Alpha Cephei, or Alderamin, is a fast-spinning star

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Alpha Cephei, also known as Alderamin, is the brightest star in the constellation Cepheus the King. Astronomers are intrigued by this star because it spins at such a fast rate. Unlike our almost-round sun, Alderamin is distorted by its rapid spin into an oblate form, like a partly deflated beach ball.

This star isn’t the brightest one around, but it’s pretty bright. It shines at about magnitude 2.5.

Its faint constellation, Cepheus, is surprisingly easy to spot in a dark sky because the stars of Cepheus trace out the shape of a child’s stick house.

Science of Alpha Cephei

Alpha Cephei is about 49 light-years away. It is a white star roughly twice as massive as our sun and about 17 times brighter. Alpha Cephei is a Class A star that is now changing as it gets older. As it uses up the hydrogen in its core, changes inside the star will cause its outer layers to expand, and it will eventually become a red giant.

Alpha Cephei rotates rapidly; observations suggest it could be as fast as 152 miles a second (246 km/s). In comparison, the sun’s rotation speed at the equator is not quite 1.2 miles a second (2 km/s). As a result of its rapid rotation, Alpha Cephei appears slightly flattened. That’s because the star’s surface rotation speed gets progressively faster as you move away from its rotation axis towards its equator.

And, as surface rotation speed increases, the star’s surface is increasingly pushed out. As a result, Alpha Cephei bulges along the equator.

A closer look at Alpha Cephei

The image at the top of this article is a model of Alpha Cephei based on data from the CHARA telescope array. It shows the star’s flattened shape and the differences in brightness across its surface.

The equatorial region appears darker because the star’s rapid rotation makes this part of the surface cooler than the poles. The poles are hotter and therefore appear brighter. This difference in temperature and brightness across a rapidly rotating star is known as gravity darkening.

Astronomer Jim Kaler wrote about this rapidly spinning star:

The spin may also be related to the star’s activity. [Our] sun is magnetically active in broad part because its outer third is churning up and down in huge convective currents, the movement helping to generate a magnetic field. Such outer zones are supposed to disappear in class A stars like Alderamin. Yet Alderamin emits about the same amount of X-ray radiation as does the sun and has other features that together suggest considerable magnetic activity. No one really knows why. Such anomalies, of course, drive the science. Understanding Alderamin will someday help us understand our own star, on which we depend for life.

How to find Alpha Cephei

On a dark night, Alpha Cephei is easily visible and relatively easy to find. Look northward for this star. It is circumpolar — meaning it never sets — throughout all of Europe, northern Asia, Canada and American cities as far south as San Diego, California. Its constellation, Cepheus, has the shape of the stick house we all drew as children.

Or you might prefer to see the shape of Cepheus as a point on the King’s crown. Cepheus is a rather faint constellation, but Alpha Cephei is by far its brightest star and is easily observable to the unaided eye, even in cities.

If you know the W or M-shaped constellation Cassiopeia the Queen, you can use the Cassiopeia stars Schedar and Caph to star-hop to Alderamin.

A once and future pole star

Alpha Cephei has been a pole star in the past. That is, a bright star close to the sky’s north pole. This is because a slow, constant wobble in Earth’s movement — known as precession — makes the stars shift in relation to our planet’s axis.

The last time Alpha Cephei was the nearest bright star to the north celestial pole was in 18,000 BCE. It will again be a pole star some 5,500 years from now. No matter what is going on at that time on Earth, the heavens will pursue their long and predictable cycles.

And Alpha Cephei will lie some 3 degrees from the sky’s north pole around the year 7500 CE. That means it won’t be as good a pole star as our present-day Polaris, which will be 0.4525 degrees from the north celestial pole on March 24, 2100. But it’ll be pretty good.

This star’s proper name, Alderamin, is from Arabic and means “the right arm,” presumably of Cepheus the King, father of Andromeda in Greek mythology.

Bottom line: Alpha Cephei, or Alderamin, is the brightest star in the faint constellation Cepheus the King. It’s spinning so fast that it appears like a slightly flattened beach ball.

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