Visible at latitudes between +60° and −90°. Best visible at 21:00 (9 p.m.) during the month ofFebruary.
Canis Major is aconstellation in thesouthern celestial hemisphere. In the second century, it was included inPtolemy's 48 constellations, and is counted among the88 modern constellations. Its name isLatin for "greater dog" in contrast toCanis Minor, the "lesser dog"; both figures are commonly represented as following the constellation ofOrion the hunter through the sky. TheMilky Way passes through Canis Major and severalopen clusters lie within its borders, most notablyM41.
Canis Major containsSirius, thebrightest star in the night sky, known as the "dog star". It is bright because of its proximity to theSolar System and itsintrinsic brightness. In contrast, the other bright stars of the constellation are stars of great distance and highluminosity. At magnitude 1.5,Epsilon Canis Majoris (Adhara) is the second-brightest star of theconstellation and the brightest source ofextreme ultraviolet radiation in the night sky. Next in brightness are the yellow-white supergiantDelta (Wezen) at 1.8, the blue-white giantBeta (Mirzam) at 2.0, blue-white supergiantsEta (Aludra) at 2.4 andOmicron2 at 3.0, and white spectroscopic binaryZeta (Furud), also at 3.0. The red hypergiantVY CMa is one of thelargest stars known, while theneutron starRX J0720.4-3125 has a radius of a mere 5 km.
In ancientMesopotamia, Sirius, named KAK.SI.SA2 by theBabylonians, was seen as an arrow aiming towards Orion, while the southern stars of Canis Major and a part ofPuppis were viewed as a bow, named BAN in theThree Stars Each tablets, dating to around 1100 BC. In the later compendium of Babylonian astronomy and astrology titledMUL.APIN, the arrow, Sirius, was also linked with the warriorNinurta, and the bow withIshtar, daughter ofEnlil.[2] Ninurta was linked to the later deityMarduk, who was said to have slain the ocean goddessTiamat with a great bow, and worshipped as the principal deity in Babylon.[3] The Ancient Greeks replaced the bow and arrow depiction with that of a dog.[4]
Sirius A, the brightest star in the night sky, lies in Canis Major.
In Greek Mythology, Canis Major represented the dogLaelaps, a gift fromZeus toEuropa; or sometimes the hound ofProcris,Diana's nymph; or the one given byAurora toCephalus, so famed for its speed that Zeus elevated it to the sky.[5] It was also considered to represent one of Orion's hunting dogs,[6] pursuingLepus theHare or helping Orion fightTaurus the Bull; and is referred to in this way byAratos,Homer andHesiod. Theancient Greeks refer only to one dog, but byRoman times,Canis Minor appears as Orion's second dog. Alternative names include Canis Sequens and Canis Alter.[5] Canis Syrius was the name used in the 1521Alfonsine tables.[5]
The Roman myth refers to Canis Major asCustos Europae, the dog guarding Europa but failing to prevent her abduction byJupiter in the form of a bull, and asJanitor Lethaeus, "the watchdog".[7] In medieval Arab astronomy, the constellation becameal-Kalb al-Akbar, "the Greater Dog", transcribed asAlcheleb Alachbar by 17th century writerEdmund Chilmead. Islamic scholarAbū Rayḥān al-Bīrūnī referred to Orion asKalb al-Jabbār, "the Dog of the Giant".[5] Among theMerazig ofTunisia, shepherds note six constellations that mark the passage of the dry, hot season. One of them, calledMerzem, includes the stars of Canis Major and Canis Minor and is the herald of two weeks of hot weather.[8]
Canis Major as depicted on theManuchihr Globe made inMashhad 1632–33 AD. Adilnor Collection, Sweden.
Canis Major, observed above Kuantan (north is towards top right)
InChinese astronomy, the modern constellation ofCanis Major is located in theVermilion Bird (南方朱雀;Nán Fāng Zhū Què), where the stars were classified in several separateasterisms of stars. The Military Market (軍市;Jūnshì) was a circular pattern of stars containingNu3,Beta,Xi1 andXi2, and some stars from Lepus.[9] The Wild Cockerel (野雞;Yějī) was at the centre of the Military Market, although it is uncertain which stars depicted what. Schlegel reported that the starsOmicron andPi Canis Majoris might have been them,[10] while Beta orNu2 have also been proposed.[11]Sirius wasTiānláng (天狼), the Celestial Wolf,[12] denoting invasion and plunder.[11] Southeast of the Wolf was the asterismHúshǐ (弧矢), the celestial Bow and Arrow, which was interpreted as containingDelta,Epsilon,Eta andKappa Canis Majoris andDelta Velorum. Alternatively, the arrow was depicted byOmicron2 and Eta and aiming at Sirius (the Wolf), while the bow comprised Kappa, Epsilon,Sigma, Delta and 164 Canis Majoris, andPi andOmicron Puppis.[13]
Both theMāori people and the people of theTuamotus recognized the figure of Canis Major as a distinct entity, though it was sometimes absorbed into other constellations.Te Huinga-o-Rehua, also calledTe Putahi-nui-o-Rehua andTe Kahui-Takurua, ("The Assembly ofRehua" or "The Assembly of Sirius") was a Māori constellation that included both Canis Minor and Canis Major, along with some surrounding stars.[14][15] Related wasTaumata-o-Rehua, also calledPukawanui, the Mirror ofRehua, formed from an undefined group of stars in Canis Major.[16][17] They called SiriusRehua andTakarua, corresponding to two of the names for the constellation, thoughRehua was a name applied to other stars in various Māori groups and other Polynesian cosmologies.[18][19] The Tuamotu people called Canis MajorMuihanga-hetika-o-Takurua, "the abiding assemblage ofTakarua".[20]
The Tharumba people of theShoalhaven River saw three stars of Canis Major asWunbula (Bat) and his two wivesMurrumbool (Mrs Brown Snake) andMoodtha (Mrs Black Snake); bored of following their husband around, the women try to bury him while he is hunting a wombat down its hole. He spears them and all three are placed in the sky as the constellationMunowra.[21] To theBoorong people of Victoria,Sigma Canis Majoris wasUnurgunite (which has become the official name of this star[22]), and its flanking stars Delta and Epsilon were his two wives.[23] The moon (Mityan, "native cat") sought to lure the further wife (Epsilon) away, butUnurgunite assaulted him and he has been wandering the sky ever since.[24]
Canis Major is a constellation in theSouthern Hemisphere's summer (or northern hemisphere's winter) sky, bordered byMonoceros (which lies between it and Canis Minor) to the north, Puppis to the east and southeast,Columba to the southwest, and Lepus to the west. The three-letter abbreviation for the constellation, as adopted by theInternational Astronomical Union in 1922, is "CMa".[25] The official constellation boundaries, as set by Belgian astronomerEugène Delporte in 1930, are defined by a quadrilateral; in theequatorial coordinate system, theright ascension coordinates of these borders lie between06h 12.5m and07h 27.5m, while thedeclination coordinates are between −11.03° and −33.25°.[1] Covering 380 square degrees or 0.921% of the sky, it ranks 43rd of the 88 currently-recognized constellations in size.[26]
Canis Major is a prominent constellation because of its many bright stars. These include Sirius (Alpha Canis Majoris), thebrightest star in the night sky, as well as three other stars above magnitude 2.0.[6] Furthermore, two other stars are thought to have previously outshone all others in the night sky—Adhara (Epsilon Canis Majoris) shone at −3.99 around 4.7 million years ago, and Mirzam (Beta Canis Majoris) peaked at −3.65 around 4.42 million years ago. Another,NR Canis Majoris, will be brightest at magnitude −0.88 in about 2.87 million years' time.[27]
The German cartographerJohann Bayer used the Greek lettersAlpha throughOmicron to label the most prominent stars in the constellation, including three adjacent stars as Nu and two further pairs as Xi and Omicron,[28] while subsequent observers designated further stars in the southern parts of the constellation that were hard to discern from Central Europe.[3] Bayer's countrymanJohann Elert Bode later added Sigma, Tau and Omega;[29] the French astronomerNicolas Louis de Lacaille added lettered stars a to k (though none are in use today).[29]John Flamsteed numbered 31 stars, with 3 Canis Majoris being placed by Lacaille into Columba asDelta Columbae (Flamsteed had not recognised Columba as a distinct constellation).[30] He also labelled two stars—his10 and13 Canis Majoris—as Kappa1 and Kappa2 respectively, but subsequent cartographers such asFrancis Baily andJohn Bevis dropped the fainter former star, leaving Kappa2 as the sole Kappa.[28] Flamsteed's listing of Nu1, Nu2, Nu3, Xi1, Xi2, Omicron1 and Omicron2 have all remained in use.[31]
Canis Major as depicted inUrania's Mirror, a set of constellation cards published in London c.1825. Next to it are Lepus and Columba (partly cut off).
Sirius is thebrightest star in the night sky atapparent magnitude −1.46 and one of theclosest stars to Earth at a distance of 8.6light-years. Its name comes from the Greek word for "scorching" or "searing". Sirius is also abinary star; its companionSirius B is awhite dwarf with a magnitude of 8.4–10,000 times fainter than Sirius A to observers on Earth.[32] The twoorbit each other every 50 years. Their closest approach last occurred in 1993 and they will be at their greatest separation between 2020 and 2025. Sirius was the basis for theancient Egyptian calendar.[6] The star marked the Great Dog's mouth on Bayer's star atlas.[33]
Flanking Sirius are Beta andGamma Canis Majoris. Also called Mirzam or Murzim, Beta is a blue-whiteBeta Cephei variable star of magnitude 2.0, which varies by a few hundredths of a magnitude over a period of six hours.[34] Mirzam is 500 light-years from Earth, and its traditional name means "the announcer", referring to its position as the "announcer" of Sirius, as it rises a few minutes before Sirius does.[6] Gamma, also known as Muliphein, is a fainter star of magnitude 4.12, in reality a blue-white bright giant ofspectral type B8IIe located 441 light-years from earth.[35]Iota Canis Majoris, lying between Sirius and Gamma, is another star that has been classified as a Beta Cephei variable, varying from magnitude 4.36 to 4.40 over a period of 1.92 hours.[36] It is a remote blue-whitesupergiant star of spectral type B3Ib, around 46,000 times as luminous as the sun and, at 2500 light-years distant, 300 times further away than Sirius.[37]
Epsilon,Omicron2, Delta, and Eta Canis Majoris were calledAl Adzari "the virgins" in medieval Arabic tradition.[38] Marking the dog's right thigh on Bayer's atlas is Epsilon Canis Majoris,[33] also known as Adhara. At magnitude 1.5, it is the second-brightest star in Canis Major and the 23rd-brightest star in the sky. It is a blue-white supergiant of spectral type B2Iab, around 404 light-years from Earth.[39] This star is one of the brightest known extreme ultraviolet sources in the sky.[40] It is a binary star; the secondary is of magnitude 7.4. Its traditional name means "the virgins", having been transferred from the group of stars to Epsilon alone.[41] Nearby is Delta Canis Majoris, also called Wezen. It is a yellow-white supergiant of spectral type F8Iab and magnitude 1.84, around 1605 light-years from Earth.[42] With a traditional name meaning "the weight", Wezen is 17 times as massive and 50,000 times as luminous as the Sun. If located in the centre of the Solar System, it would extend out to Earth as its diameter is 200 times that of the Sun. Only around 10 million years old, Wezen has stopped fusing hydrogen in its core. Its outer envelope is beginning to expand and cool, and in the next 100,000 years it will become a red supergiant as its core fuses heavier and heavier elements. Once it has a core of iron, it will collapse and explode as asupernova.[43] Nestled between Adhara and Wezen lies Sigma Canis Majoris, known as Unurgunite to the Boorong and Wotjobaluk people,[23] a red supergiant of spectral type K7Ib that varies irregularly between magnitudes 3.43 and 3.51.[44]
Also called Aludra, Eta Canis Majoris is a blue-white supergiant of spectral type B5Ia with a luminosity 176,000 times and diameter around 80 times that of the Sun.[45] Classified as anAlpha Cygni typevariable star, Aludra varies in brightness from magnitude 2.38 to 2.48 over a period of 4.7 days.[46] It is located 1120 light-years away. To the west of Adhara lies 3.0-magnitudeZeta Canis Majoris or Furud, around 362 light-years distant from Earth.[47] It is a spectroscopic binary, whose components orbit each other every 1.85 years, the combinedspectrum indicating a main star of spectral type B2.5V.[48]
Between these stars and Sirius lieOmicron1, Omicron2, andPi Canis Majoris. Omicron2 is a massive supergiant star about 21 times as massive as the Sun.[49] Only 7 million years old,[49] it has exhausted the supply of hydrogen at its core and is now processing helium.[50] It is an Alpha Cygni variable that undergoes periodic non-radial pulsations, which cause its brightness to cycle from magnitude 2.93 to 3.08 over a 24.44-day interval.[51] Omicron1 is an orangeK-type supergiant of spectral type K2.5Iab that is anirregular variable star, varying between apparent magnitudes 3.78 and 3.99.[52] Around 18 times as massive as the Sun, it shines with 65,000 times itsluminosity.[53]
North of Sirius lieTheta andMu Canis Majoris, Theta being the most northerly star with a Bayer designation in the constellation.[54] Around 8 billion years old, it is an orange giant of spectral type K4III that is around as massive as the Sun but has expanded to 30 times the Sun's diameter.[55] Mu is a multiple star system located around 1244 light-years distant,[56] its components discernible in a small telescope as a 5.3-magnitude yellow-hued and 7.1-magnitude bluish star.[57] The brighter star is a giant of spectral type K2III,[56] while the companion is a main sequence star of spectral type B9.5V.[58]Nu1 Canis Majoris is a yellow-hued giant star of magnitude 5.7, 278 light-years away; it is at the threshold of naked-eye visibility. It has a companion of magnitude 8.1.[6]
At the southern limits of the constellation lie Kappa andLambda Canis Majoris. Although of similar spectra and nearby each other as viewed from Earth, they are unrelated.[26] Kappa is aGamma Cassiopeiae variable of spectral type B2Vne,[59] which brightened by 50% between 1963 and 1978, from magnitude 3.96 or so to 3.52.[60] It is around 659 light-years distant.[61] Lambda is a blue-whiteB-typemain sequence dwarf with an apparent magnitude of 4.48 located around 423 light-years fromEarth.[62] It is 3.7 times as wide as and 5.5 times as massive as the Sun, and shines with 940 times its luminosity.[54]
Canis Major is also home to many variable stars.EZ Canis Majoris is aWolf–Rayet star of spectral type WN4 that varies between magnitudes 6.71 and 6.95 over a period of 3.766 days; the cause of its variability is unknown but thought to be related to its stellar wind and rotation.[63]VY Canis Majoris is a remotered hypergiant located approximately 3,800 light-years away fromEarth. It is one oflargest stars known (sometimes described as the largest known)[64] and is alsoone of the most luminous with a radius varying from 1,420 to 2,200 times the Sun's radius, and a luminosity around 300,000 times greater than the Sun. Its current mass is about 17 ± 8 solar masses, having shed material from an initial mass of 25–32 solar masses.[65][66] VY CMa is also surrounded by a redreflection nebula that has been made by the material expelled by the strong stellar winds of its central star.W Canis Majoris is a type of red giant known as acarbon star—a semiregular variable, it ranges between magnitudes 6.27 and 7.09 over a period of 160 days.[67] A cool star, it has a surface temperature of around 2,900 K and a radius 234 times that of the Sun, its distance estimated at 1,444–1,450 light-years from Earth.[68] At the other extreme in size isRX J0720.4-3125, aneutron star with a radius of around 5 km.[69] Exceedingly faint, it has an apparent magnitude of 26.6.[70] Its spectrum and temperature appear to be mysteriously changing over several years. The nature of the changes are unclear, but it is possible they were caused by an event such as the star's absorption of anaccretion disc.[69]
Tau Canis Majoris is aBeta Lyrae-type eclipsing multiple star system that varies from magnitude 4.32 to 4.37 over 1.28 days.[71] Its four main component stars are hot O-type stars, with a combined mass 80 times that of the Sun and shining with 500,000 times its luminosity, but little is known of their individual properties. A fifth component, a magnitude 10 star, lies at a distance of 13,000astronomical units (0.21 ly). The system is only 5 million years old.[72]UW Canis Majoris is another Beta Lyrae-type star 3000 light-years from Earth; it is aneclipsing binary that ranges in magnitude from a minimum of 5.3 to a maximum of 4.8. It has a period of 4.4 days;[6] its components are two massive hot blue stars, one ablue supergiant of spectral type O7.5–8 Iab, while its companion is a slightly cooler, less evolved and less luminous supergiant of spectral type O9.7Ib. The stars are 200,000 and 63,000 times as luminous as the Sun. However the fainter star is the more massive at 19solar masses to the primary's 16.[73]R Canis Majoris is another eclipsing binary that varies from magnitude 5.7 to 6.34 over 1.13 days,[74] with a third star orbiting these two every 93 years. The shortness of the orbital period and the low ratio between the two main components make this an unusual Algol-type system.[75]
Seven star systems have been found to have planets.Nu2 Canis Majoris is an ageing orange giant of spectral type K1III of apparent magnitude 3.91 located around 64 light-years distant.[76] Around 1.5 times as massive and 11 times as luminous as the Sun, it is orbited over a period of 763 days by a planet 2.6 times as massive as Jupiter.[77]HD 47536 is likewise an ageing orange giant found to have a planetary system—echoing the fate of the Solar System in a few billion years as the Sun ages and becomes a giant.[78] Conversely,HD 45364 is a star 107 light-years distant that is a little smaller and cooler than the Sun, of spectral type G8V, which has two planets discovered in 2008. With orbital periods of 228 and 342 days, the planets have a 3:2orbital resonance, which helps stabilise the system.[79]HD 47186 is another sunlike star with two planets; the inner—HD 47186 b—takes four days to complete an orbit and has been classified as aHot Neptune, while the outer—HD 47186 c—has an eccentric 3.7-year period orbit and has a similar mass toSaturn.[80]HD 43197 is a sunlike star around 183 light-years distant that has two planets: a hot Jupiter-size planet with an eccentric orbit. The other planet, HD 43197 c, is another massive Jovian planet with a slightly oblong orbit outside of its habitable zone.[81]
Z Canis Majoris is a star system a mere 300,000 years old composed of twopre-main-sequence stars—aFU Orionis star and aHerbig Ae/Be star,[82] which has brightened episodically by two magnitudes to magnitude 8 in 1987, 2000, 2004 and 2008.[83] The more massive Herbig Ae/Be star is enveloped in an irregular roughly spherical cocoon of dust that has an inner diameter of 20 AU (3.0×109 km) and outer diameter of 50 AU (7.5×109 km). The cocoon has a hole in it through which light shines that covers an angle of 5 to 10 degrees of its circumference. Both stars are surrounded by a large envelope of in-falling material left over from the original cloud that formed the system. Both stars are emitting jets of material, that of the Herbig Ae/Be star being much larger—11.7 light-years long.[84] Meanwhile,FS Canis Majoris is another star with infra-red emissions indicating a compact shell of dust, but it appears to be a main-sequence star that has absorbed material from a companion. These stars are thought to be significant contributors to interstellar dust.[85]
The band of theMilky Way goes through Canis Major, with only patchy obscurement byinterstellar dust clouds. It is bright in the northeastern corner of the constellation, as well as in a triangular area between Adhara, Wezen and Aludra, with many stars visible in binoculars. Canis Major boasts severalopen clusters.[86] The onlyMessier object isM41 (NGC 2287), an open cluster with a combinedvisual magnitude of 4.5, around 2300 light-years from Earth. Located 4 degrees south of Sirius, it contains contrasting blue, yellow and orange stars and covers an area the apparent size of thefull moon—in reality around 25 light-years in diameter.[87] Its most luminous stars have already evolved into giants. The brightest is a 6.3-magnitude star of spectral type K3. Located in the field is12 Canis Majoris, though this star is only 670 light-years distant.[88]NGC 2360, known as Caroline's Cluster after its discovererCaroline Herschel, is an open cluster located 3.5 degrees west of Muliphein and has a combined apparent magnitude of 7.2. Around 15 light-years in diameter, it is located 3700 light-years away from Earth,[89] and has been dated to around 2.2 billion years old.[90]NGC 2362 is a small, compact open cluster, 5200 light-years from Earth. It contains about 60 stars, of which Tau Canis Majoris is the brightest member.[6] Located around 3 degrees northeast of Wezen, it covers an area around 12 light-years in diameter, though the stars appear huddled around Tau when seen through binoculars. It is a very young open cluster as its member stars are only a few million years old. Lying 2 degrees southwest of NGC 2362 isNGC 2354 a fainter open cluster of magnitude 6.5, with around 15 member stars visible with binoculars.[87] Located around 30' northeast of NGC 2360,[91]NGC 2359 (Thor's Helmet or the Duck Nebula) is a relatively brightemission nebula in Canis Major, with an approximate magnitude of 10, which is 10,000 light-years from Earth. The nebula is shaped byHD 56925, an unstableWolf–Rayet star embedded within it.[92]
In 2003, an overdensity of stars in the region was announced to be theCanis Major Dwarf, the closestsatellite galaxy to Earth. However, there remains debate over whether it represents a disrupted dwarf galaxy or in fact a variation in the thin and thick disk and spiral arm populations of the Milky Way. Investigation of the area yielded only tenRR Lyrae variables—consistent with theMilky Way's halo and thick disk populations rather than a separate dwarf spheroidal galaxy.[94] On the other hand, aglobular cluster in Puppis,NGC 2298—which appears to be part of the Canis Major dwarf system—is extremely metal-poor, suggesting it did not arise from the Milky Way's thick disk, and instead is of extragalactic origin.[95]
NGC 2207 and IC 2163 are a pair of face-oninteracting spiral galaxies located 125 million light-years from Earth. About 40 million years ago, the two galaxies had a close encounter and are now moving farther apart; nevertheless, the smaller IC 2163 will eventually be incorporated into NGC 2207. As the interaction continues, gas and dust will be perturbed, sparking extensivestar formation in both galaxies.[96] Supernovae have been observed in NGC 2207 in 1975 (type IaSN 1975a),[97] 1999 (the type IbSN 1999ec),[98] 2003 (type 1b supernovaSN 2003H),[99] and 2013 (type II supernovaSN 2013ai).[100] Located 16 million light-years distant,[93]ESO 489-056 is an irregulardwarf- andlow-surface-brightness galaxy that has one of the lowestmetallicities known.[101]
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