Alioth (Epsilon (ε) Ursae Majoris) is easy to locate as it’s one of the seven stars that form the famous Plough or Saucepan asterism.
Imagining the pattern as a saucepan, mag. +1.8 Alioth is the first star in the handle away from the edge of the pan.
Although in one of the more minor positions in the pattern, it may come as a surprise that Alioth is actually the brightest star in the Saucepan, albeit only beating Dubhe (Alpha (α) Ursae Majoris) by 0.3 magnitudes.
‘Alioth’ translates as ‘the sheep’s fat tail’.
Facts about Alioth
Alioth has a spectral type of A1III-IVp kB9, a long string of characters that needs some explanation.
The ‘A1’ part indicates that it’s a star that appears white in colour.
The ‘III–IV’ indicates that it’s regarded as being on the normal giant–subgiant border.
The ‘p’ indicates it has a peculiar spectrum, while ‘kB9’ indicates that the star’s spectrum includes the calcium-K line, representative of a B9-class star despite it being an A1-type star.
Alioth is classed as an Alpha2 (α2) Canum Venaticorum-type variable with spectral lines that vary over a period of 5.1 days.
The variation may in part be due to a very strong magnetic field and an offset between the star’s magnetic field axis and its rotational axis, separating different elements into bands which then appear to cross our line of sight to create the observed variability.
The offset between Alioth’s magnetic and rotational poles is believed to be around 90°.
This guide appeared in the April 2025 issue of BBC Sky at Night Magazine
