Cassiopeia
What is Cassiopeia?
Constellations· Autumn–winter (N, circumpolar) · five-star W. Five stars forming a W in the northern sky. It sits opposite the Big Dipper across Polaris, so in autumn and winter, when the Dipper is low, Cassiopeia takes over as the guide to the pole star.
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Stars of the figure
| Star | Magnitude |
|---|---|
| Schedar | 2.2 |
| Caph | 2.3 |
| Gamma Cassiopeiae | 2.5 |
| Ruchbah | 2.7 |
| Segin | 3.4 |
The 5 brightest of the 6 stars used in the figure (5 lines). Lower magnitude means brighter.
Deep-sky objects near this constellation
Constellation basics
| Season | autumn–winter; circumpolar |
|---|---|
| Shape | five stars in a W |
| How to find | the W opposite the Big Dipper |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Navi, the middle of the W — an astronaut's name spelled backwards
The star at the centre of the W, Gamma Cassiopeiae, has no traditional name of its own. Instead it is widely called Navi. Astronaut Gus Grissom put the name on a list of navigation stars during the Apollo programme; it is his middle name, Ivan, spelled backwards. Grissom died in the Apollo 1 fire in 1967, but the name stayed.
The star itself is unusual. It spins so fast, with its equator moving at close to 400 km per second, that gas flows off the equator and forms a disk around it. As the disk builds up and breaks apart, the star's brightness changes. In the late 1930s it rose above magnitude 2, outshining the other stars of the W, and then soon fell below magnitude 3. So the balance of the W itself shifts over the decades.
The constellation in depth
Cassiopeia is five bright stars drawing a W, which turns into an M as it circles the pole. The clear shape makes it one of the easiest constellations to find in the northern sky. It lies on the Milky Way, so sweeping the area with binoculars turns up one star cluster after another.
The constellation sits on the opposite side of Polaris from the Big Dipper. When the Dipper drops toward the horizon, Cassiopeia rides high, and Polaris lies roughly halfway between them. As a result, at any time of year one of the two is high enough to guide the eye to the pole star.
In November 1572 a brilliant new star appeared here. The Danish astronomer Tycho Brahe measured it carefully and showed that it lay far beyond the Moon. People then believed the realm of the stars never changed, so his result shook that idea badly. Today the star is known to have been a supernova, the explosion of a star at the end of its life.
Frequently asked
- When can I see Cassiopeia?
- Cassiopeia is best seen in autumn–winter; circumpolar.
- How do I find Cassiopeia?
- To find Cassiopeia, look for the W opposite the Big Dipper.
- What is the brightest star in Cassiopeia?
- The brightest star in Cassiopeia is Schedar, at magnitude 2.2.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.