Introduction
Cassiopeia is a large and easily recognizable constellation in the northern sky. Its name comes from a vain and boastful queen in Greek mythology. Known as the W constellation, it is easily spotted because its five brightest stars form a W shape. This constellation was first cataloged by the Greek astronomer Ptolemy in the 2nd century, along with other constellations in the Perseus family (except for Lacerta).
Cassiopeia contains several interesting celestial objects, such as the open clusters Messier 52 and Messier 103, the Heart and Soul Nebulae, the supernova remnant Cassiopeia A, the star-forming cloud known as the Pacman Nebula, and the White Rose Cluster.
Facts, Location, and Map
Cassiopeia is the 25th largest constellation in the night sky, covering an area of 598 square degrees. It lies in the first quadrant of the northern hemisphere (NQ1) and is visible at latitudes between +90° and -20°. The neighboring constellations are Andromeda, Camelopardalis, Cepheus, Lacerta, and Perseus.
The constellation’s name is pronounced /ˌkæsioʊˈpiːə/. In English, it is known as Cassiopeia or the Seated Queen. The genitive form, used in star names, is Cassiopeiae (/ˌkæsioʊˈpiːiː/). The three-letter abbreviation, adopted by the International Astronomical Union (IAU) in 1922, is Cas.
Belonging to the Perseus family of constellations, it is grouped with Andromeda, Auriga, Cepheus, Cetus, Lacerta, Pegasus, Perseus, and Triangulum.
There are three stars with known planets within this constellation, and it contains two Messier objects: M103 (NGC 581) and M52 (NGC 7654). The brightest star in Cassiopeia is Schedar, also known as Alpha Cassiopeiae.
Eight stars in this constellation have proper names officially approved by the IAU. These stars are Achird, Caph, Castula, Fulu, Nushagak, Ruchbah, Schedar, and Segin.
Additionally, the Perseid meteor shower is associated with Cassiopeia.
Mythology
In Greek mythology, Cassiopeia was the wife of King Cepheus of Ethiopia, represented by the neighboring constellation Cepheus in the sky. Cassiopeia was very vain and boastful, and one day she claimed to be more beautiful than the Nereids, the 50 sea nymphs fathered by the Titan Nereus. These nymphs were furious at Cassiopeia’s comments and appealed to Poseidon, the god of the sea, to punish her.
Poseidon, who was married to one of the nymphs, Amphitrite, decided to send Cetus, a sea monster represented by the constellation Cetus (the Whale), to ravage the coast of Cepheus’ kingdom. Desperate, Cepheus consulted an oracle who told him that the only way to appease Poseidon was to sacrifice their daughter Andromeda to the monster. Reluctantly, they chained Andromeda to a rock for the monster to devour.
Just in the nick of time, the Greek hero Perseus happened to be passing by, saw Andromeda, and rescued her from the monster. After saving her, Perseus and Andromeda got married. However, during the wedding, one of her former suitors named Phineus showed up, claiming his right to marry Andromeda.
A fight broke out, and finding himself outnumbered, Perseus used the head of Medusa, which he had captured on one of his adventures, to turn Phineus and his followers to stone. Unfortunately, King Cepheus and Queen Cassiopeia also met their end because they didn’t look away from Medusa’s head in time.
Poseidon then placed Cassiopeia and Cepheus in the sky. Cassiopeia was condemned to circle the celestial pole forever, spending half the year upside down as punishment for her vanity. In the sky, she is usually depicted sitting on her throne, still combing her hair, perpetuating her image of vanity.
Major Stars in Cassiopeia
Cassiopeia is famous for its distinctive W shape, an asterism formed by five bright stars. Let’s learn a bit more about these and other important stars in this constellation.
Schedar – α Cassiopeiae (Alpha Cassiopeiae)
Schedar, sometimes called Shedir, is an orange giant of spectral type K0IIIa, located about 228 light-years away. It is a suspected variable star, meaning its brightness can vary slightly. Its apparent magnitude ranges from 2.20 to 2.23. Schedar marks the heart of Cassiopeia. The name “Schedar” comes from the Arabic şadr, which means “breast.”
Caph – β Cassiopeiae (Beta Cassiopeiae)
Caph is a subgiant or giant star of spectral type F2 III-IV, approximately 54.5 light-years away. It is a Delta Scuti type variable star, meaning its luminosity fluctuates due to pulsations on its surface. Caph has an apparent magnitude of 2.27. The name “Caph” is derived from the Arabic kaf, meaning “palm.” Caph is one of the Three Guides, three bright stars marking the equinoctial colure.
γ Cassiopeiae (Gamma Cassiopeiae)
Gamma Cassiopeiae is the central star in the W shape and currently the brightest star in the constellation. It is a blue star (spectral type B0.5 IVe), about 610 light-years away. Gamma Cassiopeiae is an eruptive variable star, serving as the prototype of its class, with brightness variations from 2.20 to 3.40 magnitudes. It spins very rapidly, creating a disk of lost mass and material around it, causing luminosity fluctuations. The Chinese call this star Tsih, “the whip,” and it also has the nickname Navi, given by astronaut Virgil Ivan Grissom.
Ruchbah – δ Cassiopeiae (Delta Cassiopeiae)
Ruchbah is an eclipsing binary star of spectral class A5, about 99 light-years away. Its brightness varies between magnitudes 2.68 and 2.74, making it the fourth brightest star in Cassiopeia. The name “Ruchbah” comes from the Arabic rukbah, meaning “the knee.” Sometimes, it is also known as Ksora.
Segin – ε Cassiopeiae (Epsilon Cassiopeiae)
Segin is a bright blue-white B-class giant, approximately 440 light-years away. It is 2,500 times more luminous than the Sun and has an apparent magnitude of 3.34. This star is notable for showing unusually weak spectral absorptions of helium.
Achird – η Cassiopeiae (Eta Cassiopeiae)
Achird is the nearest star in Cassiopeia to our solar system, lying only 19.4 light-years away. It is similar to the Sun; a yellow-white G-class hydrogen-fusing dwarf, slightly cooler than the Sun with a surface temperature of 5730 Kelvin. It has an apparent magnitude of 3.45. Achird has a dimmer companion, an orange K-class dwarf, located 11 arc seconds away. Together they form a close binary star system known as an RS Canum Venaticorum type variable star, with variations in brightness due to large stellar spots.
ζ Cassiopeiae (Zeta Cassiopeiae)
Zeta Cassiopeiae is a blue-white subgiant (spectral type B2IV), approximately 600 light-years away. It has an apparent magnitude of 3.67 and is classified as a Slow Pulsating B (SPB) variable star, the first of its kind discovered to have a magnetic field. The star’s rotation speed is 56 km/s, and its rotation period is 5.37 days.
ρ Cassiopeiae (Rho Cassiopeiae)
Rho Cassiopeiae is one of the rare yellow hypergiant stars in the Milky Way, located about 11,650 light-years away. It is 550,000 times more luminous than the Sun, with an apparent magnitude that varies from 4.1 to 6.2. Rho Cassiopeiae undergoes enormous outbursts roughly every 50 years, resulting in significant changes in luminosity. In 2000-2001, the star ejected about 10,000 Earth masses in one such outburst.
V509 Cassiopeiae
V509 Cassiopeiae is another G-type hypergiant, at least 7,800 light-years away. The yellow-white star is classified as a semi-regular variable, with its luminosity varying between magnitudes 4.75 and 5.5.
Deep Sky Objects in Cassiopeia
Cassiopeia is home to several fascinating deep sky objects, including open clusters, nebulae, and galaxies. Let’s explore some of the most notable ones.
Messier 52 (NGC 7654)
Messier 52 is an open cluster approximately 5,000 light-years away. With an apparent magnitude of 5.0, it can be seen with binoculars. The cluster is about 35 million years old and has a diameter of 13 arcminutes (19 light-years). It was discovered by Charles Messier in 1774. The brightest stars in Messier 52 are two yellow giants of magnitude 7.77 and 8.22.
Messier 103 (NGC 581)
Messier 103 is another open cluster in Cassiopeia, lying about 10,000 light-years from Earth. It contains 172 stars and is estimated to be around 25 million years old. With an apparent magnitude of 7.4, it can be seen with small telescopes. The cluster was discovered by the French astronomer Pierre Méchain in 1781 and was the last object added by Charles Messier to his catalog.
Bubble Nebula – NGC 7635
The Bubble Nebula (NGC 7635) is a large emission nebula near the border with Cepheus. It lies at an estimated distance of 7,100 to 11,000 light-years and has an apparent magnitude of 10. It appears in the same area of the sky as Messier 52. The nebula occupies an area of 15 by 8 arcminutes and can be seen in 8- or 10-inch telescopes as a large, faint shell around a central star. The central star, catalogued as SAO 20575, is a hot, massive, blue O-type star with a surface temperature of about 37,500 K, believed to be around 2 million years old.
Cassiopeia A
Cassiopeia A is a supernova remnant and is the brightest astronomical radio source in the sky outside the solar system. The supernova occurred approximately 11,000 light-years from Earth, and its light is believed to have reached us about 300 years ago. The cloud of material ejected in the supernova event is about 10 light-years across and is expanding at a rate of 4,000-6,000 km/s. It has a temperature of about 50 million degrees Fahrenheit.
Pacman Nebula – NGC 281
NGC 281, known as the Pacman Nebula, is an H II region, a large gas cloud where star formation has recently taken place. It contains a large amount of ionized atomic hydrogen (H II) illuminated by the ultraviolet light of young, hot, blue stars. The nebula is located about 9,500 light-years from Earth and was discovered by the American astronomer E.E. Barnard in 1883. It is nicknamed the Pacman Nebula because of its resemblance to the character from the popular video game.
White Rose Cluster (NGC 7789)
NGC 7789, also known as the White Rose Cluster or Caroline’s Rose, is an open star cluster about 7,600 light-years away. It has an apparent magnitude of 6.7 and was discovered by the British astronomer Caroline Herschel in 1783. The cluster is named for the loops of stars that resemble rose petals.
NGC 185 (Caldwell 18)
NGC 185 is a dwarf spheroidal galaxy approximately 2.08 million light-years from Earth. It is a satellite galaxy of the Andromeda Galaxy (Messier 31) and forms a physical pair with NGC 147. NGC 185 is classified as a type 2 Seyfert galaxy with an active galactic nucleus (AGN). It was discovered by the English astronomer John Herschel in 1829. Unlike typical dwarf elliptical galaxies, NGC 185 contains young star clusters and shows evidence of recent star formation.
NGC 147 (Caldwell 17)
NGC 147 is another dwarf spheroidal galaxy in Cassiopeia, located about 2.53 million light-years away. Like NGC 185, it is a satellite galaxy of the Andromeda Galaxy (Messier 31) and a member of the Local Group of galaxies. NGC 147 has an apparent magnitude of 10.5 and was discovered by John Herschel in 1829. It contains young star clusters, indicating recent star formation activities.
These deep sky objects make Cassiopeia a rich and exciting constellation to explore, offering a variety of astronomical phenomena for both amateur and professional astronomers alike.
