Lying just 18° from the north celestial pole, Pherkad is circumpolar from the UK and never sets.
It’s a bright, giant of spectral class A3 III and sits 487 lightyears from the Sun. Pherkad is 15 times larger than the Sun and 1,100 times more luminous.
With a mass almost five times that of the Sun, Pherkad is likely close to 100 million years old.
![Pherkad is a circumpolar star in the constellation Ursa Minor. Credit: Pete Lawrence](https://c02.purpledshub.com/uploads/sites/48/2024/05/pherkad-ursa-minor.jpg?webp=1&w=1200)
How Pherkad compares to its constellation companions
Most people know what the Plough or Saucepan looks like, a distinctive seven-star pattern that is part of the Great Bear, Ursa Major.
The Little Bear, Ursa Minor, similarly has seven stars, three being in a curve starting from Polaris (Alpha (α) Ursae Minoris) that joins to a box shape.
As with the Plough, the pattern represents the little bear’s rump and tail, but unlike the Great Bear, the rest of the creature is not well represented.
![Chart showing the constellation Ursa Major](https://c02.purpledshub.com/uploads/sites/48/2023/01/ursa-major-constellation-4c25ef4.jpg?webp=1&w=1200)
The box’s two corner stars located closest to Polaris are fairly dim and easily lost from a site with medium light pollution.
These are mag. +4.3 Zeta (ζ) and mag. +4.9 Eta (η) Ursae Minoris.
The other two are brighter: mag. +2.1 Kochab (Beta (β) Ursae Minoris) and mag. +3.0 Pherkad (Gamma (γ) Ursae Minoris).
Together, Kochab and Pherkad are known enigmatically as the Guardians of the Pole.
![Star Pherkad in Ursa Minor. Credit WikiSky / DSS2](https://c02.purpledshub.com/uploads/sites/48/2024/05/star-pherkad.jpg?fit=800%2C366&webp=1&w=1200)
A variable star
Pherkad rotates at 180km/s and is a variable star, but exhibits an odd form of variability.
Its brightness can change by less than a tenth of a magnitude over just a few hours.
The cause of this isn’t well known.
Originally it was classed as a shell star, which is one with a circumstellar disc of gas around its equator, but this seems to have disappeared.
This guide appeared in the May 2024 issue of BBC Sky at Night Magazine.