Cassiopeia is a constellation in the northern sky, named after the vain queen Cassiopeia in Greek mythology, who boasted about her unrivaled beauty.
Key Facts & Summary
- Cassiopeia is among the 48 constellations first listed by the Greek astronomer Ptolemy, in the 2nd century CE.
- Cassiopeia is now among the 88 modern constellations and it is easily recognizable due to its distinctive W shape – formed by five of its brightest stars.
- Cassiopeia is the 25th largest constellation in the sky, stretching across for around 598 square degrees.
- Currently, there are around 14 known stars that host exoplanets in Cassiopeia.
- The star HR 8832 is believed to host seven planets.
- There are two Messier objects in Cassiopeia, Messier 52, and Messier 103 – both are open clusters.
- The brightest star in Cassiopeia is Schedar – Alpha Cassiopeiae – which has an apparent magnitude of 2.2, and it is 676 times brighter than our Sun.
- Cassiopeia is located in the northern sky and from latitudes above 34oN it is visible all year-round.
- Alpha Cassiopeiae is occasionally outshone by the variable star Gamma Cassiopeiae, which sometimes reaches magnitude 1.6.
- The constellation of Cassiopeia hosts some of the most luminous stars known, including the yellow hypergiants Rho Cassiopeiae, and V509 Cassiopeiae, and the white hypergiant 6 Cassiopeiae.
- A supernova remnant designated as Cassiopeia A – is the brightest extrasolar radio source in the sky at frequencies above 1 GHz.
- A rich section of the Milky Way runs through Cassiopeia – containing several open clusters, young luminous galactic disc stars, and nebulae.
- The irregular galaxy IC 10 is situated in Cassiopeia – it is the closest known starburst galaxy and the only one in the Local Group of galaxies
The constellation of Cassiopeia is named after the queen of Aethiopia. Cassiopeia was the wife of King Cepheus of Aethiopia and the mother of princess Andromeda.
Andromeda, Cepheus, and Cassiopeia were placed next to each other among the stars as punishment after enraging Poseidon, the ocean god after Cassiopeia boasted that her daughter, Andromeda, was more beautiful than the Nereids.
In an alternative story, Cassiopeia boasted that she herself has more beautiful than the sea nymphs. Poseidon forced her to wheel around the North Celestial Pole on her throne, spending half of her time clinging to it to avoid falling off.
Poseidon decreed that Andromeda be bound by a rock as prey for the sea-monster Cetus (a near constellation), however, she was rescued by the hero Perseus.
Cassiopeia has been depicted throughout history as a constellation. In ancient Persia, she was drawn by al-Shufi as a queen holding a staff with a crescent moon in her right hand, wearing a crown, and as a two-humped camel.
The constellation of Cassiopeia is the 25th largest constellation in the sky. Cassiopeia spreads out for over 598 square degrees. Cassiopeia lies in the first quadrant of the northern hemisphere, and it can easily be identified due to its W or M shape.
- Right Ascension: 22.57 hours
- Declination: 77.69 degrees
- Visible: between latitudes +90 degrees and -20 degrees
- Best viewed: During November, at 21:00 – 9 p.m.
The constellation of Cassiopeia belongs to the Perseus family of constellations, along with Andromeda, Auriga, Cepheus, Cetus, Lacerta, Pegasus, Perseus, and Triangulum.
In the constellation of Cassiopeia, there are only two Messier objects, they are the open clusters Messier 52, and Messier 103
Messier 52, also designated as NGC 7654, is an open cluster of stars in the northern constellation of Cassiopeia. It was discovered in 1774 by Charles Messier.
Messier 52 can be seen from Earth with binoculars, having an apparent magnitude of 5.0. The brightness of the cluster is influenced by extinction, which is stronger in the southern half.
This cluster is rich with central concentration and a medium-range in the brightness of the stars. Messier 52’s core radius is 2.97 light-years across, having a tidal radius of 42.7 light-years and a mass of 1,200 times that of the Sun.
Messier 52 is 158.5 million years old and it is around 4.6 light-years / 1.4 parsecs away from us. The stellar population of Messier 52 includes 18 candidate slowly pulsating B stars, one of which is a Delta Scuti variable star, three candidate Gamma Doradus variables, and possibly three Be stars
Messier 52 core shows a lack of interstellar matter, which may be the result of supernovae explosions in the cluster’s early history.
Messier 103, also designated as NGC581, is an open cluster where a few thousands of stars formed in the constellation of Cassiopeia. The cluster was discovered in 1781 by Charles Messier’s friend and collaborator Pierre Mechain.
Messier 103 is among the most distant known open clusters, with distances of 8,000 to 10,000 light-years from the Earth, and ranging about 15 light-years apart.
Messier 103 has an apparent magnitude of 7.4, it can be seen even with binoculars. Around 172 stars are members of Messier 103, two of which have magnitudes 10.5, and a 10.8 red giant, which is the brightest within the cluster. Messier 103 is around 25 million years old.
The five brightest stars in the constellation of Cassiopeia are Alpha – Schedar, Beta – Caph, Gamma – Navi, Delta – Ruchbah or Rukbat, and Epsilon – Segin Cassiopeiae.
These five bright stars form the famous W-shaped asterism. All five stars are visible to the naked eye, three are noticeably variable, and a fourth is a suspected low amplitude variable.
This asterism is shaped like a W-when it is below Polaris during the northern spring and summer nights. In northern winter, and when seen from southern latitudes, the asterism appears above Polaris and the W-shape appears inverted, as an M.
Alpha Cassiopeiae, also known as Schedar, is a four-star system with the primary star being an orange-hued giant of magnitude 2.2, situated at 228 light-years away from Earth.
Schedar is around 771 times brighter than our Sun, having around 400% of our Sun’s mass, 4200% its radius, and temperatures of around 4,530 K – it is cooler than our Sun. Schedar is occasionally outshone by the variable star Gamma Cassiopeiae, which sometimes reaches magnitude 1.6.
Beta Cassiopeiae, also known as Caph, is a white-hued star of magnitude 2.3. Caph is situated at around 54 light-years away from the Sun.
Caph is a Delta Scuti variable star, and since it is rotating at around 92% of its critical speed, the star has an oblate spheroid shape with an equatorial bulge that is 24% larger than its polar radius.
Caph is 1.9 times more massive than our Sun, and it has around 343% of our Sun’s radius. Caph is 21.3 times more luminous than our Sun. Caph has surface average temperatures of around 7,079 K, much hotter than our Sun.
Gamma Cassiopeiae, also known as Navi, is the prototype Gamma Cassiopeiae variable star, a type of star that has a variable disc of material flung off by the high rotation rate of the star.
Navi occasionally outshines the brightest star in Cassiopeia – Alpha Cassiopeiae, reaching a magnitude of 1.6, due to its variability in brightness.
Navi has a minimum magnitude of 3.0, a maximum of 1.6, while generally, it stays near magnitude 2.2. Navi is a Be star, being around 17 times more massive than our Sun, having 1000% of its radius, while being 34,000 times brighter.
Navi is situated at around 550 light-years away from us. The temperatures on Navi reach 25,000 K, around 4.3 times hotter than our Sun.
Delta Cassiopeiae, also known as Ruchbah or Rukbat, is a possible Algol-type eclipsing binary star with a maximum magnitude of 2.7. Ruchbah is 99.4 light-years away from Earth.
Ruchbah is a subgiant star on its way into becoming a giant star. It has 249% of our Sun’s mass, 390% its radius, and it is 77 times brighter.
Ruchbah is also hotter than our Sun, having average surface temperatures of around 7,980 K.
Epsilon Cassiopeiae, also known as Segin, is a hot blue-white star with an apparent magnitude of 3.3. It is located at around 410 light-years away from us.
Segin is also a Be star – rapidly spinning stars that throw off a ring or shell of matter around them. Segin is 9 times more massive than our Sun, having 6 times its radius.
It is also considerably hotter than our Sun, having average surface temperatures of around 15,680 K. Segin is also 2,500 times brighter than our Sun.
Fainter Notable Stars
Though they are not the brightest, either due to distance or their light being emitted in the invisible part of the spectrum, the following stars deserve some recognition as well.
Eta Cassiopeiae, also known as Achird, is a binary star system located at around 19 light-years away from Earth. The primary star, Achird, is a G-type main-sequence star, similar to our Sun.
It has an apparent magnitude of 3.44. Achird is slightly lighter than our Sun, having 90% of its mass, but wider, having a radius of 103% that of our Sun’s.
Achird is hotter than our Sun, having temperatures of around 5,973 K, and it is radiating around 129% of our Sun’s luminosity. Achird is also a suspected Canum Venaticorum variable star.
Rho Cassiopeiae is a yellow hypergiant star situated at around 3,400 light-years away from us, yet still visible to the naked eye. Rho Cassiopeiae is 500,000 times brighter than our Sun.
As a yellow hypergiant, it is among the rarest types of stars, only around a few dozen have been discovered in the Milky Way. However, the constellation of Cassiopeia also hosts another yellow hypergiant, V509 Cassiopeiae.
Rho Cassiopeiae has between 400 to 500 times the diameter of our Sun. The star would be around twice the size of our planet’s orbit. The apparent magnitude of Rho Cassiopeiae is 4.1, and it is also categorized as a semiregular variable star. This star is cooler than our Sun, yet it is 40 times heavier.
Kappa Cassiopeiae is a blue supergiant situated at around 4,000 light-years away from us. It is 302,000 times brighter than our Sun, having 3300% its mass, and 3300% its radius.
Kappa Cassiopeiae is a runaway star, moving at around 2.5 million mph / 1,100 km per second, relative to its neighbors. Its magnetic field and wind of particles create a visible bow shock 4 light-years ahead of it, that collides with the diffuse, and usually invisible, interstellar gas and dust.
The dimensions of Kappa Cassiopeiae’s bow shock are vast – around 12 light-years long and 1.8 light-years wide.
V509 Cassiopeiae is an extremely rare yellow hypergiant star, which is around 400,000 times brighter than our Sun, and 14 times as massive.
V509 Cassiopeiae is around 4,500 light-years away from us, and it has an apparent magnitude that has varied from below +6 in historical times to a peak of +4.6, and now around +5.3.
It is visible to the naked eye, having a radius between 400 to 900 times that of our Sun. V509 Cassiopeiae is undergoing strong mass loss as part of its rapid evolution.
PZ Cassiopeiae is a red supergiant semi-regular variable star. It is among the largest stars currently known. PZ Cassiopeiae has 1,062 times our Sun’s radius, and it is also 229,00 times brighter.
PZ Cassiopeiae is located at around 2,810 light-years away from us. The star’s apparent magnitude is 8.9, and it varies from 8.2 to 10.2.
Theta Cassiopeiae, also known as Marfak, is an A-type main-sequence star with an apparent magnitude of 4.3. Marfak is located at around 134 light-years away from us.
This star is 1.8 times more massive than our Sun, having 2.6 times its radius, and it is also considerably hotter, having average surface temperatures of around 8,202 K.
6 Cassiopeiae is a hot white hypergiant star, around 25 times more massive than our Sun, and 200,000 times more luminous. It is an Alpha Cygni variable star, having an apparent magnitude of +5.43.
6 Cassiopeiae is situated at around 8,000 light-years away from us. Its average surface temperatures have been recorded at around 10,023 K.
Other Deep Sky Objects
The constellation of Cassiopeia hosts many interesting deep-sky objects. This is because a rich part of the Milky Way runs through this constellation, stretching from Perseus towards Cygnus.
It contains several open clusters, young luminous galactic disc stars, nebulae, galaxies, and supernova remnants. Apart from the aforementioned Messier 52 and 103 open clusters, the other prominent clusters are NGC 457 and NGC 663.
Both clusters have around 80 discovered stars. NGC 457 is looser, and its brightest star, Phi Cassiopeiae, is a white-hued supergiant star of magnitude 5.0.
The stars of NGC 457 are around 10,000 light-years away from us, while NGC 663 is closer, at 8,200 light-years, and larger, at 0.25 degrees in diameter.
The Heart Nebula, also designated as IC 1805, is situated at around 7,500 light-years away from Earth. It is located in the Perseus Arm of our galaxy.
It was discovered in 1787 by William Herschel. The Heart Nebula is an emission nebula, showing glowing ionized hydrogen gas and darker dust lanes.
The Heart Nebula has an apparent magnitude of 18.3, and an absolute magnitude of 6.5. It has a radius of around 165 light-years.
The Soul Nebula, also designated as Westerhout 5, is an emission nebula located at around 7,500 light-years away from us. The Soul Nebula has an absolute magnitude of 6.5.
The Pacman Nebula, designated as NGC 281 – is an H II region, a large gas cloud in which star formation has recently taken place. This nebula contains large amounts of ionized atomic hydrogen which is lit by the ultraviolet light of young, hot, blue stars.
It was nicknamed the Pacman Nebula due to its resemblance to the character from the popular video game.
The constellation of Cassiopeia hosts two supernova remnants. They are:
3C 10, also known as Tycho’s Supernova Remnant, is the aftermath of the supernova called Tycho’s star. It was a supernova of Type la, and it is one of the eight supernovae visible to the naked eye in historical records.
Its first appearance was noted in November 1572, being independently discovered by many individuals. The remnant of the supernova has been observed ever since, but it was first detected at radio wavelengths.
This supernova remnant is the brightest extrasolar radio source in the sky at frequencies above 1 GHz. The supernova occurred around 11,000 light-years away within the Milky Way.
The expanding cloud of material left over now appears to be around 10 light-years across from Earth’s perspective. In wavelengths of visible light, it has been seen with amateur telescopes down to 234 mm / 9.25 in – with filters.
It is estimated that the light from this giant explosion first reached us approximately 300 years ago, however, there are no historical records of this event.
There are three members of the Local Group – a group of galaxies of which our Milky Way is also part of – in Cassiopeia. They are the elliptical galaxies NGC 185, NGC 147, and the IC 10 starburst galaxy.
NGC 185 is 2 million light-years away from us, having a magnitude of 9.2. NGC 147 is slightly dimmer, at magnitude 9.3, and it is even farther away, at 2.3 million light-years.
Both dwarf galaxies are gravitationally bound to the Andromeda Galaxy. Another interesting galaxy in Cassiopeia is the irregular galaxy IC 10.
IC 10 is the closest starburst galaxy to us, at around 2.2 million light-years, and the only starburst galaxy in the Local Group of galaxies. Cassiopeia also contains part of the closest galaxy group to our Local Group, the IC 342/ Maffei Group.
These galaxies, Maffei 1 and Maffei 2 are located south of the Heart and Soul Nebula, being within 10 million light-years away from us. Maffei 2 is below the range of most amateur telescopes.
The December Phi Cassiopeiids is a recently discovered early-December meteor shower that radiates from the constellation of Cassiopeia.
Phi Cassiopeiids are quite slow, with an entry velocity of around 16.7 km / 10.3 mi per second. The parent body of this meteor shower is a Jupiter family comet, though its specific identity is currently unknown.
Did you know?
- In France, queen Cassiopeia 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 of Cassiopeia are found among three areas: the Purple Forbidden enclosure, the Black Tortoise of the North, and the White Tiger of the West.
- Chinese astronomers saw several figures in Cassiopeia. Kappa, Eta, and Mu Cassiopeiae formed a constellation called the Bridge of the Kings.
- In Welsh Mythology, Cassiopeia was Llys Don – The Court of Don.
- Several cultures see a hand or moose antlers instead of the W-pattern. The Lapps people see the W as an elk antler. The Chuckchi saw the five main stars as reindeer stags.
- The people of the Marshall Islands saw Cassiopeia as part of a great tortoise constellation. The five main stars mark its tail, Andromeda and Triangulum form its body, and Aries marks its head