Editing Circinus

{{Short description|Constellation in the southern celestial hemisphere}}
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{{Other uses}}
{{Infobox constellation
| name = Circinus
| abbreviation = Cir
| genitive = Circini
| pronounce = {{IPAc-en|ˈ|s|ɜːr|s|ᵻ|n|ə|s}} ''Círcinus,''<br/>genitive {{IPAc-en|ˈ|s|ɜːr|s|ᵻ|n|aɪ}}
| symbolism = [[Compass (drafting)|Compass]]
| RA = {{RA|13|38.4}} to {{RA|15|30.2}}<ref name=boundary/>
| dec = −55.43° to −70.62°<ref name=boundary/>
| areatotal = 93
| arearank = 85th
| numbermainstars = 3
| numberbfstars = 9
| numberstarsplanets = 2
| numberbrightstars = 0
| numbernearbystars = 0
| brighteststarname = [[Alpha Circini|α Cir]]
| starmagnitude = 3.19
| neareststarname = [[Alpha Circini|α Cir]]
| stardistancely = 53.50
| stardistancepc = 16.40
| numbermessierobjects = 0
| meteorshowers = [[Alpha Circinids]] (ACI)
| bordering = [[Centaurus]]<br />[[Musca]]<br />[[Apus]]<br />[[Triangulum Australe]]<br />[[Norma (constellation)|Norma]]<br />[[Lupus (constellation)|Lupus]]
| latmax = [[30th parallel north|30]]
| latmin = [[South Pole|90]]
| month = June
| notes=
}}
'''Circinus''' is a small, faint [[constellation]] in the [[southern sky]], first defined in 1756 by the French astronomer [[Nicolas-Louis de Lacaille]]. Its name is [[Latin]] for [[compass (drawing tool)|compass]], referring to the [[Technical drawing|drafting]] tool used for drawing circles (it should not be confused with [[Pyxis]], a constellation that represents a [[mariner's compass]] which points north). Its brightest star is [[Alpha Circini]], with an [[apparent magnitude]] of 3.19. Slightly [[variable star|variable]], it is the brightest [[rapidly oscillating Ap star]] in the night sky. [[AX Circini]] is a [[Cepheid variable]] visible with the unaided eye, and [[BX Circini]] is a faint star thought to have been formed from the merger of two [[white dwarf]]s. Two sun-like stars have planetary systems: [[HD 134060]] has two small planets, and [[HD 129445]] has a [[Jupiter]]-like planet. [[Supernova]] [[SN 185]] appeared in Circinus in 185&nbsp;AD and was recorded by Chinese observers. Two [[nova]]e have been observed more recently, in the 20th century.

The [[Milky Way]] runs through the constellation, featuring prominent objects such as the [[open cluster]] [[NGC 5823]] and the [[planetary nebula]] [[NGC 5315]]. Circinus hosts a notable [[spiral galaxy]], the [[Circinus Galaxy]], discovered in 1977; it is the closest [[Seyfert galaxy]] to the Milky Way. The [[Alpha Circinids]] (ACI), a [[meteor shower]] also discovered in 1977, radiate from this constellation.

==History==
In 1756, French astronomer [[Nicolas-Louis de Lacaille]] introduced the [[constellation]] of Circinus with the French name ''le Compas'', representing a pair of dividing [[compass (drawing tool)|compass]]es, on a chart of the southern sky.<ref name="ridpath2"/> On that chart, Lacaille portrayed the constellations of [[Norma (constellation)|Norma]], Circinus, and [[Triangulum Australe]], respectively, as a set square and ruler, a compass, and a surveyor's level in a set of draughtsman's instruments.<ref name="ridpath">{{cite web |url=http://www.ianridpath.com/startales/lacaillenormaetc.html |last=Ridpath |first=Ian |author-link=Ian Ridpath |title=Lacaille's grouping of Norma, Circinus, and Triangulum Australe |access-date=27 June 2012 |work=Star Tales}}</ref> Circinus was given its current name in 1763, when Lacaille published an updated sky map with Latin names for the constellations he introduced.<ref name="ridpath2">{{cite web |url=http://www.ianridpath.com/startales/circinus.html |last=Ridpath |first=Ian |author-link=Ian Ridpath |title=Circinus |access-date=27 June 2012 |work=Star Tales}}</ref>

==Characteristics==
Bordered by [[Centaurus]], [[Musca]], [[Apus]], Triangulum Australe, Norma and [[Lupus (constellation)|Lupus]], Circinus lies adjacent to the [[Alpha Centauri|Alpha]] and [[Beta Centauri]] stars. As it is at [[declination]] −50° to −70°, the whole constellation is only visible south of [[latitude]] [[30th parallel north|30° N]]. The official constellation boundaries, as set by Belgian astronomer [[Eugène Joseph Delporte|Eugène Delporte]] in 1930, are defined by a [[polygon]] of 14&nbsp;segments. In the [[equatorial coordinate system]], the [[right ascension]] coordinates of these borders lie between {{RA|13|38.4}} and {{RA|15|30.2}}, and the declination coordinates are between −55.43° and −70.62°.<ref name=boundary>{{Cite journal | title=Circinus, constellation boundary | journal=The Constellations | publisher=International Astronomical Union | url=https://www.iau.org/public/themes/constellations/#cir | access-date=27 June 2012}}</ref> Circinus [[Culmination|culminates]] each year at 9&nbsp;p.m. on 30&nbsp;July.<ref>{{cite web | url= http://www.southastrodel.com/Page20502.htm | title='The '"Constellations : Part 2 Culmination Times"'
 | work=Southern Astronomical Delights | date= 7 February 2011| access-date=12 October 2012|author=James, Andrew|location=Sydney, New South Wales}}</ref> The recommended three-letter abbreviation for the constellation, as adopted by the [[International Astronomical Union]] in 1922, is "Cir".<ref name=pa30_469>{{cite journal |last=Russell |first=Henry Norris |author-link=Henry Norris Russell |title=The New International Symbols for the Constellations |journal=[[Popular Astronomy (US magazine)|Popular Astronomy]] |volume=30 |page=469 |bibcode=1922PA.....30..469R |year=1922}}</ref>

==Features==
[[Image:Constellation Circinus.jpg|thumb|left|The constellation Circinus as it can be seen by the naked eye.]]

===Stars===
{{Further|List of stars in Circinus}}
<!-- first para - two brightest stars, both A-type main sequence stars -->
Circinus is a faint constellation, with only one star brighter than fourth magnitude.<ref name=moore11>{{cite book |last=Moore |first=Patrick |author-link=Patrick Moore |title=Patrick Moore's Data Book of Astronomy |publisher=Cambridge University Press |year=2011 |page=410 |isbn=978-0-521-89935-2 |url=https://books.google.com/books?id=2FNfjWKBZx8C&pg=PA410}}</ref> [[Alpha Circini]], a white [[main sequence]] star with an [[apparent magnitude]] of 3.19, is 54&nbsp;[[light-year]]s away and 4° south of [[Alpha Centauri]].<ref name="motz">{{cite book |last1=Motz |first1=Lloyd |author-link1=Lloyd Motz |last2=Nathanson |first2=Carol |title=The Constellations: An Enthusiast's Guide to the Night Sky |publisher=Aurum Press |location=London, United Kingdom |year=1991 |page=387 |isbn=978-1-85410-088-7}}</ref> Not only the brightest star in the constellation, it is also the brightest example of a [[Rapidly oscillating Ap star|rapidly oscillating Ap (RoAp) star]] in the night sky. It has the unusual [[spectral type]] A7&nbsp;Vp SrCrEu, showing increased emissions of [[strontium]], [[chromium]] and [[europium]]. Stars of this type have oddly localised [[magnetic field]]s and are slightly [[variable star|variable]].<ref name=kaler>{{cite web |url=http://stars.astro.illinois.edu/sow/alphacir.html |title=Alpha Circini |last=Kaler |first=Jim |author-link=James B. Kaler |work=Stars |publisher=University of Illinois |access-date=26 October 2012}}</ref> Alpha Circini forms a [[binary star|binary]] [[star system]] with an [[K-type main-sequence star|orange dwarf]] companion of spectral type K5 and magnitude 8.5,<ref name=kaler/> which with a separation of 5.7&nbsp;[[arcsecond]]s is only discernible with a telescope.<ref name=moore11/><ref name=redath07>{{cite book |last1=Ridpath |first1=Ian |author-link1=Ian Ridpath |last2=Tirion |first2=Wil |author-link2=Wil Tirion |year=2017 |title=Stars and Planets Guide (5th ed.) |publisher=Princeton University Press |location=Princeton |isbn=978-0-69-117788-5 |page=120}}</ref> The distance between the two stars is 260&nbsp;[[Astronomical unit|AU]] and they take 2600&nbsp;years to rotate around a common centre of gravity.<ref name=kaler/> The second brightest star is [[Beta Circini]], a white main sequence star of spectral type A3Va and a magnitude of 4.07, about 100&nbsp;light-years away.<ref name=sb0>{{cite web |url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Beta++Circini&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |title=Beta Circini |work=[[SIMBAD|SIMBAD Astronomical Database]] |publisher=Centre de Données astronomiques de Strasbourg |access-date=30 June 2012}}</ref> It has around 1.8&nbsp;times the diameter of the Sun.<ref name=bagnall>{{cite book |last=Bagnall |first=Philip M. |title=The Star Atlas Companion: What You Need to Know about the Constellations |url=https://archive.org/details/staratlascompani00bagn |url-access=limited |publisher=Springer  |location=New York, New York |year=2012 |isbn=978-1-4614-0830-7 |pages=[https://archive.org/details/staratlascompani00bagn/page/n170 160]–62}}</ref>

[[Gamma Circini]] is a binary star 450&nbsp;light-years away,<ref name=sb1/> whose components need a telescope of 150&nbsp;mm to be seen, as they are only 0.8 arcseconds apart.<ref name=moore11/><ref name=redath07/> The brighter component is a bluish [[Be star]] of spectral type B5IV+ and magnitude 4.51,<ref name=sb1>{{cite web |url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Gamma++Circini&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |title=Gamma Circini |work=[[SIMBAD|SIMBAD Astronomical Database]] |publisher=Centre de Données astronomiques de Strasbourg |access-date=30 June 2012}}</ref> while the dimmer component is a [[G-type main-sequence star|yellow star]] of magnitude 5.5.<ref name="ridpath2001">{{Cite book |title=Stars and Planets Guide |last1=Ridpath |first1=Ian |author-link1=Ian Ridpath |last2=Tirion |first2=Wil |author-link2=Wil Tirion|year=2001 |publisher=Princeton University Press |isbn=978-0-691-08913-3 |pages=118–19}}</ref> They orbit each other every 180 years.<ref name=moore11/> [[Delta Circini]] is also a multiple star whose components have magnitudes of 5.1 and 13.4 and orbit around a common centre of gravity every 3.9 days. The brighter component is a close [[eclipsing binary]] (specifically, a [[rotating ellipsoidal variable]]),<ref name=moore11/> with a minor dip of magnitude (0.1). Both are hot [[O-type main-sequence star|blue stars]] of spectral types O7III-V and O9.5V, respectively, and are estimated to have around 22 and 12 times the Sun's mass.<ref>{{cite journal |year=2001 |title=Tomographic Separation of Composite Spectra. VII. The Physical Properties of the Massive Triple System HD 135240 (Delta Circini) |journal=The Astrophysical Journal |volume=548 |pages=889–99 |doi=10.1086/319031 |last1=Penny |first1=Laura R. |last2=Seyle |first2=Debra |last3=Gies |first3=Douglas R. |last4=Harvin |first4=James A. |last5=Bagnuolo |first5=Jr. |last6=Thaller |first6=M. L. |last7=Fullerton |first7=A. W. |last8=Kaper |first8=L. |issue=2|bibcode = 2001ApJ...548..889P |doi-access=free |url=https://pure.uva.nl/ws/files/3754521/18634_99997y.pdf }}</ref> Over 3600&nbsp;light-years away,<ref name=sb2>{{cite web |url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Delta++Circini&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |title=Delta Circini |work=[[SIMBAD|SIMBAD Astronomical Database]] |publisher=Centre de Données astronomiques de Strasbourg |access-date=25 October 2012}}</ref> this system would outshine [[Venus]] at magnitude −4.8 if it were 32&nbsp;light-years (10&nbsp;[[parsec]]s) distant.<ref name=bagnall/> The two main components are separated by 50 arcseconds, resolvable to the naked eye for individuals with good vision and easily discernible with a telescope.<ref name=moore11/>

[[Eta Circini]] is a yellow giant of spectral type G8III and magnitude 5.17, located around 276&nbsp;light-years distant,<ref name=sb3>{{cite web |url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Eta++Circini&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |title=Eta Circini |work=[[SIMBAD|SIMBAD Astronomical Database]] |publisher=Centre de Données astronomiques de Strasbourg |access-date=26 October 2012}}</ref> and [[Zeta Circini]] is a blue-white main sequence star of spectral type B3V and magnitude 6.09, located around 1273&nbsp;light-years away.<ref name=sb4>{{cite web |url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Zeta++Circini&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |title=Zeta Circini |work=[[SIMBAD|SIMBAD Astronomical Database]] |publisher=Centre de Données astronomiques de Strasbourg |access-date=26 October 2012}}</ref>

493 variable stars have been recorded in Circinus, but most have a very small range or are quite dim.<ref>{{cite web |url=http://www.aavso.org/vsx/index.php?view=results.submit0 |title=Circinus search results |publisher=AAVSO |work=International Variable Star Index |access-date=31 January 2013}}</ref> Three prominent examples are [[Theta Circini]], [[T Circini]], and [[AX Circini]].<ref name="simpson">{{cite book |pages = 743–47 |title = Guidebook to the Constellations : Telescopic Sights, Tales, and Myths |first = Phil |last = Simpson |isbn = 978-1-4419-6941-5 |publisher = Springer New York |year = 2012}}</ref><ref name="inglis"/> Theta Circini is a B-class irregular variable, ranging in magnitude from 5.0 to 5.4.<ref name=moore11/> T Circini has a B-type spectrum, ranging in magnitude from 10.6 to 9.3 over a period of 3.298&nbsp;days,<ref name="simpson"/> although it is actually an eclipsing binary system rather than a pulsating star.<ref name=AAVSOT>{{cite web|url=http://www.aavso.org/vsx/index.php?view=detail.top&oid=9402 |title=T Circini |author =VSX |date=4 January 2010|work=AAVSO Website|publisher=American Association of Variable Star Observers|access-date=24 January 2014}}</ref> AX is a [[Cepheid variable]] that varies between magnitudes 5.6 and 6.19 over 5.3&nbsp;days.<ref name="inglis">{{cite book |last=Inglis |first=Mike |title=Astronomy of the Milky Way: Observer's Guide to the Southern Sky |publisher=Springer |location=New York, New York |year=2004 |page=31 |isbn=978-1-85233-742-1 |url=https://books.google.com/books?id=1r0qvMjSCGAC&pg=SA3-PA31}}</ref> It is a yellow-white supergiant of spectral type F8II+, 1600 light-years away.<ref name=sb5>{{cite web |url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=AX+Circini&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |title=AX Circini |work=[[SIMBAD|SIMBAD Astronomical Database]] |publisher=Centre de Données astronomiques de Strasbourg |access-date=26 October 2012}}</ref> [[BP Circini]] is another [[Cepheid variable]] with an apparent magnitude ranging from 7.37 to 7.71 over 2.4 days.<ref name=AAVSOBP>{{cite web|url=http://www.aavso.org/vsx/index.php?view=detail.top&oid=9492 |title=BP Circini |last=BSJ|date=30 November 2013|work=AAVSO Website|publisher=American Association of Variable Star Observers|access-date=24 January 2014}}</ref> Both cepheids are spectroscopic binaries, with companions that are blue-white stars of spectral type B6 and 5 and 4.7 solar masses, respectively.<ref name="evans13">{{cite journal|author1=Evans, Nancy Remage |author2=Bond, Howard E. |author3=Schaefer, Gail H. |author4=Mason, Brian D. |author5=Karovska, Margarita |author6=Tingle, Evan |date=2013|title=Binary Cepheids: Separations and Mass Ratios in 5M ⊙ Binaries|journal=Astronomical Journal|volume=146|issue=4|pages=93, 10 pp|arxiv=1307.7123|doi=10.1088/0004-6256/146/4/93 |bibcode = 2013AJ....146...93E |s2cid=34133110 }}</ref> [[BX Circini]] is a faint star that fluctuates between magnitudes 12.57 and 12.62 over a period of 2 hours 33 minutes.<ref name=AAVSOBX>{{cite web|url=http://www.aavso.org/vsx/index.php?view=detail.top&oid=9500 |title=BX Circini |author =Otero, Sebastian Alberto |date=30 October 2011 |work=AAVSO Website|publisher=American Association of Variable Star Observers|access-date=24 January 2014}}</ref> Over 99% of its composition appears to be helium. Its origin is unclear, but thought to be the result of the merger of a helium and a carbon/oxygen white dwarf.<ref name="woolf02">{{cite journal|author1=Woolf, V. M. |author2=Jeffery, C. S. |year=2002|title=Temperature and gravity of the pulsating extreme helium star LSS 3184 (BX Cir) through its pulsation cycle|journal=Astronomy & Astrophysics|volume=395|issue=2 |pages=535–40|url=http://www.aanda.org/index.php?option=com_article&access=bibcode&Itemid=129&bibcode=2002A%2526A...395..535WFUL|doi=10.1051/0004-6361:20021113|arxiv = astro-ph/0208269 |bibcode = 2002A&A...395..535W |s2cid=16753328 }}</ref>

Several stars with planetary systems lie within the borders of Circinus, although none of the host stars are particularly prominent. [[HD 134060]] is a sun-like yellow dwarf star of spectral type G0VFe+0.4 and magnitude 6.29, around 79&nbsp;light-years away.<ref name=sbHD134060>{{cite web |url=http://simbad.u-strasbg.fr/simbad/sim-id?protocol=html&Ident=HD%20134060 |title=LTT 6035 – High proper-motion Star |work=[[SIMBAD|SIMBAD Astronomical Database]] |publisher=Centre de Données astronomiques de Strasbourg |access-date=25 October 2012}}</ref> Its two planets were discovered in 2011 through the [[radial velocity method]]: the smaller, [[HD 134060 b]], has a mass of 0.0351&nbsp;[[Jupiter mass|M<sub>J</sub>]] (Jupiter masses) and orbits its star every 3.27&nbsp;days, at 0.0444 AU;<ref>{{cite encyclopedia |url=http://voparis-exoplanet-new.obspm.fr/catalog/hd_134060_b/ |archive-url=https://web.archive.org/web/20140202125600/http://voparis-exoplanet-new.obspm.fr/catalog/hd_134060_b/ |archive-date=2014-02-02 |title=HD 134060 b |publisher=Paris Observatory |encyclopedia=The Extrasolar Planet Encyclopedia |date=12 September 2011 |access-date=31 January 2013}}</ref> the larger, [[HD 134060 c]] (0.15&nbsp;M<sub>J</sub>), orbits farther out at 2.226&nbsp;AU, with a period of approximately 1161 days.<ref>{{cite encyclopedia |url=http://voparis-exoplanet-new.obspm.fr/catalog/hd_134060_c/ |archive-url=https://web.archive.org/web/20140202131017/http://voparis-exoplanet-new.obspm.fr/catalog/hd_134060_c/ |archive-date=2014-02-02 |title=HD 134060 c |publisher=Paris Observatory |encyclopedia=The Extrasolar Planet Encyclopedia |date=12 September 2011 |access-date=31 January 2013}}</ref> Even fainter, at magnitude 8.8, [[HD 129445]] is 220&nbsp;light-years away and has 99% of the Sun's mass and a similar spectral type of G8V. [[HD 129445 b]], a [[Jupiter]]-like planet (1.6 M<sub>J</sub>) discovered in 2010 via the radial velocity method, orbits this star at a distance of 2.9 AU, approximately every 1840&nbsp;days.<ref>{{cite encyclopedia |url=http://voparis-exoplanet-new.obspm.fr/catalog/hd_129445_b/ |archive-url=https://web.archive.org/web/20140202131657/http://voparis-exoplanet-new.obspm.fr/catalog/hd_129445_b/ |archive-date=2014-02-02 |title=HD 129445 b |publisher=Paris Observatory |encyclopedia=The Extrasolar Planet Encyclopedia |date=26 January 2010 |access-date=31 January 2013}}</ref>

As this constellation intersects the plane of the Milky Way, there are many massive stars located in this constellation, including [[GKF2010 MN18]] (or simply MN18), a blue supergiant located in a bipolar nebula,<ref>{{Cite journal|last1=Gvaramadze|first1=V. V.|last2=Kniazev|first2=A. Y.|last3=Bestenlehner|first3=J. M.|last4=Bodensteiner|first4=J.|last5=Langer|first5=N.|last6=Greiner|first6=J.|last7=Grebel|first7=E. K.|last8=Berdnikov|first8=L. N.|last9=Beletsky|first9=Y.|date=2015-11-01|title=The blue supergiant MN18 and its bipolar circumstellar nebula|journal=Monthly Notices of the Royal Astronomical Society|volume=454|issue=1 |pages=219–237|doi=10.1093/mnras/stv1995|arxiv=1508.06288 |bibcode=2015MNRAS.454..219G |issn=0035-8711|doi-access=free}}</ref> as well as 9 [[Wolf–Rayet star|Wolf-Rayet stars]], a very high number for such a small constellation.

===Deep-sky objects===
[[File:NGC 5823 in Cir.jpg|thumb|An astrophotograph of [[NGC 5823]], showing its backwards S shape]]
Three [[open cluster]]s and a [[planetary nebula|planetary nebulae]] are found within the borders of Circinus, all visible with amateur telescopes of varying sizes. [[NGC 5823]], also called [[Caldwell catalogue|Caldwell]] 88,<ref name=moore11/> is an 800-million-year-old open cluster, located 3500&nbsp;light-years away and spanning a 12-light-year region along the constellation's northern border.<ref name="mobberley"/> Despite having an integrated magnitude of 7.9,<ref name=moore11/> the cluster can be seen by [[star hopping]] from Beta Circini or from Alpha Centauri.<ref name="photoatlas"/> It contains 80–100 stars of 10th magnitude and fainter, which are spread out over a diameter of 10&nbsp;arcseconds.<ref name="inglis"/> The brighter stars, however, are not true members of the cluster, as they are closer to the Earth than the dimmer ones.<ref name="o'meara">{{cite book |title=The Caldwell Objects |first=Stephen James |last=O'Meara |publisher=Cambridge University Press |year=2002 |pages=349–50 |isbn=978-0-521-82796-6}}</ref> NGC 5823 appears distinct to the observer, sometimes seen as a reversed "S", as described by [[John Herschel]],<ref name="mobberley">{{cite book |title=The Caldwell Objects And How to Observe Them |first=Martin |last=Mobberley |author-link=Martin Mobberley |page=184 |publisher=Springer |year=1999 |isbn=978-1-4419-0326-6}}</ref><ref>{{cite book |title=1001 Celestial Wonders to See Before You Die: The Best Sky Objects for Star Gazers |first=Michael E. |last=Bakich |publisher=Springer |year=2010 |isbn=978-1-4419-1777-5 |page=174}}</ref> although it has also been described as "tulip-shaped" and "boxy".<ref name="o'meara"/> That cluster can be easily mistaken with a similar cluster, [[NGC 5822]], nearby in [[Lupus (constellation)|Lupus]].<ref name="photoatlas">{{cite book |title=The Photographic Atlas of the Stars |first1=H.J.P. |last1=Arnold |first2=P.D. |last2=Doherty |first3=Patrick |last3=Moore |author-link3=Patrick Moore |publisher=CRC Press |year=1997 |isbn=978-0-7503-0654-6 |page=176}}</ref> Comparatively, open cluster [[NGC 5715]] is fainter (integrated magnitude of 9.8)—its brightest star is only 11th magnitude—and smaller (7.0&nbsp;arcminutes), comprising only 30 stars. The third open cluster, [[Pismis 20]], contains 12 stars in a diameter of 4.5&nbsp;arcseconds but exhibits a magnitude similar to NGC 5823 (7.8). At 8270&nbsp;light-years, it requires an amateur telescope with an aperture over 300&nbsp;mm to be easily discerned.<ref name="simpson"/>

[[File:NGC 5315HSTfull.jpg|left|thumb|[[Hubble Space Telescope]]'s view of NGC 5315, showing its intricate structure and central star]]
The planetary nebula [[NGC 5315]] has a magnitude of 9.8 around a central star of magnitude 14.2, located 5.2&nbsp;degrees west-southwest of Alpha Circini. It is only visible as a disc at magnifications over 200-fold.<ref name=bakich2010>{{cite book |first=Michael E. |last=Bakich |year=2010 |page=162 |title=1001 Celestial Wonders to See Before You Die: The Best Sky Objects for Star Gazers |series=Patrick Moore's Practical Astronomy Series |publisher=Springer |isbn=978-1-4419-1776-8 |url=https://books.google.com/books?id=qEhpS7d5ZdAC&pg=PA475}}</ref> [[Bernes 145]] is a [[dark nebula|dark]] and [[reflection nebula]] first listed in the 1971 Bernes Catalog. The dark nebula component is easily visible in a large amateur telescope, and it measures 12&nbsp;by&nbsp;5&nbsp;arcminutes. The smaller reflection nebula component requires a larger instrument and [[averted vision]] to be seen.<ref>{{cite book |title=Imaging the Southern Sky: An Amateur Astronomer's Guide |first1=Stephen |last1=Chadwick |first2=Ian |last2=Cooper |page=139 |isbn=978-1-4614-4750-4 |year=2012|publisher=Springer }}</ref>

Circinus also houses ESO 97-G13, commonly known as the [[Circinus Galaxy]]. Discovered in 1977,<ref name="inglis"/> it is a relatively unobscured galaxy (magnitude 10.6), which is unusual for galaxies located in constellations near the [[Milky Way]], since their dim light is obscured by gas and dust. This oblong [[spiral galaxy]] with 6.9&nbsp;by&nbsp;3.0&nbsp;arcminutes and 26,000&nbsp;light-years in diameter, is located 13 million light-years away from Earth and lies 4 degrees off the [[galactic plane]].<ref name="simpson"/> It is the closest [[Seyfert galaxy]] to the Milky Way,<ref>{{cite journal |last1=Maiolino |first1=R |last2=Krabbe |first2=A. |last3=Thatte |first3=N. |last4=Genzel |first4=R. |year=1998 |title=Seyfert Activity and Nuclear Star Formation in the Circinus Galaxy |journal=The Astrophysical Journal |volume=493 |issue=2 |pages=650–65 |bibcode=1998ApJ...493..650M |doi=10.1086/305150|arxiv = astro-ph/9709091 |s2cid=16365899 }}</ref> and therefore hosts an [[active galactic nucleus]].<ref name="for12">{{cite journal |year=2012 |title=Gas and Star Formation in the Circinus Galaxy |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=425 |issue=3 |pages=1934–50 [1934] |arxiv=1206.4102 |bibcode=2012MNRAS.425.1934F |last1=For |first1=B.-Q. |last2=Koribalski|first2=B. S.|last3=Jarrett |first3=T. H. |doi=10.1111/j.1365-2966.2012.21416.x|doi-access=free |s2cid=41853947 }}</ref>

[[File:Circinus X-1.jpg|thumb|The [[Chandra X-ray Observatory]]'s false-color image of Circinus X-1 showing its [[astrophysical jet|jets]]]]
[[Circinus X-1]] is an [[X-ray binary]] star system that includes a [[neutron star]]. Observations of Circinus X-1 in July 2007 revealed the presence of X-ray jets normally found in [[black hole]] systems.<ref>{{cite web |url=http://chandra.harvard.edu/photo/2007/cirx1/ |title=Circinus X-1: Neutron Stars Join the Black Hole Set |access-date=8 January 2009 |work=[[Chandra X-ray Observatory]] |publisher=[[Harvard University]] / [[NASA]] |year=2007}}</ref> Located at 19,000&nbsp;light-years, the pulsar [[PSR B1509-58]], also called the Circinus Pulsar, has expelled a 20-light-year-long jet of material from its southern pole, clearly visible in the [[X-ray]] spectrum.<ref>{{Cite APOD |title=X-Rays and the Circinus Pulsar |access-date=31 January 2013 |date=13 September 2001 }}</ref> Another [[supernova remnant]] in Circinus is that of [[SN 185]]. Recorded by Chinese observers in 185 AD, SN 185 was visible in the night sky for around eight months; its remnants, known as RCW 86, cover an area larger than the typical full moon.<ref name="nasa-2011-10-24">{{cite news |title=NASA Telescopes Help Solve Ancient Supernova Mystery |publisher=[[NASA]] |date=24 November 2011 |url=http://www.nasa.gov/mission_pages/spitzer/news/spitzer20111024.html |access-date=30 June 2012 |archive-date=17 June 2019 |archive-url=https://web.archive.org/web/20190617105712/https://www.nasa.gov/mission_pages/spitzer/news/spitzer20111024.html |url-status=dead }}</ref>

A white dwarf star in a close binary system can accumulate material from its companion until it ignites and blows off in a thermonuclear explosion, known as a [[nova]].<ref name=greeley95/> These stars generally brighten by 7 to 16 magnitudes.<ref>{{cite web |url=http://www.aavso.org/types-variables |title=Types of Variables |date=18 June 2012 |work=AAVSO |publisher=American Association of Variable Star Observers |access-date=2 February 2013 |location=Cambridge, Massachusetts |archive-date=17 October 2018 |archive-url=https://web.archive.org/web/20181017170335/http://www.aavso.org/types-variables |url-status=dead }}</ref> Nova Circini 1926, also known as [[X Circini]], was observed at magnitude 6.5 on 3 September 1926, before fading and fluctuating between magnitudes 11.7 and 12.5, during 1928, and magnitude 13, in 1929.<ref>{{cite journal |title=Becker's Nova Circini No. 2 1926 |last=Cannon |first=A.J.|journal=Harvard College Observatory Bulletin |issue=872 |pages=1–2 |bibcode=1930BHarO.872....1C |volume=872 |year=1930}}</ref> Nova Circini 1995 ([[BY Circini]]) reached a maximum apparent magnitude of 7.2 in January 1995.<ref name=greeley95>{{cite journal |last1=Greeley |first1=Bradford W. |last2=Blair |first2=William P. |last3=Long |first3=Knox S. |year=1995 |title=Hopkins Ultraviolet Telescope Observations of Nova Circini 1995 and Nova Aquilae 1995 |journal=Astrophysical Journal Letters |volume=454 |pages=L43–46 |bibcode=1995ApJ...454L..43G |doi=10.1086/309767|doi-access=free }}</ref> [[BW Circini]] is a low mass X-ray binary system, comprising a black hole of around 8 solar masses and a yellow G0III-G5III subgiant star.<ref name="kreidberg12">{{cite journal|last=Kreidberg|first=Laura|author2=Bailyn, Charles D. |author3=Farr, Will M. |author4= Kalogera, Vicky  |year=2012|title=Mass Measurements of Black Holes in X-ray Transients: is There a Mass Gap? |journal=The Astrophysical Journal|volume=757|issue=36|pages=17pp|bibcode=2012ApJ...757...36K|doi=10.1088/0004-637x/757/1/36|arxiv = 1205.1805 |s2cid=118452794}}</ref> X-ray outbursts were recorded in 1987 and 1997, and possibly 1971–72.<ref>{{cite journal|author1=Casares, J. |author2=Zurita, C. |author3=Shahbaz, T. |author4=Charles, P.A. |author5=Fender, R.P. |year=2004|title=Evidence of a Black Hole in the X-ray Transient GS 1354–64 (=BW Circini)|journal=The Astrophysical Journal|volume=613|issue=2 |pages=L133–L136|doi=10.1086/425145|arxiv = astro-ph/0408331 |bibcode = 2004ApJ...613L.133C |s2cid=14766056 }}</ref>

===Meteor showers===
Circinus is the [[radiant (meteor shower)|radiant]] of an annual [[meteor shower]], the [[Alpha Circinids]] (ACI). First observed in [[Queensland]] in 1977,<ref name=jenniskens>{{cite book |last=Jenniskens |first=Peter |title=Meteor Showers and Their Parent Comets |year=2006 |publisher=Cambridge University Press |isbn=978-0-521-85349-1}}</ref> the meteors have an average velocity of 27.1&nbsp;km/s and are thought to be associated with a [[long-period comet]].<ref name=jenniskens/><ref>{{cite web |url=http://www.astro.amu.edu.pl/~jopek/MDC2007/Roje/pojedynczy_obiekt.php?kodstrumienia=00162&colecimy=0 |publisher=IAU |work=Meteor Data Center |title=Alpha Circinids |date=15 November 2012 |access-date=31 January 2013 |url-status=dead |archive-url=https://web.archive.org/web/20140221213056/http://www.astro.amu.edu.pl/~jopek/MDC2007/Roje/pojedynczy_obiekt.php?kodstrumienia=00162&colecimy=0 |archive-date=21 February 2014 }}</ref> In 2011, [[Peter Jenniskens]] proposed that the debris trail of comet C/1969 T1 could intersect with the Earth's orbit and generate a meteor outburst coming from a radiant close to Beta Circini.<ref>{{cite web |url=https://www.ngdc.noaa.gov/stp/space-weather/online-publications/igc_calendars/2011/igc_calendar_2011_long.pdf |title=2011 International Geophysical Year Calendar |last=Jenniskens |first=Peter |access-date=31 January 2013 |publisher=[[NASA]] }}</ref> The ACI shower peaks on 4 June, the day it was first observed.<ref>{{cite book |title=A Complete Manual of Amateur Astronomy: Tools and Techniques for Astronomical Observations |first1=P. Clay |last1=Sherrod |first2=Thomas L. |last2=Koed |publisher=Courier Dover Publications |year=2003 |isbn=978-0-486-42820-8}}</ref>

==See also==
* [[Circinus (Chinese astronomy)]]

==References==
{{Reflist}}

== External links ==
{{Commons}}
* [http://www.allthesky.com/constellations/centaurus/ The Deep Photographic Guide to the Constellations: Circinus]

{{Stars of Circinus}}
{{Constellations}}
{{ConstellationsByLacaille}}
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[[Category:Circinus| ]]
[[Category:Southern constellations]]
[[Category:Constellations listed by Lacaille]]