Visible at latitudes between +90° and −12°. Best visible at 21:00 (9 p.m.) during the month ofNovember.
Cassiopeia (listenⓘ) is aconstellation andasterism in the northern sky named after the vain queenCassiopeia, mother ofAndromeda, inGreek mythology, who boasted about her unrivaled beauty. Cassiopeia was one of the 48 constellations listed by the 2nd-century Greek astronomerPtolemy, and it remains one of the 88 modern constellations today. It is easily recognizable due to its distinctive 'W' shape, formed by five bright stars.
Cassiopeia is located in thenorthern sky and from latitudes above34°N it is visible year-round. In the (sub)tropics it can be seen at its clearest from September to early November, and at low southern, tropical, latitudes of less than25°S it can be seen, seasonally, low in the North.
At magnitude 2.2,Alpha Cassiopeiae, or Schedar, is the brightest star in Cassiopeia. The constellation hosts some of the most luminous stars known, including the yellow hypergiantsRho Cassiopeiae andV509 Cassiopeiae and white hypergiant6 Cassiopeiae. In 1572,Tycho Brahe's supernova flared brightly in Cassiopeia.[4]Cassiopeia A is asupernova remnant and the brightest extrasolarradio source in the sky at frequencies above 1 GHz. Fourteen star systems have been found to haveexoplanets, one of which – HD 219134 – is thought to host six planets. A rich section of theMilky Way runs through Cassiopeia, containing a number ofopen clusters, young luminous galactic disc stars, andnebulae.IC 10 is an irregular galaxy that is the closest knownstarburst galaxy and the only one in the Local Group of galaxies.
The constellation is named after Cassiopeia, the queen ofAethiopia. Cassiopeia was the wife ofKing Cepheus of Aethiopia[4] and mother of PrincessAndromeda. Cepheus and Cassiopeia were placed next to each other among the stars, along with Andromeda. She was placed in the sky as a punishment after enragingPoseidon with the boast that her daughter Andromeda was more beautiful than theNereids or, alternatively, that she herself was more beautiful than the sea nymphs.[5] She was forced to wheel around thenorth celestial pole on her throne, spending half of her time clinging to it so she does not fall off, and Poseidon decreed that Andromeda should be bound to a rock as prey for the monsterCetus. Andromeda was then rescued by thehero Perseus, whom she later married.[6][7]
Cassiopeia has been variously portrayed throughout her history as a constellation. InPersia, she was drawn byal-Sufi as a queen holding a staff with acrescent moon in her right hand, wearing a crown, as well as a two-humped camel. In France, she was portrayed as having a marble throne and a palm leaf in her left hand, holding her robe in her right hand. This depiction is fromAugustin Royer's 1679 atlas.[6]
The Chinese astronomers saw several figures in what is modern-day Cassiopeia. Kappa, Eta, and Mu Cassiopeiae formed a constellation called the Bridge of the Kings; when seen along with Alpha and Beta Cassiopeiae, they formed the great chariotWang-Liang. The charioteer's whip was represented by Gamma Cassiopeiae, sometimes called "Tsih", the Chinese word for "whip".[6]
InWelsh MythologyLlys Dôn (literally "The Court of Dôn") is the traditionalWelsh name for the constellation. At least three of Dôn's children also have astronomical associations: Caer Gwydion ("The fortress ofGwydion") is the traditional Welsh name for theMilky Way, and Caer Arianrhod ("The Fortress ofArianrhod") being the constellation ofCorona Borealis.[8]
A figure called the "Tinted Hand" also appeared in the stars of Cassiopeia in some Arabic atlases. This is variously said to represent a woman's hand dyed red withhenna, as well as the bloodied hand ofMuhammad's daughterFatima. The hand is made up of the starsα Cas,β Cas,γ Cas,δ Cas,ε Cas, andη Cas. The arm is made up of the starsα Per,γ Per,δ Per,ε Per,η Per, andν Per.[6]
Another Arabic constellation that incorporated the stars of Cassiopeia was the Camel. Its head was composed of Lambda, Kappa, Iota, and Phi Andromedae; its hump was Beta Cassiopeiae; its body was the rest of Cassiopeia, and the legs were composed of stars in Perseus and Andromeda.[6]
Other cultures see a hand or moose antlers in the pattern.[9] These include theSámi, for whom the W of Cassiopeia forms an elk antler. TheChukchi ofSiberia similarly saw the five main stars as five reindeer stags.[6]
The people of theMarshall Islands saw Cassiopeia as part of a great porpoise constellation. The main stars of Cassiopeia make its tail, Andromeda andTriangulum form its body, andAries makes its head.[6] InHawaii, Alpha, Beta, and Gamma Cassiopeiae were named. Alpha Cassiopeiae was calledPoloahilani, Beta Cassiopeiae was calledPolula, and Gamma Cassiopeiae was calledMulehu. The people ofPukapuka saw the figure of Cassiopeia as a distinct constellation calledNa Taki-tolu-a-Mataliki.[10]
Covering 598.4 square degrees and hence 1.451% of the sky, Cassiopeia ranks25th of the 88 constellations in area.[11] It is bordered by Cepheus to the north and west, Andromeda to the south and west, Perseus to the southeast and Camelopardalis to the east, and also shares a short border with Lacerta to the west.
The three-letter abbreviation for the constellation, as adopted by theInternational Astronomical Union in 1922, is "Cas".[12] The official constellation boundaries, as set by Belgian astronomerEugène Delporte in 1930,[b] are defined by a polygon of 30 segments. In theequatorial coordinate system, theright ascension coordinates of these borders lie between00h 27m 03s and23h 41m 06s, while thedeclination coordinates are between 77.69° and 46.68°.[3] Its position in the Northern Celestial Hemisphere means that the whole constellation is visible to observers north of 12°S.[11][c] High in the northern sky, it is circumpolar (that is, it never sets in the night sky) to viewers in the British Isles, Canada and the northern United States.[14]
The constellation Cassiopeia as it can be seen by the naked eye from a northern location
The German cartographerJohann Bayer used the Greek lettersAlpha throughOmega, and then A and B, to label the most prominent 26 stars in the constellation.Upsilon was later found to be two stars and labelled Upsilon1 and Upsilon2 byJohn Flamsteed. B Cassiopeiae was in fact the supernova known asTycho's Supernova.[15] Within the constellation's borders, there are 157 stars brighter than or equal toapparent magnitude 6.5.[d][11]
The five brightest stars of Cassiopeia – Alpha, Beta, Gamma, Delta, and Epsilon Cassiopeiae – form the characteristic W-shapedasterism.[14] All five are prominent naked eye stars, three are noticeablyvariable, and a fourth is a suspected low amplitude variable. The asterism is oriented as a W when belowPolaris during northern spring and summer nights. In northern winter, and when seen from southern latitudes, it is "above" Polaris (i.e. closer to thezenith) and the W appears inverted.
Alpha Cassiopeiae, traditionally called Schedar (from the ArabicAl Sadr, "the breast"), is commonly mistaken as afour-star system, but is actually a single star with three physically distantoptical components. The primary dominates, an orange-huedgiant of magnitude 2.2,228±2 light-years from Earth.[17] With aluminosity of around 771 times that of the Sun, it has swollen and cooled after exhausting its core hydrogen over its 100 to 200 million-year lifespan, spending much of it as a blue-whiteB-type main-sequence star.[18] Magnitude 8.9yellow dwarf companion (B) is widely separated; companions (C and D) are closer and magnitudes 13 and 14 respectively.[19]
Beta Cassiopeiae, or Caph (meaning "hand"), is a white-hued star of magnitude 2.3,54.7±0.3 light-years from Earth.[17] Around 1.2 billion years old, it has used up its core hydrogen and begun expanding and cooling off the main sequence. It is around 1.9 times as massive as the Sun, and around 21.3 times as luminous. Rotating at about 92% of itscritical speed, Caph completes a full rotation every 1.12 days. This is giving the star anoblate spheroid shape with an equatorial bulge that is 24% larger than the polar radius.[20] It is aDelta Scuti variable with a small amplitude and period of 2.5 hours.[21]
Gamma Cassiopeiae now known as Tiansi is the prototypeGamma Cassiopeiae variable star, a type of variable star that has a variable disc of material flung off by the high rotation rate of the star. Gamma Cassiopeiae has a minimum magnitude of 3.0 and a maximum magnitude of 1.6, but is generally near magnitude 2.2, with unpredictable fades and brightenings. It is a spectroscopic binary, with an orbital period of 203.59 days and a companion with a calculated mass about the same as the Sun. However, no direct evidence of this companion has been found, leading to speculation that it might be a white dwarf or other degenerate star.[22] It is550±10 light-years from Earth.
Delta Cassiopeiae, also known as Ruchbah or Rukbat, meaning "knee," is a possibleAlgol-typeeclipsing binary star with a maximum brightness of magnitude 2.7. It has been reported to show eclipses of less than 0.1 magnitudes with a period of 2 years and 1 month.,[23] but this has never been confirmed. It is99.4±0.4 light-years from Earth.[17] Delta Cassiopeiae was the star used byJean Picard in 1669 to establish the length on the Earth's surface corresponding to one degree, and hence theradius of the Earth.[24]
Epsilon Cassiopeiae also known as Segin has an apparent magnitude of 3.3. Located410±20 light-years from Earth,[17] it is a hot blue-white star of spectral type B3 III with a surface temperature of15,680 K. It is 6.5 times as massive and 4.2 times as wide as the Sun, and belongs to a class of stars known asBe stars—rapidly spinning stars that throw off aring or shell of matter.[25]
Kappa Cassiopeiae and its bow shock.Spitzer infrared image (NASA/JPL-Caltech)
The next seven brightest stars in Cassiopeia are also all confirmed or suspected variable stars, including50 Cassiopeiae which was not given a Greek letter by Bayer and is a suspected variable with a very small amplitude.Zeta Cassiopeiae (Fulu[26]) is a suspectedslowly pulsating B-type star.Kappa Cassiopeiae (Cexing) is ablue supergiant of spectral type BC0.7Ia that is some 302,000 times as luminous as the Sun and has 33 times its diameter.[27] It is arunaway star, moving at around 2.5 million mph relative to its neighbors (1,100 kilometers per second).[28] Its magnetic field and wind of particles creates a visiblebow shock 4 light-years ahead of it, colliding with the diffuse, and usually invisible, interstellar gas and dust. The dimensions of the bow shock are vast: around 12 light-years long and 1.8 light-years wide.[29]Theta Cassiopeiae, named Marfak, is a suspected variable star whose brightness changes by less than a tenth of a magnitude.Iota Cassiopeiae is atriple star 142 light-years from Earth. The primary is a white-hued star of magnitude 4.5 and anα2 Canum Venaticorum variable, the secondary is a yellow-hued star of magnitude 6.9, and the tertiary is a star of magnitude 8.4. The primary and secondary are close together but the primary and tertiary are widely separated.Omicron Cassiopeiae is a triple star and the primary is another γ Cassiopeiae variable.
Eta Cassiopeiae (Achird) is the nearest star in Cassiopeia, located some 19.13 ly away.[30] It is abinary star made up of aG-dwarf star similar to the Sun, of apparent magnitude 3.45, and aK-dwarf star of apparent magnitude 7.51.[31] It is visually located between Alpha and Gamma Cassiopeiae and are one of thenearest G-type /K-type stars to Earth.
Sigma Cassiopeiae is a binary star 1500 light-years from Earth. It has a green-hued primary of magnitude 5.0 and a blue-hued secondary of magnitude 7.3.Psi Cassiopeiae is a triple star 193 light-years from Earth. The primary is an orange-hued giant star of magnitude 4.7 and the secondary is a close pair of stars that appears to be of magnitude 9.0.[23]
Rho Cassiopeiae is asemi-regular pulsating variableyellow hypergiant, and is among the most luminous stars in the galaxy at approximately 500,000 L☉.[32] It has a minimum magnitude of 6.2 and a maximum magnitude of 4.1; its period is approximately 320 days. It has around 450 times the Sun's diameter and 17 times its mass, having begun life 45 times as massive as the Sun. Rho Cassiopeiae is about 10,000 light-years from Earth. Cassiopeia includesV509 Cassiopeiae, a second example of the extremely rare yellow hypergiants, which is around 400,000 times as luminous as the Sun and 14 times as massive,[32] as well as6 Cassiopeiae which is a hotter whitehypergiant. It also hosts the red supergiantPZ Cassiopeiae, which is one of thelargest known stars with an estimate of 1,190–1,940 R☉ and is also a semiregular variable.[33] Between 240,000 and 270,000 times as luminous as the Sun, it is around 9,160 light-years distant from Earth.[34]
AO Cassiopeiae is abinary system composed of an O8 main sequence star and an O9.2 bright giant that respectively weigh anywhere between 20.30 and 57.75 times and 14.8 and 31.73 times the mass of theSun.[35] The two massive stars are so close to each other they distort each other into egg-shapes.[36]
Tycho Brahe's supernova was visible within Cassiopeia, and the starTycho G is suspected of being the donor of the material that triggered that explosion.
Planetary nebulaIC 289 is a cloud of ionised gas being pushed out into space by the remnants of the star's core.
A rich section of theMilky Way runs through Cassiopeia, stretching fromPerseus towardsCygnus, and it contains a number ofopen clusters, young luminous galactic disc stars, andnebulae.
TwoMessier objects,M52 (NGC 7654) andM103 (NGC 581), are located in Cassiopeia; both are open clusters. M52, once described as a "kidney-shaped" cluster, contains approximately 100 stars and is4,600 light-years from Earth.[37] Its most prominent member is an orange-hued star of magnitude 8.0 near the cluster's edge. M103 is far poorer than M52, with only about 25 stars included. It is also more distant, between 8000 and 9500 light-years from Earth.[38] Its most prominent member is actually a closer, superimposed double star; it consists of a 7th-magnitude primary and 10th-magnitude secondary.[23]
Four clusters in Cassiopeia. IC 166, NGC 654,NGC 663, andNGC 659
The other prominent open clusters in Cassiopeia areNGC 457 and NGC 663, both of which have about 80 stars. NGC 457 is looser, and its brightest member isPhi Cassiopeiae, a white-hued supergiant star of magnitude 5.0. However, it is uncertain whether Phi Cassiopeiae is part of the open cluster or not.[39] The stars of NGC 457, arrayed in chains, are approximately 10,000 light-years from Earth. NGC 663 is both closer, at 8200 light-years from Earth, and larger, at 0.25 degrees in diameter.[23]
There are twosupernova remnants in Cassiopeia. The first, designated3C 10 or justTycho's Supernova Remnant, is the aftermath of the supernova calledTycho's Star. It was observed in 1572 byTycho Brahe and now exists as a bright object in theradio spectrum.[23] Within the 'W' asterism formed by Cassiopeia's five major stars liesCassiopeia A (Cas A). It is the remnant of asupernova that took place approximately 300 years ago (as observed now from Earth; it is 10,000 light-years away),[40] and has the distinction of being the strongest radio source observable outside theSolar System. It was perhaps observed as a faint star in 1680 byJohn Flamsteed. It was also the subject of the first image returned by theChandra X-Ray Observatory in the late 1990s. The shell of matter expelled from the star is moving at 4,000 kilometres (2,500 mi) per second; it has a temperature of30,000 K on average.[40]
NGC 457 is another open cluster in Cassiopeia, also called theE.T. Cluster, the Owl Cluster, andCaldwell 13. The cluster was discovered in 1787 byWilliam Herschel. It has an overall magnitude of 6.4 and is approximately 10,000 light-years from Earth, lying in thePerseus Arm of theMilky Way. However, its most prominent member, thedouble starPhi Cassiopeiae, is far closer – between 1000 and 4000 light-years away. NGC 457 is fairly rich; it is aShapley class e andTrumpler class I 3 r cluster. It is concentrated towards its center and detached from the star field. It contains more than 100 stars, which vary widely in brightness.[41]
Two members of theLocal Group of galaxies are in Cassiopeia.NGC 185 is a magnitude 9.2elliptical galaxy of type E0, 2 million light-years away. Slightly dimmer and more distantNGC 147 is a magnitude 9.3 elliptical galaxy, like NGC 185 it is an elliptical of type E0; it is 2.3 million light-years from Earth. Though they do not appear inAndromeda, bothdwarf galaxies are gravitationally bound to the far largerAndromeda Galaxy.[42]
IC 10 is an irregular galaxy that is the closest knownstarburst galaxy and the only one in the Local Group of galaxies.[43]
Cassiopeia also contains part of the closest galaxy group to our Local Group, theIC 342/Maffei Group. The galaxiesMaffei 1 andMaffei 2 are located just to the south of the Heart and Soul nebulae. As a result of this location in theZone of Avoidance, both are surprisingly faint despite both being within 10 million light-years away (Maffei 2 is below the range of most amateur telescopes).[44]
TheDecember Phi Cassiopeiids are a recently discovered early Decembermeteor shower thatradiates from Cassiopeia. Phi Cassiopeiids are very slow, with an entry velocity of approximately 16.7 kilometers per second. The shower's parent body isBiela's Comet, although the identity of the parent body was unknown for a number of years.[45]
InPokémon Scarlet and Violet, the villainous team, Team Star, is divided into five squads named after the brightest stars in the constellation:Segin Squad,Schedar Squad,Ruchbah Squad,Navi Squad, andCaph Squad. The group's leader uses the alias Cassiopeia.
Cassiopeia is also the name of a song by London-based bandBears in Trees. Although the lyrics of the song mainly refer to the ancient Greek woman, the album cover shows the constellation.[46]
Cassiopeia is the name of a champion inLeague of Legends. Her beauty and vanity mirror the character in Greek mythology.
Casiopea is the name of a Japanese Jazz-Fusion group formed in 1976. The name was chosen by the guitarist's mother, solely so the band members could remember the name.
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^Delporte had proposed standardising the constellation boundaries to the International Astronomical Union, who had agreed and gave him the lead role[13]
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^Objects of magnitude 6.5 are among the faintest visible to the unaided eye in suburban-rural transition night skies.[16]
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