Alderamin, the brightest star in Cepheus
Although Cepheus represents a mythical king in the sky, the constellation is more frequently described in terms of its shape: it resembles the outline of a house formed from a square base and pointed roof. The bottom-right corner of the base (southwest corner) is mag. +2.4 Alderamin (Alpha (α) Cephei). This is one of our nearer neighbours, lying at a distance of 49 lightyears. The name Alderamin translates as ‘the right arm’.
Alderamin is a white star of spectral class A8Vn, ‘A8’ placing it in the blue-white spectral region, closer to the white or yellow-white end.
‘V’ indicates it’s a main sequence dwarf, the ‘n’ indicating that its spectrum contains broad absorption lines caused by a fast spinrate. With a physical size 2.5 times larger than the Sun, the spin-rate for this star is high, one rotation taking just 12 hours. This translates to a rotational velocity around 283 km/s, compared to the Sun’s rather leisurely 2km/s. An alternative spectral designation for Alderamin is A7V–IV, which hints that it’s moving off the main sequence branch and evolving into a subgiant (the ‘IV’ designation), something that happens when hydrogen fusion in a star’s core begins to wane.
Located around 3˚ from the North Celestial Pole,
Alderamin was the pole star around 18,000 BC and will be again in another 5,500 years.
Alderamin emits a similar amount of X-radiation as the Sun, something which is unexpected for an A-class star such as this. The reason is likely the rapid rotation rate which gives rise to huge convective currents within the star’s interior.