Cassiopeia
Cassiopeia is one of the easiest constellations to spot in the northern sky. Its five bright stars form an unmistakable shape: a giant W (or M when you see it the other way up). It sits almost opposite the Big Dipper across the North Star, so when one is high in the sky, the other is low, but at least one of them is always visible from most of the Northern Hemisphere. Cassiopeia is named after a vain queen from ancient Greek mythology and is best seen in the autumn.
- Best seen fromNorthern hemisphereAlways visible north of approx. 30 degrees N
- Best viewing timeAutumn evenings(Northern hemisphere)
- ShapeGiant W or MFive bright stars in a zig-zag
- Brightest starSchedarOrange giant, about 228 light years away
- Famous eventTycho's supernovaExploded in Cassiopeia in 1572
- Greek mythVain queenMother of Andromeda, punished by the gods
The W-shape
The five main stars of Cassiopeia form the famous W (or M, if you are looking at it later in the night when it has rotated upside-down). From left to right when the constellation is in its "W" position, the five stars are:
- Segin (Epsilon Cassiopeiae)
- Ruchbah (Delta Cassiopeiae)
- Gamma Cassiopeiae (also nicknamed Navi by the Apollo astronaut Gus Grissom)
- Schedar (Alpha Cassiopeiae), the brightest
- Caph (Beta Cassiopeiae)
All five are different types of star. Schedar is an orange giant, Caph is a yellow-white star a bit hotter than our Sun, Gamma Cassiopeiae is a fast-spinning blue star throwing off rings of hot gas, and the other two are blue and white giants. Cassiopeia's W gives you a quick tour through almost every type of bright star in one short walk across the sky.
How to find Cassiopeia
Like the Big Dipper, Cassiopeia is circumpolar from most of the Northern Hemisphere: visible all year round from somewhere in the sky. The easiest way to find it is to first find Polaris (the North Star, see Ursa Minor). Cassiopeia sits on the opposite side of Polaris from the Big Dipper.
If you can see the Big Dipper high in the sky, Cassiopeia will be low. If the Big Dipper is low, Cassiopeia is high. The two constellations basically swing around Polaris like opposite sides of a slow-moving clock. Either way, the bright zig-zag W is hard to mistake for anything else.
The myth of the vain queen
In Greek mythology, Cassiopeia was the queen of Aethiopia and the mother of the princess Andromeda. Cassiopeia was famously beautiful and famously vain. One day she boasted that she (or, in some versions, her daughter) was more beautiful than the sea nymphs called the Nereids. The Nereids were offended and complained to the sea god Poseidon, who sent the sea monster Cetus to ravage the coast as punishment.
To save her kingdom, Cassiopeia's husband King Cepheus had to chain Andromeda to a rock as a sacrifice to the monster. Andromeda was rescued at the last moment by the hero Perseus, and they later married. The gods placed all of them in the sky as constellations: Cassiopeia, Cepheus, Andromeda, Perseus and Cetus are all near each other and all share part of the same story. As a final touch of punishment, Cassiopeia was placed in the sky in a chair that turns upside-down for half the year, leaving her hanging from it.
The Milky Way runs through it
If you look at Cassiopeia from a really dark sky, you will notice that the band of the Milky Way passes straight through the middle of it. That means the constellation is rich in star clusters and nebulae visible with binoculars or a small telescope. Famous targets in Cassiopeia include the Heart Nebula, the Soul Nebula, and the bright open clusters M52 and M103.
Deeper dive: Tycho's supernova and the end of the "perfect" heavens
When the bright new star appeared in Cassiopeia on 11 November 1572, it caused a serious problem for European science. At the time most astronomers still followed the ideas of Aristotle, who had taught that the heavens above the Moon were perfect and unchanging, the same forever. The new star clearly broke that rule. It was right there in Cassiopeia, a constellation that anyone could see, and it was brighter than any normal star.
The Danish nobleman and astronomer Tycho Brahe (1546-1601) measured the new star's position every night with the most precise instruments of his age. He proved that it was further away than the Moon (it showed no parallax shift compared to nearby stars) and therefore had to be a real change in the supposedly perfect heavens. His careful work helped end the old Aristotelian view and paved the way for the scientific revolution.
Modern astronomers used X-ray and radio telescopes in the 2000s to study the gas left over from Tycho's supernova. They figured out it was a Type Ia supernova, which happens when a white dwarf star steals too much gas from a companion and explodes. Even more impressively, in 2008 they detected the light from the original explosion bouncing off dust clouds further out in space, an effect called a light echo. It gave us a chance to "see" a 1572 event over 430 years later.
To navigate from Cassiopeia, drop down to Ursa Minor and Polaris, or jump across the sky to Orion. See what is a constellation for the basics.