Cassiopeia
“The Queen” · Northern · best around November evenings
Real star positions and magnitudes (HYG database via D3-Celestial, BSD-3). Lines are the conventional stick figure; north is up.
The story
The unmistakable W (or M) of five bright stars — a vain mythical queen chained to her throne, circling the pole forever, sometimes hanging upside down as punishment for boasting that she outshone the sea nymphs. Cassiopeia sits opposite the Big Dipper across Polaris, so one of the two is always well placed. The Milky Way runs straight through it, and in 1572 Tycho Brahe watched a supernova blaze here brighter than Venus — the 'new star' that cracked the doctrine of unchanging heavens.
How to find it
Look north: whichever of the Big Dipper or the W is higher, the other is lower on the opposite side of Polaris.
✦ What lives inside it
- Tycho's supernova remnant
- Rich Milky Way star fields
- The Heart & Soul Nebulae
The deep dive
Researched for the Atlas from Wikipedia — Cassiopeia (constellation) (18,234 characters read) · updated Sep 20, 2026
01 A queen punished to spin forever
The Greek myth behind Cassiopeia is less a tale of heroism than of cosmic comeuppance. Queen Cassiopeia of Aethiopia boasted that either she herself, or her daughter Andromeda, surpassed the Nereids — the fifty sea-nymph daughters of the god Nereus — in beauty. Poseidon, enraged on the nymphs' behalf, placed Cassiopeia in the sky as punishment, chained to a throne that revolves endlessly around the north celestial pole. Because the pole never dips below the horizon in northern latitudes, the throne completes a full circuit every day, meaning Cassiopeia spends roughly half her time upside down, clinging to her seat so she does not fall off. The punishment did not stop there: Poseidon also decreed that Andromeda be bound to a rock as prey for the sea monster Cetus — a fate averted only when the hero Perseus arrived and slew the creature. Perseus and Andromeda later married, and all the principals — Cepheus, Cassiopeia, Andromeda, Perseus, and Cetus — were placed among the constellations, arranged across the sky as a permanent tableau of the episode.
02 How the world's cultures read the W
Greek myth is only one reading of the five stars that form the W. In Chinese astronomy they were divided among three separate sky regions — the Purple Forbidden Enclosure, the Black Tortoise of the North, and the White Tiger of the West — and several distinct figures were traced within them. Kappa, Eta, and Mu Cassiopeiae together formed the Bridge of the Kings; joined with Alpha and Beta, they became the great chariot Wang-Liang; and Gamma Cassiopeiae alone represented the charioteer's whip, earning it the Chinese name Tsih, meaning exactly that. In Arabic sky-lore, the same stars were read as the "Tinted Hand" — variously a woman's palm dyed red with henna or the bloodied hand of Fatima, daughter of Muhammad — made up of Alpha through Epsilon and Eta Cassiopeiae. Separately, Arabic astronomers also traced a camel whose hump fell on Beta Cassiopeiae and whose body stretched across the rest of the constellation. The Sámi of northern Europe saw elk antlers; the Chukchi of Siberia counted five reindeer stags; and the people of the Marshall Islands treated Cassiopeia's stars as the tail of a great porpoise constellation whose body was Andromeda and Triangulum and whose head was Aries.
03 Welsh, Hindu, and Biblical identities
Beyond Greece and the Arabic world, Cassiopeia collected still more identities across centuries and continents. In Welsh tradition, the constellation is known as Llys Dôn — "The Court of Dôn" — named for a mother goddess whose children include figures associated with the Milky Way and the constellation Corona Borealis. Caer Gwydion, the Welsh name for the Milky Way, and Caer Arianrhod, the Welsh name for Corona Borealis, belong to two of Dôn's children, placing Cassiopeia at the center of a Welsh family of sky figures. In Hindu mythology, the pattern was associated with Sharmishtha, daughter of the Daitya king Vrishparva and companion to Devayani, who corresponds to Andromeda. During the 17th century, European Christian tradition layered yet more figures onto the stars: Cassiopeia was at various times identified with Bathsheba, mother of Solomon; Deborah, the Old Testament prophet; and Mary Magdalene. The Hawaiian people named three of its stars individually — Alpha as Poloahilani, Beta as Polula, and Gamma as Mulehu — while the people of Pukapuka recognized a distinct constellation they called Na Taki-tolu-a-Mataliki. This diversity of readings, spanning at minimum a dozen cultures, makes Cassiopeia one of the most multiply interpreted patterns in the entire sky.
04 The extreme stars hiding in the W Deeper
Beneath the familiar W lie some of the most extreme individual stars known to astronomers. Rho Cassiopeiae is a yellow hypergiant shining at roughly 500,000 times the Sun's luminosity, with a diameter about 450 times the Sun's — a sphere so large that if placed at the center of the Solar System it would engulf the orbit of Mars. It has only about 17 solar masses today, but is thought to have begun its life with around 45 solar masses, meaning it has shed most of its bulk through powerful mass-loss events. Nearby V509 Cassiopeiae is a second yellow hypergiant — an extraordinarily rare stellar type — radiating around 400,000 solar luminosities with 14 times the Sun's mass, while 6 Cassiopeiae is a hotter white hypergiant. The red supergiant PZ Cassiopeiae carries an estimated diameter of 1,190 to 1,940 solar radii, placing it among the largest known stars, and it too is a semiregular variable shining between 240,000 and 270,000 times as brightly as the Sun from about 9,160 light-years away. Kappa Cassiopeiae is a runaway blue supergiant, 302,000 times as luminous as the Sun, tearing through space at about 2.5 million miles per hour (1,100 kilometers per second) and carving a bow shock 12 light-years long and 1.8 light-years wide ahead of it in the interstellar medium.
05 Tycho's supernova and what it proved
In November 1572, a new star blazed into visibility within Cassiopeia, eventually outshining Venus. The Danish astronomer Tycho Brahe observed it meticulously, and his records helped establish the startling conclusion that the heavens — believed since Aristotle to be unchanging and perfect — could in fact change dramatically. The star, now called Tycho's Supernova or SN 1572, has left behind a remnant designated 3C 10 or Tycho's Supernova Remnant, visible today as a bright source in the radio spectrum. Within the constellation's W asterism lies a second, far younger-looking remnant: Cassiopeia A, which originated in a supernova that occurred approximately 300 years ago as measured from when its light reached Earth. Cassiopeia A sits about 10,000 light-years away, and the shell of matter it ejected is still expanding at 4,000 kilometres per second with an average temperature of 30,000,000 K. It is the strongest radio source visible from Earth beyond the Solar System at frequencies above 1 GHz — a distinction that made it one of the first cosmic radio sources ever identified. It was also the subject of the very first image returned by the Chandra X-Ray Observatory in the late 1990s. A star designated Tycho G is suspected to have been the donor star whose transferred material triggered the original 1572 explosion.
06 Caph: the fastest-spinning bright star Deeper
Beta Cassiopeiae, known as Caph — Arabic for "hand" — is one of the more physically remarkable stars in the W despite often being overlooked in favor of its neighbors. At only 54.7 light-years from Earth it is the closest of the five W stars, making it also one of the more carefully studied. At approximately 1.2 billion years old, it has exhausted its core hydrogen and begun expanding off the main sequence, now carrying about 1.9 solar masses and shining at 21.3 solar luminosities. What sets Caph apart is its rotation: it spins at roughly 92 percent of its critical speed — the velocity at which centrifugal force would begin to tear the star apart — completing a full rotation every 1.12 days. This extreme spin has distorted Caph into an oblate spheroid whose equatorial radius is 24 percent larger than its polar radius, giving it a noticeably squashed shape. On top of that, Caph is a Delta Scuti variable, a class of pulsating star, with a pulsation period of just 2.5 hours and a small amplitude. Its variability makes it a useful calibration object for studies of stellar oscillation, and its shape makes it a benchmark for models of rapidly rotating stars in the late stages of hydrogen burning.
07 Gamma Cassiopeiae and its mysterious companion Deeper
Gamma Cassiopeiae, now formally named Tiansi, is the prototype of an entire class of variable stars that bear its name. It spins fast enough to fling material off its equator, building a disc around itself that expands and contracts unpredictably, causing its brightness to swing between magnitude 3.0 and 1.6 — a range large enough to be obvious to the naked eye — with no regular schedule. It generally hovers near magnitude 2.2 but can fade or brighten at any time. Located 550 light-years from Earth, it is also a spectroscopic binary with an orbital period of 203.59 days, but the companion has never been directly detected. Its calculated mass is roughly equal to the Sun's, yet it produces no spectral signature that can be straightforwardly explained by a normal star of that mass. This absence has fueled a genuine astronomical debate: the companion may be a white dwarf, a neutron star, or another type of compact degenerate object — none of which has been confirmed. The mystery makes Gamma Cassiopeiae a valuable test case for understanding how mass transfer and disc dynamics operate in close binary systems, and a subject of ongoing observational campaigns that have so far not settled the question.
08 Delta Cassiopeiae and the meter of Earth
Delta Cassiopeiae, also called Ruchbah or Rukbat — Arabic for "knee" — is a possible Algol-type eclipsing binary star with a maximum brightness of magnitude 2.7. Reports suggest it undergoes eclipses of less than 0.1 magnitudes with a period of about two years and one month, but this variability has never been definitively confirmed, leaving its binary nature somewhat uncertain. It lies 99.4 light-years from Earth. What makes Delta Cassiopeiae particularly notable in the history of science is not its stellar physics but its role in geodesy: the French astronomer Jean Picard used it in 1669 as a positional reference to precisely establish the length of one degree of arc along Earth's surface, from which the radius of the Earth could be calculated. Picard's measurement was one of the most accurate of its era and provided Isaac Newton with a key observational input when Newton later sought to verify his theory of universal gravitation. The calculation required knowing the Earth's size, and it was a bright, well-placed star in Cassiopeia that supplied the anchor point for that measurement — a connection between a modest binary star and one of the founding results of modern physics.
09 IC 10 and the starburst next door Deeper
Among Cassiopeia's galaxies, IC 10 holds a distinction that no other galaxy in the Local Group can claim: it is the only known starburst galaxy within our immediate cosmic neighborhood. A starburst galaxy is one undergoing star formation at a rate far above normal, consuming gas and producing massive young stars in regions of intense activity. IC 10 is an irregular galaxy, meaning it lacks the ordered spiral arms or elliptical structure of its more massive neighbors. Its place within the Local Group makes it a uniquely accessible laboratory for studying starburst conditions without the blurring effects of vast distances, and astronomers have used it to probe how star-forming regions behave under conditions that resemble those of earlier epochs of cosmic history, when starburst activity was far more common across the universe. Cassiopeia also contains part of the closest galaxy group beyond the Local Group: the IC 342/Maffei Group. Both Maffei 1 and Maffei 2 lie within 10 million light-years and are positioned just south of the Heart and Soul Nebulae, but their location deep within the Zone of Avoidance — the band of sky obscured by the dust of our own Milky Way's disc — renders them surprisingly faint, and Maffei 2 is largely beyond the reach of most amateur instruments.
10 The slowest meteor shower and a lost comet
Cassiopeia is the radiant point for the December Phi Cassiopeiids, a meteor shower discovered relatively recently that peaks in early December. The shower is notable for one physical characteristic above all others: the meteoroids enter Earth's atmosphere at only about 16.7 kilometers per second, making them among the slowest-moving meteors observable. The low entry velocity is a consequence of the orbital geometry of the parent body — slow meteors come from sources whose orbits are broadly similar in direction and speed to Earth's own, reducing the relative collision velocity. That parent body is now identified as Biela's Comet, a comet that famously broke apart in the 19th century and has never been recovered as a coherent object since. The identification was not made immediately; the shower's association with Biela's Comet was unknown for a number of years after the shower itself was recognized. The Phi Cassiopeiids thus represent observable debris from a comet that no longer exists as a discrete body — meteors that are, in effect, the ghost of Biela's Comet, still tracing its old orbit and encountering Earth each December in a faint but measurable display.
11 Exoplanet riches in a star-forming corridor
Cassiopeia sits astride one of the richest sections of the Milky Way accessible from northern latitudes, with the galactic plane stretching through it from Perseus toward Cygnus and carrying open clusters, young luminous stars, and nebulae including the Heart Nebula and the Soul Nebula, both about 7,500 light-years away. Within this galactic corridor, fourteen star systems in Cassiopeia have been confirmed to host exoplanets. The most populous of these is HD 219134, which is thought to support six planets in orbit around it — one of the larger confirmed retinues in any nearby system. The constellation's star-forming environment means it is also a productive hunting ground for understanding how planetary systems emerge: young stellar clusters like NGC 457, sitting in the Perseus Arm of the Milky Way at around 10,000 light-years, represent earlier phases of stellar evolution from which planetary systems are still assembling. At the other extreme, the nearby binary Eta Cassiopeiae lies only 19.13 light-years from Earth, making its Sun-like G-dwarf primary one of the closest solar analogs in the sky and a natural target in the search for habitable-zone planets around stars that resemble our own.
12 Measuring 598 square degrees of sky
Cassiopeia covers 598.4 square degrees of sky — 1.451 percent of the entire celestial sphere — and ranks 25th in area among the 88 modern constellations. Its borders were formally defined in 1930 by the Belgian astronomer Eugène Delporte, who drew the boundaries using a 30-sided polygon aligned with the coordinate grid of his era. The right ascension of the constellation's borders runs from 00h 27m 03s to 23h 41m 06s, straddling the zero-hour line of right ascension, while declination runs between 46.68° and 77.69° North — placing it firmly in the high northern sky. The International Astronomical Union had already assigned the three-letter abbreviation "Cas" in 1922. From latitudes north of 34°N, Cassiopeia never sets — it is circumpolar, visible every night of the year at any hour, which is why observers in the British Isles, Canada, and the northern United States see it wheeling around Polaris without interruption. Within its generous boundaries, 157 stars reach or exceed apparent magnitude 6.5, making it unusually rich for casual naked-eye exploration, while Johann Bayer's original labeling of the constellation used the full Greek alphabet from Alpha through Omega and then added the letters A and B to cover its 26 most prominent members.