Meet Alpha Cephei, a rapidly rotating star

While not one of the most conspicuous stars in the night sky, Alderamin – aka Alpha Cephei – is easy to spot, and is interesting for its rapid rotation on its axis.

Fuzzy-edged flattened orange globe with latitude and longitude lines drawn on.

Astronomers used the CHARA array at Georgia State University – optical interferometer – to learn the inclination, polar and equatorial radius and temperature, as well as the rotation speed of Alpha Cephei. Read about this work here. Image via M. Zao.

The constellation Cepheus the King is not terribly conspicuous and can boast of only one relatively bright star. That star is Alderamin – aka Alpha Cephei – which is by far the brightest star in Cepheus, lighting up one corner of an otherwise faint house-shaped pattern of stars. While not one of the most conspicuous stars in the night sky, this star is easy to spot, and it is interesting for its rapid rotation on its axis.

Science of Alpha Cephei. Alderamin is a white star; it’s considered a Class A star, which is now evolving off the main sequence into a subgiant. It’s thought that this star is now on its way to becoming a red giant as its internal supply of hydrogen fuel runs low.

According to the star expert Jim Kaler, Alderamin shines with the luminosity of 18 suns.

Alpha Cephei rotates rapidly. It completes one revolution in less than 12 hours, in contrast to nearly a month for our sun to turn on its axis. Jim Kaler writes of this 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.

By the way, Alpha Cephei is not a very powerful star in contrast to Cepheus’ two king-sized stars: Mu Cephei (the Garnet Star) and VV (two V’s) Cephei. Mu Cephei and VV Cephei are supergiants – among the largest and brightest in our Milky Way galaxy – shining with the firepower of hundreds of thousands of suns. If either star were to replace the sun in our solar system, its diameter would extend beyond the orbit of the planet Jupiter, which lies a good five times farther out from our sun than Earth does. Although both of these stars appear faint, only visible to the unaided eye on a dark, moonless night, it’s because they’re so distant, residing a few thousand light-years away.

Meanwhile, Alderamin is only 49 light-years away.

Star chart of constellations Cepheus and Cassiopeia with star names.

View larger. The constellation Cepheus the King has the shape of the house we all drew as children. Alderamin, or Alpha Cephei, is by far the brightest star in this constellation. A line drawn between Schedar and Caph in the constellation Cassiopeia will lead you to Alpha Cephei.

How to find Alpha Cephei. On a dark night, Alpha Cephei is easily visible and also relatively easy to find. Look northward for this star. It is circumpolar 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. 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.

Sky chart of the constellation Cepheus with stars in black on white.

Sky chart of the constellation Cepheus the King.

Alpha Cephei in the history of astronomy. Alpha Cephei has been a pole star in the past, that is, a star close to the sky’s north pole. The last time was in 18,000 B.C. It will again be a pole star some 5,500 years from now. What kind of world will Earth be then? No matter. The heavens will pursue their long cycles, and Alpha Cephei will lie some three degrees from the sky’s north pole around the year 7500 A.D. 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 in on March 24, 2100. But it’ll be pretty good.

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

Bottom line: Cepheus the King is not a very conspicuous constellation and has only one relatively bright star, Alderamin – aka Alpha Cephei. This star is interesting for its rapid rotation on its axis.

Bruce McClure