Cassiopeia () is a constellation and asterism in the northern sky named after the vain queen Cassiopeia, mother of Andromeda, in Greek mythology, who boasted about her unrivaled beauty. Cassiopeia was one of the 48 constellations listed by the 2nd-century Greek astronomer Ptolemy, 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 the northern sky and from latitudes above 34°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 than 25°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 hypergiants Rho Cassiopeiae and V509 Cassiopeiae and white hypergiant 6 Cassiopeiae. In 1572, Tycho Brahe's supernova flared brightly in Cassiopeia. Cassiopeia A is a supernova remnant and the brightest extrasolar radio source in the sky at frequencies above 1 GHz. Fourteen star systems have been found to have exoplanets, one of which – HD 219134 – is thought to host six planets. A rich section of the Milky Way runs through Cassiopeia, containing a number of open clusters, young luminous galactic disc stars, and nebulae. IC 10 is an irregular galaxy that is the closest known starburst galaxy and the only one in the Local Group of galaxies.
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Mythology
The constellation is named after Cassiopeia, the queen of Aethiopia. Cassiopeia was the wife of King Cepheus of Aethiopia and mother of Princess Andromeda. 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 enraging Poseidon with the boast that her daughter Andromeda was more beautiful than the Nereids or, alternatively, that she herself was more beautiful than the sea nymphs. She was forced to wheel around the north 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 monster Cetus. Andromeda was then rescued by the hero Perseus, whom she later married.
Cassiopeia has been variously portrayed throughout her history as a constellation. In Persia, she was drawn by al-Sufi as a queen holding a staff with a crescent 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 from Augustin Royer's 1679 atlas.
In Chinese astronomy, the stars forming the constellation Cassiopeia are found among three areas: the Purple Forbidden enclosure (紫微垣, Zǐ Wēi Yuán), the Black Tortoise of the North (北方玄武, Běi Fāng Xuán Wǔ), and the White Tiger of the West (西方白虎, Xī Fāng Bái Hǔ).
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 chariot Wang-Liang. The charioteer's whip was represented by Gamma Cassiopeiae, sometimes called "Tsih", the Chinese word for "whip".
In Hindu Mythology, Cassiopeia was associated with the mythological figure Sharmishtha (Sanskrit: शर्मिष्ठा)– the daughter of the great Devil (Daitya) King Vrishparva and a friend to Devayani (Sanskrit: देवयानी) (Andromeda).
In Welsh Mythology Llys Dôn (literally "The Court of Dôn") is the traditional Welsh name for the constellation. At least three of Dôn's children also have astronomical associations: Caer Gwydion ("The fortress of Gwydion") is the traditional Welsh name for the Milky Way, and Caer Arianrhod ("The Fortress of Arianrhod") being the constellation of Corona Borealis.
Characteristics
Cassiopeia had a supernova, Cassiopeia A, SN 1572.
Covering 598.4 square degrees and hence 1.451% of the sky, Cassiopeia ranks 25th of the 88 constellations in area. 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 the International Astronomical Union in 1922, is "Cas". The official constellation boundaries, as set by Belgian astronomer Eugène Delporte in 1930, are defined by a polygon of 30 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 00h 27m 03s and 23h 41m 06s, while the declination coordinates are between 77.69° and 46.68°. Its position in the Northern Celestial Hemisphere means that the whole constellation is visible to observers north of 12°S. 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.
Features
Stars
The German cartographer Johann Bayer used the Greek letters Alpha through Omega, 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 by John Flamsteed. B Cassiopeiae was in fact the supernova known as Tycho's Supernova. Within the constellation's borders, there are 157 stars brighter than or equal to apparent magnitude 6.5.
The five brightest stars of Cassiopeia – Alpha, Beta, Gamma, Delta, and Epsilon Cassiopeiae – form the characteristic W-shaped asterism. All five are prominent naked eye stars, three are noticeably variable, and a fourth is a suspected low amplitude variable. The asterism is oriented as a W when below Polaris during northern spring and summer nights. In northern winter, and when seen from southern latitudes, it is "above" Polaris (i.e. closer to the zenith) and the W appears inverted.
Alpha Cassiopeiae, traditionally called Schedar (from the Arabic Al Sadr, "the breast"), is commonly mistaken as a four-star system, but is actually a single star with three physically distant optical components. The primary dominates, an orange-hued giant of magnitude 2.2, 228±2 light-years from Earth. With a luminosity 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-white B-type main-sequence star. Magnitude 8.9 yellow dwarf companion (B) is widely separated; companions (C and D) are closer and magnitudes 13 and 14 respectively.
Beta Cassiopeiae, or Caph (meaning "hand"), is a white-hued star of magnitude 2.3, 54.7±0.3 light-years from Earth. 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 its critical speed, Caph completes a full rotation every 1.12 days. This is giving the star an oblate spheroid shape with an equatorial bulge that is 24% larger than the polar radius. It is a Delta Scuti variable with a small amplitude and period of 2.5 hours.
Gamma Cassiopeiae now known as Tiansi is the prototype Gamma 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. It is 550±10 light-years from Earth.
Deep-sky objects
A rich section of the Milky Way runs through Cassiopeia, stretching from Perseus towards Cygnus, and it contains a number of open clusters, young luminous galactic disc stars, and nebulae.
The Heart Nebula and the Soul Nebula are two neighboring emission nebulae about 7,500 light-years away.
Two Messier objects, M52 (NGC 7654) and M103 (NGC 581), are located in Cassiopeia; both are open clusters. M52, once described as a "kidney-shaped" cluster, contains approximately 100 stars and is 4,600 light-years from Earth. 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. Its most prominent member is actually a closer, superimposed double star; it consists of a 7th-magnitude primary and 10th-magnitude secondary.
The other prominent open clusters in Cassiopeia are NGC 457 and NGC 663, both of which have about 80 stars. NGC 457 is looser, and its brightest member is Phi 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. 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.
There are two supernova remnants in Cassiopeia. The first, designated 3C 10 or just Tycho's Supernova Remnant, is the aftermath of the supernova called Tycho's Star. It was observed in 1572 by Tycho Brahe and now exists as a bright object in the radio spectrum. Within the 'W' asterism formed by Cassiopeia's five major stars lies Cassiopeia A (Cas A). It is the remnant of a supernova that took place approximately 300 years ago (as observed now from Earth; it is 10,000 light-years away), and has the distinction of being the strongest radio source observable outside the Solar System. It was perhaps observed as a faint star in 1680 by John Flamsteed. It was also the subject of the first image returned by the Chandra 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 of 30,000,000 K on average.
Meteor showers
The December Phi Cassiopeiids are a recently discovered early December meteor shower that radiates from Cassiopeia. Phi Cassiopeiids are very slow, with an entry velocity of approximately 16.7 kilometers per second. The shower's parent body is Biela's Comet, although the identity of the parent body was unknown for a number of years.