Cepheus
What is Cepheus?
Constellations· Autumn evenings (N) · a house-shaped pentagon beside Cassiopeia. The king of Ethiopia and father of Andromeda, drawn as a house with a pointed roof between Cassiopeia and Polaris. Its star Delta Cephei brightens and fades every 5.4 days and gave its name to the Cepheid variables, the stars that became a ruler for cosmic distances.
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Stars of the figure
| Star | Magnitude |
|---|---|
| Alderamin | 2.5 |
| Alfirk | 3.2 |
| Errai | 3.2 |
| Zeta Cephei | 3.4 |
The 4 brightest of the 6 stars used in the figure (6 lines). Lower magnitude means brighter.
Deep-sky objects near this constellation
Constellation basics
| Season | circumpolar from the north, highest on autumn evenings |
|---|---|
| Brightest star | Alderamin (2.5) |
| How to find | the pointed "house" between Cassiopeia's W and Polaris |
| Area rank | 27th of 88 |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Herschel's Garnet Star — the red supergiant of Cepheus
At the southern edge of Cepheus lies Mu Cephei, a 4th-magnitude star. William Herschel compared it to a red gemstone and called it the Garnet Star. In binoculars it looks strikingly red among the white stars around it. It is a red supergiant, a star that has swollen enormously as it nears the end of its life.
Estimates of its size range from about 760 to more than 1,600 times the Sun's diameter, depending on the method. Even the smallest figure means that, placed where the Sun is, it would swallow the orbit of Mars and reach the asteroid belt. That size and brightness are why we can see it with the naked eye from so far away. Over several years it varies irregularly between magnitude 3.4 and 5.1.
A star this large has loose outer layers and is slowly losing material. Astronomers expect Mu Cephei to explode as a supernova one day, but no one can say when. Cepheus thus holds both Delta Cephei, the star behind our cosmic yardstick, and a red supergiant nearing the end of its life.
The constellation in depth
Cepheus is the king of Ethiopia in Greek myth. With Queen Cassiopeia, their daughter Andromeda and her husband Perseus, he makes up a family of constellations in the autumn sky. He has no bright stars and is easy to miss. But he sits close to the north celestial pole, so from Europe, Canada and the northern United States he never sets.
Delta Cephei is the star John Goodricke found to vary in 1784. It swells and shrinks, and its brightness changes as it does. In 1912 Henrietta Leavitt found that stars like this are truly brighter the more slowly they vary. Measuring the period therefore tells you how bright the star really is, and from that, how far away it is. Edwin Hubble used this method to prove that Andromeda is a galaxy beyond our own.
On autumn evenings, look north between Cassiopeia's W and Polaris for five stars shaped like a house with a pointed roof, sometimes upside down. Alderamin marks one lower corner. Compare Delta Cephei with its neighbours over several nights and you can see it brighten and fade with the naked eye. Set the Sky view to an October night to find the house high in the north.
Frequently asked
- When can I see Cepheus?
- Cepheus is best seen in circumpolar from the north, highest on autumn evenings.
- How do I find Cepheus?
- To find Cepheus, look for the pointed "house" between Cassiopeia's W and Polaris.
- What is the brightest star in Cepheus?
- The brightest star in Cepheus is Alderamin, at magnitude 2.5.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.