Alpha Cephei

Alpha Cephei, also known by its official name Alderamin, is a second-magnitude star located in the northern constellation Cepheus. Its Bayer designation is Alpha Cephei, Latinized from α Cephei, and it also carries the Flamsteed designation 5 Cephei. The star lies about 49 light-years from Earth and is moving closer to us, with a heliocentric radial velocity of roughly −16 km/s. It is the brightest star in Cepheus and can be seen with the naked eye, even from areas with significant light pollution.

Nomenclature

The traditional name Alderamin comes from a contraction of the Arabic phrase al-dhirā‘ al-yamīn, meaning "the right arm." In 2016, the International Astronomical Union’s Working Group on Star Names (WGSN) officially approved Alderamin as the star’s proper name.

Visibility

Due to its declination of over 62 degrees north, Alpha Cephei never rises south of about −27° latitude, keeping it below the horizon for much of South America, Australia, South Africa, and all of New Zealand. It is circumpolar throughout Europe, northern Asia, Canada, and as far south as San Diego in the United States.

Pole star

Alpha Cephei lies close to the precessional path traced by Earth’s north pole across the celestial sphere. As a result, it periodically comes within 3° of being a pole star—a role currently held by Polaris. It will next become the North Star around the year 7500 AD. Interestingly, the north pole of Mars points to the midpoint of the line connecting Alpha Cephei and the star Deneb.

Properties

Alderamin is a white class A star that is evolving off the main sequence into a subgiant, likely on its way to becoming a red giant as its hydrogen supply diminishes. Its average apparent magnitude is 2.46. Parallax measurements from 2007 place it at a distance of about 15 parsecs, or 49 light-years. Interferometry by the CHARA array reveals that the star has an oblate shape, with an equatorial size and a polar radius that differ.

Its temperature varies from 8,558 K at the poles to 6,574 K at the equator. A later, less detailed measurement by the Navy Precision Optical Interferometer likely corresponds to the equatorial radius. The star is a hybrid pulsator, exhibiting both Delta Scuti and Gamma Doradus pulsations, though the amplitude is only 0.002 magnitudes.