Editing Cancer (constellation) (section)

==Features==

===Stars===
{{See also|List of stars in Cancer}}
Cancer is the dimmest of the [[zodiacal constellations]], having only two stars above the fourth magnitude.<ref name="ridpath"/> The German cartographer [[Johann Bayer]] used the Greek letters [[Alpha]] through [[Omega]] to label the most prominent stars in the constellation, followed by the letter A, then lowercase b, c and d.{{sfn|Wagman|2003|p=60}}  Within the constellation's borders, there are 104 stars brighter than or equal to [[apparent magnitude]]&nbsp;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]]|access-date=28 August 2017|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>{{cite web| url=http://www.ianridpath.com/constellations1.html | title=Constellations: Andromeda–Indus | work= Star Tales |author=Ridpath, Ian |author-link=Ian Ridpath|publisher=self-published | access-date= 26 August 2015}}</ref>

[[File:CancerCC cropped.jpg|thumb|left|256px|The constellation Cancer as it can be seen by the naked eye.]]
Also known as Altarf or Tarf,<ref name="IAU-LSN">{{cite web | url=https://www.iau.org/public/themes/naming_stars/ | title=Naming Stars |publisher=IAU.org |access-date=30 July 2018}}</ref> [[Beta Cancri]] is the brightest star in Cancer at [[apparent magnitude]] 3.5.<ref name="kalerbeta">{{cite web | title=Al Tarf (Beta Cancri) | work=Stars | publisher=University of Illinois | first1=James B. | last1=Kaler | url=http://stars.astro.illinois.edu/sow/altarf.html | access-date=20 March 2014}}</ref> Located 290 ± 30 [[light-year]]s from Earth,<ref name="vanLeeuwen2007"/> it is a [[binary star]] system, its main component an [[orange giant]] of spectral type K4III that is varies slightly from a baseline magnitude of 3.53—dipping by 0.005 magnitude over a period of 6 days.<ref name=AAVSObeta>{{cite web|url=http://www.aavso.org/vsx/index.php?view=detail.top&oid=42597 |title=NSV 3973 |author =Watson, Christopher |date=3 May 2013 |work=AAVSO Website|publisher=American Association of Variable Star Observers|access-date=20 March 2014}}</ref> An aging star, it has expanded to around 50 times the Sun's diameter and shines with 660 times its luminosity. It has a faint magnitude 14 [[red dwarf]] companion located 29 arcseconds away that takes 76,000 years to complete an orbit.<ref name="kalerbeta"/> [[Altarf]] represents a part of Cancer's body.

At magnitude 3.9 is [[Delta Cancri]], also known as Asellus Australis.<ref name="kalerdelta"/> Located 131±1 light-years from Earth,<ref name="vanLeeuwen2007">{{cite journal | first=F. | last=van Leeuwen | title=Validation of the New Hipparcos Reduction | journal=Astronomy and Astrophysics | volume=474 | issue=2 | pages=653–64 | date=2007 | bibcode=2007A&A...474..653V | doi=10.1051/0004-6361:20078357 | arxiv=0708.1752| s2cid=18759600 }}</ref> it is an orange-hued giant star that has swollen and cooled off the main sequence to become an orange giant with a radius 11 times and luminosity 53 times that of the Sun.<ref name="kalerdelta">{{cite web | title=Asellus Australis (Delta Cancri) | work=Stars | publisher=University of Illinois | first1=James B. | last1=Kaler | url=http://stars.astro.illinois.edu/sow/asellusaus.html |date=14 May 2010 | access-date=7 April 2015}}</ref> Its common name means "southern donkey".<ref name="ridpath"/> The star also holds a record for the longest name, "Arkushanangarushashutu," derived from ancient Babylonian language, which translates to "the southeast star in the Crab."{{Citation needed|date=January 2024}} Delta Cancri also makes it easy to find [[X Cancri]], the reddest star in the sky.  Known as Asellus Borealis "northern donkey", [[Gamma Cancri]] is a white-hued A-type subgiant of spectral type A1IV and magnitude 4.67,<ref name=SIMBAD>{{cite simbad|title = gam Cnc |access-date = 8 April 2015}}</ref> that is 35 times as luminous as of the Sun.<ref name=Mcdonald>{{cite journal|author=McDonald, I.|author2=Zijlstra, A. A.|author3=Boyer, M. L.|date=2012|title=Fundamental Parameters and Infrared Excesses of Hipparcos Stars|journal=Monthly Notices of the Royal Astronomical Society|volume=427|issue=1|pages=343–57|bibcode=2012MNRAS.427..343M|arxiv = 1208.2037 |doi = 10.1111/j.1365-2966.2012.21873.x |doi-access=free |s2cid=118665352}}</ref> It is located 181 ± 2 light-years from Earth.<ref name="vanLeeuwen2007"/>

[[Iota Cancri]] is a wide double star. The primary is a yellow-hued G-type bright giant star of magnitude 4.0,<ref name="kaleriota"/> located 330 ± 20 light-years from Earth.<ref name="vanLeeuwen2007"/> It spent much of its stellar life as a B-type main sequence star before expanding and cooling to its current state as it spent its core hydrogen. The secondary is a [[A-type main sequence star|white main sequence star]] of spectral type A3V and magnitude 6.57. Despite having different distances when measured by the HIPPARCOS satellite, the two stars share a common proper motion and appear to be a natural binary system.<ref name="kaleriota">{{cite web | title=Iota Cancri | work=Stars | publisher=University of Illinois | first1=James B. | last1=Kaler | url=http://stars.astro.illinois.edu/sow/iotacnc.html | access-date=29 May 2015}}</ref>

Located 181 ± 2 [[light-years]] from Earth,<ref name="vanLeeuwen2007"/> [[Alpha Cancri]] (Acubens) is a multiple star with a primary component an apparent white main sequence star of spectral type A5 and magnitude 4.26. The secondary is of magnitude 12.0 and is visible in small amateur [[telescopes]]. Its common name means "the claw".<ref name="ridpath"/> The primary is actually two very similar white main sequence stars that are 5.3 AU distant from each other and the secondary is two small main sequence stars, most likely red dwarfs, that are 600 AU from the main pair. Hence the system is a quadruple one.<ref name="kaleralpha">{{cite web | title=Acubens | work=Stars | publisher=University of Illinois | first1=James B. | last1=Kaler | url=http://stars.astro.illinois.edu/sow/acubens.html | access-date=29 May 2015}}</ref>

[[Zeta Cancri]] or Tegmine ("the shell") is a [[multiple star system]] that contains at least four stars located 82 light-years from Earth. The two brightest components are a binary star with an [[orbital period]] of 1100 years; the brighter component is a yellow-hued binary pair and the dimmer component is a yellow-hued star of magnitude 6.2. The brighter component is itself a binary star with a period of 59.6 years; its primary is of magnitude 5.6 and its secondary is of magnitude 6.0. This pair is at its greatest separation around 2019.<ref name="ridpath"/>

Ten star systems have been found to have planets. Rho<sup>1</sup> Cancri or [[55 Cancri]] (or Copernicus<ref name="IAU-LSN"/>) is a binary star approximately 40.9 light-years distant from Earth. 55 Cancri consists of a yellow dwarf and a smaller red dwarf, with five planets orbiting the primary star; one low-mass planet that may be either a hot, water-rich world or a carbon planet and four gas giants. 55 Cancri A, classified as a rare "super metal-rich" star, is one of the top 100 target stars for NASA's Terrestrial Planet Finder mission, ranked 63rd on the list. The red dwarf 55 Cancri B, a suspected binary, appears to be gravitationally bound to the primary star, as the two share common proper motion.

[[YBP 1194]] is a sunlike star in the open cluster [[Messier 67|M67]] that has been found to have three planets.

===Deep-sky objects===
[[File:Messier 44 2018.jpg|alt=Image of Messier 44 (the Beehive Cluster)|thumb|Messier 44 (the Beehive Cluster)]]
Cancer is best known among stargazers as the home of [[Beehive Cluster|Praesepe]] (Messier 44), an [[open cluster]] also called the ''Beehive Cluster'', located right in the centre of the constellation. Located about 590 light-years from Earth, it is one of the nearest open clusters to our Solar System. M 44 contains about 50 stars, the brightest of which are of the sixth magnitude. Epsilon Cancri is the brightest member at magnitude 6.3. Praesepe is also one of the larger open clusters visible; it has an area of 1.5 square degrees, or three times the size of the full Moon.<ref name="ridpath"/> It is most easily observed when Cancer is high in the sky. North of the Equator, this period stretches from February to May. 

[[Ptolemy]] described the Beehive Cluster as "the nebulous mass in the breast of Cancer." It was one of the first objects [[Galileo]] observed with his telescope in 1609, spotting 40 stars in the cluster. Today, there are about 1010 high-probability members, most of them (68 percent) red dwarfs. The Greeks and Romans identified the nebulous object as a manger from which two donkeys, represented by the neighbouring stars [1213] Asellus Borealis and [1210] Asellus Australis, were eating. The stars represent the donkeys that the god [[Dionysus]] and his tutor [[Silenus]] rode in the war against the [[Titan (mythology)|Titan]]s. The ancient Chinese interpreted the object as a ghost or demon riding in a carriage, calling it a "cloud of pollen blown from under willow catkins."{{cn|date=March 2024}}

The smaller, denser open cluster [[Messier 67]] can also be found in Cancer, 2600 light-years from Earth. It has an area of approximately 0.5 square degrees, the size of the full Moon. It contains approximately 200 stars, the brightest of which are of the tenth magnitude.<ref name="ridpath"/>

[[QSO J0842+1835]] is a [[quasar]] used to measure the [[speed of gravity]] in [[VLBI]] experiment conducted by [[Edward Fomalont]] and [[Sergei Kopeikin]] in September 2002.

[[OJ 287]] is a [[BL Lacertae object]] located 3.5 [[1000000000 (number)|billion]] [[light years]] away that has produced quasi-periodic optical outbursts going back approximately 120 years, as  first apparent on photographic plates from 1891.  It was first detected at radio wavelengths during the course of the [[Ohio Sky Survey]]. Its central [[supermassive black hole]] is [[List of most massive black holes|among the largest known]], with a mass of 18 billion [[solar masses]],<ref>{{Cite journal | last1 = Valtonen | first1 = M. J. | last2 = Lehto | first2 = H. J. | last3 = Nilsson | first3 = K. | last4 = Heidt | first4 = J. | last5 = Takalo | first5 = L. O. | last6 = Sillanpää | first6 = A. | last7 = Villforth | first7 = C. | last8 = Kidger | first8 = M. | last9 = Poyner | first9 = G. | last10 = Pursimo | doi = 10.1038/nature06896 | first10 = T. | last11 = Zola | first11 = S. | last12 = Wu | first12 = J. -H. | last13 = Zhou | first13 = X. | last14 = Sadakane | first14 = K. | last15 = Drozdz | first15 = M. | last16 = Koziel | first16 = D. | last17 = Marchev | first17 = D. | last18 = Ogloza | first18 = W. | last19 = Porowski | first19 = C. | last20 = Siwak | first20 = M. | last21 = Stachowski | first21 = G. | last22 = Winiarski | first22 = M. | last23 = Hentunen | first23 = V. -P. | last24 = Nissinen | first24 = M. | last25 = Liakos | first25 = A. | last26 = Dogru | first26 = S. | title = A massive binary black-hole system in OJ 287 and a test of general relativity | journal = Nature | volume = 452 | issue = 7189 | pages = 851–853 | year = 2008 | pmid = 18421348 | url = http://astrophysics.rit.edu/pastjclub/nature06896.pdf | arxiv = 0809.1280 | bibcode = 2008Natur.452..851V | s2cid = 4412396 | access-date = 1 September 2015 | archive-date = 8 September 2015 | archive-url = https://web.archive.org/web/20150908193115/http://astrophysics.rit.edu/pastjclub/nature06896.pdf | url-status = dead }}</ref> more than six times the value calculated for the previous largest object.<ref name="newscientist">{{cite news| url=https://www.newscientist.com/article/dn13166-biggest-black-hole-in-the-cosmos-discovered.html|title= Biggest black hole in the cosmos discovered| date=10 January 2008| publisher= NewScientist.com news service| first= David| last= Shiga}}</ref>