Constellations: Overview | Andromeda | Antlia | Apus | Aquarius | Aquila | Ara | Aries | Auriga | Boötes | Caelum
Camelopardalis | Cancer | Canes Venatici | Canis Major | Canis Minor | Capricornus | Carina | Cassiopeia | Centaurus
Cepheus | Cetus | Chamæleon | Circinus | Columba | Coma Berenices | Corona Australis | Corona Borealis | Corvus
Crater | Crux | Cygnus | Delphinus | Dorado | Draco | Equuleus | Eridanus | Fornax | Gemini | Grus | Hercules | Horologium
Hydra | Hydrus | Indus | Lacerta | Leo | Leo Minor | Lepus | Libra | Lupus | Lynx | Lyra | Mensa | Microscopium | Monoceros
Musca | Norma | Octans | Ophiuchus | Orion | Pavo | Pegasus | Perseus | Phoenix | Pictor | Pisces | Piscis Austrinus
Puppis | Pyxis | Reticulum | Sagitta | Sagittarius | Scorpius | Sculptor | Scutum | Serpens | Sextans | Taurus
Telescopium | Triangulum | Triangulum Australe | Tucana | Ursa Major | Ursa Minor | Vela | Virgo | Volans | Vulpecula

Shine forth a constellation, full and bright,
Bless the poor heavens with more majestic light,
Who in recital shall present you there:
Ariadne's crown and Cassiopeia's chair.
- The Lucasta Poems (1640) by Richard Lovelace (1618 - 57)

Cassiopeia features in the 48 constellations originally listed by the Greek astronomer Ptolemy (c90 - 168AD), and is one of the 88 modern constellations - although in Lapp (Sámi) mythology², Cassiopeia isn't a separate constellation but a part of a larger one. Their 'Sarva the Moose' also encompasses Perseus and Auriga.

Cassiopeia is a northern hemisphere constellation. Situated between Perseus and Cepheus, it is easy to find and point out to casual observers: when the main stars are joined up they are shaped like a letter 'W'. Locate Ursa Major (the Great Bear AKA the Plough); the 'pointers' of the Plough point to the North Star, and Cassiopeia is on the other side of it. Our galaxy the Milky Way goes through Cassiopeia.

Objects of interest include:

• Two Messier objects: M52 and M103.
• Two supernova remnants.
• Two yellow hypergiant stars.
• A Wolf-Rayet star which has created a large ring nebula.
• Two planetary nebulae.
• A pulsar.

Trivia

When viewed from the nearest star system, Alpha Centauri, our Sun would be part of the constellation of Cassiopeia, extending the W into a zig-zag.

The Legend of Cassiopeia

In Greek mythology Cassiopeia was a queen of Æthiopia, the wife of King Cepheus. She boasted that their daughter, Andromeda, was more beautiful than the Nereids (sea-nymphs), which angered the sea god Poseidon. His punishment was to bombard the kingdom with storms and forbid the Nereids from rescuing sailors in the Red Sea. According to an Oracle, the only way to appease Poseidon was to sacrifice their daughter to Cetus, a sea monster. The naked Andromeda was duly chained to a rock at low tide³ to await her fate. Perseus, who was riding Pegasus, rescued the fair maiden, slew the monster and married Andromeda. Their descendants founded the Persian race, and they were the ancestors of Hercules.

When they died, all of the players were placed in the heavens as constellations, but the vain Queen Cassiopeia, whose bragging brought such misery to her people, had not been forgiven. Still seated upon her throne, she was positioned so near the pole star that for half the year she hangs upside down in order to learn humility.

Stars

The scientific star names are simple to understand (if you know your Greek alphabet). 'Alpha Cas' means that it is the brightest star in the constellation Cassiopeia. The next brightest is 'beta Cas', etc. This is known as the 'Bayer designation'. Some are named stars, like alpha Cas is Shedar; others are known by their catalogue number.

Rho Cassiopeiae, one of the brightest yellow hypergiants, is the best candidate to undergo a supernova explosion in the near future, according to astronomers using the Utrecht Echelle Spectrograph on the William Herschel Telescope. This star is 550,000 times brighter than our Sun, and although it is over 8,000 light-years distant, it can be seen without optical aids.

Gamma Cassiopeiae is a prototype variable star.

Pulsar J0002 is moving at over 1,100 km/s, fast enough to leave our galaxy. It left the supernova remnant CTB 1, which was created by an exploding star some 10,000 years ago. The pulsar was discovered in 2017 through Citizen Science, using a programme called Einstein@Home.

New General Catalogue Objects

The constellation Cassiopeia has some interesting deep-sky objects, two of which feature in the famous Messier catalogue.

Nebulae

• IC 1795 is commonly known as The Fishhead Nebula, at 70 light years' width it's highly likely this is the biggest fish anyone has ever seen!

• Sh 2-176 registers 11.35 luminosity and its brightest star is 18.1 magnitude.

• IC 1747 has an irregularly-shaped inner disk with some ring structure but unusually, no known outer shell. Measuring 8.5 surface brightness, the brightest star is 15.4 magnitude.

The Bubble Nebula

NGC 7635 is known as the Bubble Nebula, for a rather obvious reason, it looks just like a bubble! A rapidly-expanding cosmic bubble of ionized gas, created by the stellar wind of the O 6.5 III giant BD+602522, fuels this nebula.

Sh 2-157

There is a Wolf-Rayet star in this area of the sky called WR 157, and it has created a large ring nebula, Sh 2-157. Wolf-Rayet (WR) stars are rare blue giants named after their discoverers Charles Wolf and Georges Rayet. The stars are massive, over 20 times the size of our Sun. The stellar wind they produce has been measured at speeds of several million kilometres per hour. They have a high rate of mass loss, equivalent to an Earth mass per year. This shortens the star's life and will eventually cause them to go supernova.

Supernova

A supernova is the death of a massive star, which collapses under the weight of its own gravity after it has used up all its fuel. The star blows its outer layers into space in an explosion that can be seen over thousands of light-years, outshining everything surrounding it. One that was recorded meticulously in 1054 by Chinese astronomers happened in the constellation Taurus the Bull and eventually created the beautiful Crab Nebula that we see today.

Supernovae in Cassiopeia

There have been two supernovae in Cassiopeia in recent history, the first was in 1572, and it lasted for between six months to around a year. The astronomer Tycho Brahe recorded scientific observations and published them in De Stella Nova. At that time in history, the celestial sphere was believed to have been created all at once (well, within six days) by God as divinely perfect and unchanging, so to exploit a notion that it was any other than Ptolemaic⁶ where the Sun, (our) Moon, other planets (in our solar system) and stars (no new ones, please) orbit a fixed Earth (geocentric), was tantamount to sacrilege and blasphemy. That was not a period in time when it was wise to point out gaping holes in the fabric of peoples' belief-systems, especially one as strong as religion, and if one were brave enough, refusal to recant meant enduring a fate worse than death. In the same year that Fr Giordano Bruno was burned at the stake for such a crime, 1600, William Shakespeare mentioned the supernova when he wrote about a 'bright star' - and its position in the sky - in the opening scene of Hamlet⁷.

Cassiopeia A exploded as a supernova around 340 years ago, one of the youngest remnants that we know of. Around 1665 AD from our vantage point that is, because the light took about 10,000 years to reach us. Cassiopeia A was the first object that the Chandra X-ray observatory ever located and studied. This star in Cassiopeia that blew itself apart shone down in its old age upon people such as William Shakespeare, who, if he'd known it was in its death-throes, would no doubt have written a sonnet about it. As it is, no-one recorded the supernova, not even Sir Isaac Newton (1643 - 1727) who was a young adult around the time it appeared. Around that time in the mid-1600s there was a lot of nasty stuff going on, particularly in London. The old mnemonic:

In 1665 hardly any of London was left alive;
In 1666 London burned like a bundle of sticks.

gives some idea. Certainly in medieval times and the middle ages comets were thought to be 'harbingers of momentous events', how much worse would a supernova have seemed? It's just possible that if any astrologers or soothsayers noticed the exploding star they could have made some dire predictions and been spot-on!

Extrasolar Planets

Extrasolar planets orbit other stars outside our own Solar System. Over four hundred have been discovered up to 2009. Cassiopeia is home to the following:

• HD 7924 b is a 'super-Earth' (rocky planet) which orbits its orange dwarf star in just under 130 hours. Since its discovery, two more planets have been detected in the HD 7924 system.
• HD 17156 b is a hot gas giant in an eccentric orbit around its yellow giant star. In 2008 a second planet was found to be in orbit around this star. HD 17156 c has about the same mass as our planet Neptune but is much closer to the parent star, completing a year in just over 111 Earth days.
• HD 240210 b is a superjovian planet orbiting an orange giant star in around 500 days.
• HR 8832 is an orange dwarf star purported to host seven planets in orbit.
• HD 2952 b is a superjovian world orbiting its star at 1.2 AU, possibly the system's habitable zone.
• HD 220074 b is a superjovian world some 11× the mass of Jupiter.
• HD 11755 b is a superjovian world orbiting a sun-like star.
• HD 221585 b is a superjovian world orbiting a sun-like star within its habitable zone.
• HD 13908 is a sun-like star hosting a gas giant HD 13908 b and a superjovian world, HD 13908 c which is in the habitable zone.
• HD 220842 b is one of a batch of twelve planets identified by the SOPHIE search for northern extrasolar planets.
• HD 240237 b was detected by the Penn State-Torun Planet Search Team (PTPS).
• WASP-93 b is a superjovian world in a tidally-locked orbit around its sun-like star.
• HD 219415 b is a Jupiter-mass world residing within the habitable zone of its star.
• HAT-P-44 is a yellow dwarf star which has two planets in orbit, both were discovered by the HATNet transit survey.

Cassiopeia in Modern Culture

• In the Carl Sagan book and film Contact, the heroine Ellie Arroway (played by Jodie Foster) points out the supernova remnant in Cassiopeia to Palmer Joss, the President's spiritual advisor (played by Matthew McConaughey).

• There is a gemstone called Cassiopeia Topaz, which is a soft blue colour that flashes from aquamarine through to lavender shades when it catches the light.

• 'Cassiopeia' is the title of a song from the 2006 album Astronomy by Swedish metal band Dragonland.

• Savers of old cigarette cards would do well to hang on to one featuring Cassiopeia and the Pole Star, as it's quite rare.

• The 1960 science-fiction book The Cassiopeia Affair by Chloë Zerwick and Harrison Brown explores the consequences of radio contact with intelligent extra-terrestrial beings.

Cassiopeian Creature

A family of upside-down  jellyfish has been given the scientific name Cassiopeidae. They are a favourite meal of the red-listed leatherback and green sea turtles, but aren't themselves listed as endangered. Most creatures that aren't interested in eating the jellyfish avoid them because getting too close can result in a nasty sting. However, an ingenious hermit crab, Dorippe frascone, has developed a habit of kidnapping Cassiopeia jellyfish and carrying them upon their backs. The effect upon the crab's predators is the same - they give the crab-jellyfish tandem a wide berth.