, and is one of the closest [[red giant]]s to Earth. Its secondary component is magnitude 6.5, 264 light-years from Earth.
* [[δ Crucis]] (Imai) is a magnitude 2.8 blue-white hued star about 345 light-years from Earth.{{sfn|Ridpath|Tirion|2017|pp=134–135}} Like Mimosa it is a Beta Cepheid variable.<ref name=bagnall/>

There is also a fifth star, that is often included with the Southern Cross.
* [[ε Crucis]] (Ginan) is an orange-hued giant star of magnitude 3.6, 228 light-years from Earth.

There are several other naked-eye stars within the borders of Crux, especially:
* [[Iota Crucis]] is a visual [[double star]] 125 light-years from Earth. The primary is an orange-hued giant of magnitude 4.6 and the secondary at magnitude 9.5.
* [[Mu Crucis]] or Mu<sup>1,2</sup> Crucis is a wide double star where the components are about 370 light-years from Earth. Equally blue-white in colour, the components are magnitude 4.0 and 5.1 respectively, and are easily divisible in small amateur telescopes or large binoculars.{{sfn|Ridpath|Tirion|2017|pp=134–135}}

===Scorpius–Centaurus association===

Unusually, a total of 15 of the 23 brightest stars in Crux are spectrally blue-white B-type stars.<ref name=bagnall/> Among the five main bright stars, Delta, and probably Alpha and Beta, are likely co-moving B-type members of the [[Scorpius–Centaurus association]], the nearest [[stellar association|OB association]] to the [[Sun]].<ref name=Preibisch>{{cite journal|title=The Nearest OB Association: Scorpius–Centaurus (Sco OB2)|author1=Preibisch, T. |author2=Mamajek, E. |year=2008|journal=Handbook of Star-Forming Regions|volume=2|page=235|bibcode=2008hsf2.book..235P|arxiv=0809.0407}}</ref><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> They are among the highest-mass stellar members of the Lower Centaurus–Crux subgroup of the association, with ages of roughly 10 to 20&nbsp;million years.<ref>{{cite journal | bibcode=1989A&A...216...44D |author1=de Geus, E. J. |author2=de Zeeuw, P. T. |author3= Lub, J. |name-list-style=amp | title= Physical Parameters of Stars in the Scorpio-Centaurus OB Association| journal=[[Astronomy & Astrophysics]]| volume=216 | issue=3 | year=1989 | pages=44–61 }}</ref><ref>{{cite journal |bibcode=2002AJ....124.1670M |doi=10.1086/341952 |author1=Mamajek, E. E. |author2=Meyer, M. R. |author3=Liebert, James |title=Post-T Tauri Stars in the Nearest OB Association |journal=[[Astronomical Journal]] |volume=124 |issue=3 |year=2002 |pages=1670–1694 |arxiv=astro-ph/0205417 |s2cid=16855894 }}</ref> Other members include the blue-white stars [[Zeta Crucis|Zeta]], [[Lambda Crucis|Lambda]] and both the components of the visual [[double star]], [[Mu Crucis|Mu]].<ref name=dezeeuw99>{{cite journal|title=A Hipparcos Census of Nearby OB Associations|author1=de Zeeuw, P.T. |author2=Hoogerwerf, R. |author3=de Bruijne, J.H.J. |author4=Brown, A.G.A. |author5=Blaauw, A. |year=1999|journal=Astronomical Journal|volume=117|issue=1|pages=354–99|bibcode=1999AJ....117..354D|doi=10.1086/300682|arxiv = astro-ph/9809227 |s2cid=16098861 }}</ref>

===Variable stars===

Crux contains many [[variable star]]s. It boasts four [[Cepheid variable]]s that may all reach naked eye visibility.
* [[BG Crucis]] ranges from magnitude 5.34 to 5.58 over 3.3428 days,<ref name=AAVSOBG>{{cite web|url=http://www.aavso.org/vsx/index.php?view=detail.top&oid=10853 |title=BG Crucis |last=Watson, Christopher |date=4 January 2010 |work=AAVSO Website|publisher=American Association of Variable Star Observers|access-date=12 March 2014}}</ref>
* [[T Crucis]] ranges from 6.32 to 6.83 over 6.73331 days,<ref name=AAVSOT>{{cite web|url=http://www.aavso.org/vsx/index.php?view=detail.top&oid=10771 |title=T Crucis |last=Watson, Christopher |date=4 January 2010 |work=AAVSO Website|publisher=American Association of Variable Star Observers|access-date=12 March 2014}}</ref>
* [[S Crucis]] ranges from 6.22 to 6.92 over 4.68997 days,<ref name=AAVSOS>{{cite web|url=http://www.aavso.org/vsx/index.php?view=detail.top&oid=10770 |title=S Crucis |last=Watson, Christopher |date=4 January 2010 |work=AAVSO Website|publisher=American Association of Variable Star Observers|access-date=12 March 2014}}</ref>
* [[R Crucis]] ranges from 6.4 to 7.23 over 5.82575 days.<ref name=AAVSOR>{{cite web|url=http://www.aavso.org/vsx/index.php?view=detail.top&oid=10769 |title=R Crucis |last=Watson, Christopher |date=4 January 2010 |work=AAVSO Website|publisher=American Association of Variable Star Observers|access-date=12 March 2014}}</ref>

Other well studied variable stars includes:
* [[Lambda Crucis]] and [[Theta2 Crucis|Theta<sup>2</sup> Crucis]], that are both Beta Cepheid type variable stars.<ref name=bagnall/>
* [[BH Crucis]], also known as Welch's Red Variable, is a [[Mira variable]] that ranges from magnitude 6.6 to 9.8 over 530 days.<ref name=AAVSOBH>{{cite web|url=http://www.aavso.org/vsx/index.php?view=detail.top&oid=10854 |title=BH Crucis |last=Otero, Sebastian |date=6 January 2011 |work=AAVSO Website|publisher=American Association of Variable Star Observers|access-date=23 June 2014}}</ref>  Discovered in October 1969, it has become redder and brighter (mean magnitude changing from 8.047 to 7.762) and its period lengthened by 25% in the first thirty years since its discovery.<ref name=walker09>{{cite journal | bibcode=2009JAVSO..37...87W   | journal = J. Am. Assoc. Variable Star Obs. |volume= 37| issue = 2 |pages= 87–95 |year=2009|title= BH Crucis : Period, Magnitude, and Color Changes |author= Walker, W.S.G.}}</ref>

===Host star exoplanets in Crux===

The star [[HD 106906]] has been found to have a planet—[[HD 106906 b]]—that has one of the widest orbits of any currently known planetary-mass companions.<ref name="Bailey2014">{{cite journal |title=HD 106906 b: A planetary-mass companion outside a massive debris disk |journal=[[The Astrophysical Journal Letters]] |first1=Vanessa |last1=Bailey |first2=Tiffany |last2=Meshkat |first3=Megan |last3=Reiter |first4=Katie |last4=Morzinski |first5=Jared |last5=Males |first6=Kate Y. L. |last6=Su |first7=Philip M. |last7=Hinz |first8=Matthew |last8=Kenworthy |first9=Daniel |last9=Stark |first10=Eric |last10=Mamajek |first11=Runa |last11=Briguglio |first12=Laird M. |last12=Close |first13=Katherine B. |last13=Follette |first14=Alfio |last14=Puglisi |first15=Timothy |last15=Rodigas |first16=Alycia J. |last16=Weinberger |first17=Marco |last17=Xompero |display-authors=1 |volume=780 |issue=1 |page=L4 |date=January 2014 |arxiv=1312.1265 |doi=10.1088/2041-8205/780/1/L4 |bibcode=2014ApJ...780L...4B|s2cid=119113709 }}</ref>

===Objects beyond the Local Arm===

Crux is backlit by the multitude of stars of the Scutum-Crux Arm (more commonly called the [[Scutum-Centaurus Arm]]) of the Milky Way.  This is the main inner arm in the local radial quarter of the galaxy.  Part-obscuring this is:
* The [[Coalsack Nebula]] lies partially within Crux and partly in the neighboring constellations of [[Musca]] and [[Centaurus]].{{sfn|Ridpath|Tirion|2017|pp=134–135}} It is the most prominent [[dark nebula]] in the skies, and is easily visible to the naked eye as a prominent dark patch in the southern Milky Way. It can be found 6.5° southeast from the centre of Crux or 3° east from α Crucis. Its large area covers about 7° by 5°, and is {{convert|180|pc|ly|lk=on}} away from [[Earth]].<ref name="Franco2000">{{cite journal
|last1=Franco
|first1=G.P.A.
|year=2000
|title=Interstellar Na I D lines towards the Southern Coalsack
|journal=Monthly Notices of the Royal Astronomical Society
|volume=315
|issue=3
|pages=611–621
|bibcode=2000MNRAS.315..611F
|doi=10.1046/j.1365-8711.2000.03434.x|doi-access=free
}}</ref>

A key feature of the Scutum-Crux Arm is:
* The [[Jewel Box (star cluster)|Jewel Box]], κ Crucis Cluster or NGC 4755, is a small but bright [[open cluster]] that appears as a fuzzy star to the naked eye and is very close to the easternmost boundary of Crux: about 1° southeast of Beta Crucis. The combined or total magnitude is 4.2 and it lies at a distance of {{convert|1.95|kpc|ly|lk=on}} from Earth.{{sfn|Ridpath|Tirion|2017|pp=134–135}} The cluster was given its name by [[John Herschel]],{{sfn|Ridpath|Tirion|2017|pp=134–135}} based on the range of colours visible throughout the star cluster in his telescope. About seven million years old,<ref name="webda">{{cite web
|title=WEBDA Open cluster
|work=Results for NGC 4755
|url=http://www.univie.ac.at/webda/cgi-bin/ocl_page.cgi?dirname=ngc4755
|access-date=30 April 2019}}</ref> it is one of the youngest open clusters in the Milky Way, and it appears to have the shape of a letter 'A'. The Jewel Box Cluster is classified as Shapley class 'g' and Trumpler class 'I 3 r -' cluster; it is a very rich, centrally-concentrated cluster detached from the surrounding star field. It has more than 100 stars that range significantly in brightness.{{sfn|Levy|2005|p=87}} The brightest cluster stars are mostly [[blue supergiant]]s, though the cluster contains at least one [[red supergiant]]. [[Kappa Crucis (star)|Kappa Crucis]] is a true member of the cluster that bears its name, and is one of the brighter stars at magnitude 5.9.{{sfn|Ridpath|Tirion|2017|pp=134–135}}