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We investigate to what extent the spin axes of stars in young open clusters are aligned. Assuming that the spin vectors lie uniformly within a conical section, with an opening half‐angle between  λ= 0°  (perfectly aligned) and  λ= 90°  (completely random), we describe a Monte Carlo modelling technique that returns a probability density for this opening angle given a set of measured  sin  i   values, where  i  is the unknown inclination angle between a stellar spin vector and the line of sight. Using simulations we demonstrate that although azimuthal information is lost, it is easily possible to discriminate between strongly aligned spin axes and a random distribution, providing that the mean spin‐axis inclination lies outside the range  45°–75° . We apply the technique to G‐ and K‐type stars in the young Pleiades and Alpha Per clusters. The  sin  i   values are derived using rotation periods and projected equatorial velocities, combined with radii estimated from the cluster distances and a surface brightness/colour relationship. For both clusters we find no evidence for spin‐axis alignment:  λ= 90°  is the most probable model and  λ &gt;40°  with 90 per cent confidence. Assuming a random spin‐axis alignment, we redetermine the distances to both clusters, obtaining  133 ± 7 pc  for the Pleiades and  182 ± 11 pc  for Alpha Per. If the assumption of random spin‐axis alignment is discarded however, whilst the distance estimate remains unchanged, it has an additional   +18  −32   per cent uncertainty.

Are the spin axes of stars randomly aligned within a cluster?