Am starsPeculiar main-sequence stars (see peculiar stars) of spectral types from A0 to F0 in which there is an over-abundance of heavier elements and rare earths and (less so) of iron, and an apparent under-abundance of calcium. They rotate slower than normal A stars and are almost all short-period spectroscopic binaries. Current theory suggests that tidal effects slow the A star's rotation, leading to an unusually stable atmosphere where heavy elements can diffuse up from the interior. Compare Ap stars.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006