Cassiopeia is one of the easiest constellations to recognize in the northern sky, thanks to the five bright stars that form its distinctive W or M shape. It is also much more than a convenient pattern of stars: Cassiopeia can help you navigate the northern sky, while the Milky Way runs directly through the constellation, filling the region with rich star fields, clusters and nebulae.
How to Find Cassiopeia in the Night Sky
Cassiopeia is easiest to recognize by the five bright stars forming its distinctive W shape. However, the W is not the whole constellation. It is simply its most conspicuous pattern of stars within the much larger official boundaries of Cassiopeia.
The constellation itself covers about 598 square degrees of sky, making it the 25th largest of the 88 recognized constellations. It lies in the far northern sky and is visible in its entirety from locations down to about 20° south. For observers across much of the Northern Hemisphere, Cassiopeia is circumpolar, meaning it never sets below the horizon. Instead, it appears to circle the north celestial pole as Earth rotates.
Depending on where Cassiopeia is during that journey, the five-star pattern can resemble a W, an M, a sideways W or a crooked zigzag. It completes one apparent circuit of the celestial pole in about 24 hours, although daylight hides the stars for part of that journey. Its orientation therefore changes continually rather than simply appearing as a W in one season and an M in another.
Cassiopeia in Greek Mythology
According to Greek mythology, Cassiopeia was a vain queen who boasted that she or her daughter Andromeda was more beautiful than the sea nymphs known as the Nereids. This angered the sea god Poseidon, who sent the sea monster Cetus to ravage the kingdom. To end the punishment, Andromeda was chained to a rock as a sacrifice to Cetus, but she was rescued by Perseus. The story is also mapped across this part of the night sky. Cassiopeia’s husband Cepheus lies beside her, with Andromeda and Perseus nearby and Cetus farther south.
Cassiopeia was punished for her vanity by being placed in the heavens on a throne or chair. As the constellation circles the north celestial pole, the queen periodically appears upside down — a fitting continuation of her punishment in the night sky.
The Five Stars That Form Cassiopeia’s W
The five stars responsible for Cassiopeia’s familiar W are very different objects that only appear to form a pattern from our viewpoint.
- Schedar (Alpha Cassiopeiae) is generally Cassiopeia’s brightest star, shining at around magnitude 2.2. Its Arabic-derived name means “breast,” reflecting its traditional position on the figure of the queen. Located about 230 light-years away, Schedar is an orange giant about 42 times the Sun’s radius and around 675 times as luminous.
- Caph (Beta Cassiopeiae) shines at around magnitude 2.3 and lies only about 55 light-years away, making it by far the nearest of the five W stars. Its traditional name refers to the palm of the hand. Caph is also a pulsating variable star whose brightness changes slightly over a period of only about 2.5 hours.
- Gamma Cassiopeiae forms the middle point of the W. It is a hot blue-white variable star roughly 550 light-years away — and by far the most unusual of the five.
- Ruchbah (Delta Cassiopeiae) lies about 100 light-years away. Its Arabic-derived name means “knee,” corresponding to its position in the traditional figure of Cassiopeia.
- Segin (Epsilon Cassiopeiae) is the faintest of the five at about magnitude 3.4, but it is intrinsically much more powerful than it appears. This hot blue-white star lies roughly 410 light-years away, has about nine times the Sun’s mass and is around 2,500 times as luminous.
Their very different distances also show why constellations are patterns created by our viewpoint from Earth rather than physical groups of neighboring stars.
Use Cassiopeia to Find Your Way Around the Sky
Once you can recognize Cassiopeia’s W, it becomes a useful signpost for finding your way around the northern sky. Cassiopeia and the Big Dipper in Ursa Major lie on opposite sides of Polaris and appear to circle around it. This means that when the Big Dipper is low in the northern sky, Cassiopeia is generally higher, and vice versa. A line from the Big Dipper’s pointer stars through Polaris can also be continued across the sky
From Cassiopeia, you can explore farther outward. Perseus lies nearby, while the W can also help orient you toward Andromeda and the Great Square of Pegasus. Once you know this part of the sky, Cassiopeia becomes a useful starting landmark for locating the Andromeda Galaxy, the most distant object readily visible to the naked eye under dark skies.
The Middle Star of the W Is a Very Strange Star
The middle point of Cassiopeia’s W looks like an ordinary bright star, but Gamma Cassiopeiae is anything but ordinary. Its brightness varies enough for it sometimes to become Cassiopeia’s brightest star and at other times fall well behind Schedar.
Gamma Cas is a hot blue-white star that spins extremely rapidly — at more than 400 km/s (250 mi/s) at its equator — and throws material from around its equator into a surrounding disk of gas. In 1866, astronomer Angelo Secchi detected bright hydrogen emission from this gas — the first such discovery in a star. It revealed that some hot stars could be surrounded by their own glowing material, leading to the class now called Be stars. Gamma Cas is unusual even among them. It produces far stronger and more rapidly changing X-rays than typical Be stars, a behavior that has puzzled astronomers for decades.
The Milky Way Runs Through Cassiopeia
The Milky Way runs directly through Cassiopeia, so the W appears against a particularly rich background of stars. This helps explain why the constellation is so rich in star clusters, nebulae and other deep-sky objects.
One of the most striking is Caroline’s Rose (NGC 7789), a rich open star cluster discovered by Caroline Herschel in 1783. Located about 8,000 light-years away, the cluster spans roughly the same apparent width as the full Moon, with chains of stars and darker gaps creating the appearance of rose petals.
Cassiopeia also contains the Pacman Nebula (NGC 281), a large cloud of gas and dust where new stars are forming. Its nickname comes from the dark region cutting into its glowing material, giving it a resemblance to the video-game character Pac-Man. Remarkably, NGC 281 sits almost 1,000 light-years above the Milky Way’s main plane, giving astronomers a relatively unobstructed view of the star formation taking place inside it.
Two Stars Exploded in Cassiopeia Centuries Apart
In November 1572, observers noticed an extraordinarily bright “new star” in Cassiopeia where none had previously been seen. It eventually became comparable to Venus in brightness and could even be seen during daylight. Danish astronomer Tycho Brahe studied the object and showed that it lay far beyond the Moon, among the stars. That was important because astronomers had long believed that the heavens beyond the Moon never changed. A new object appearing there was direct evidence that they did. We now know that the “new star” was a Type Ia supernova, known as SN 1572 or Tycho’s Supernova.
Another star exploded in Cassiopeia roughly a century later, but this time there is no widely accepted record of anyone seeing it. Its expanding remains, Cassiopeia A, lie about 11,000 light-years away and are now among the most intensively studied supernova remnants in astronomy. The contrast is remarkable: one explosion became one of the most famous astronomical events of the 16th century, while another in the same constellation apparently passed almost unnoticed.