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).

chart showing the location of star alderamin in the constellation cepheus
Credit: Pete Lawrence

This is one of our nearer neighbours, lying at a distance of 49 lightyears. The name Alderamin translates as ‘the right arm’.

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 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 spin-rate.

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.

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.


This guide originally appeared in the September 2022 issue of BBC Sky at Night Magazine.


Pete Lawrence, astronomer and BBC The Sky at Night presenter.
Pete LawrenceAstronomer and presenter

Pete Lawrence is an experienced astronomer and astrophotographer, and a presenter on BBC's The Sky at Night.