Ap stars(ay-pee ) Peculiar main-sequence stars (see peculiar stars) of spectral types from B5 to F5 in which the spectral lines of certain elements (mainly Mn, Si, Eu, Cr, and Sr) are selectively enhanced and are sometimes of varying intensity (see spectrum variables). The enhancement and its variation is apparently associated with strong and often variable magnetic fields. Like the Am stars, Ap stars are generally slow rotators, but they differ in being single stars rather than spectroscopic binaries. The relative enhancement may be due to diffusion in the stable atmosphere caused by slow rotation, modified by the effect of strong magnetism. The spectral peculiarities could also relate to stellar temperature, the manganese stars being hotter than the silicon stars, which in turn are hotter than the europium-chromium-strontium stars. See also magnetic stars.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006