In astrophysics, peculiar stars have distinctly unusual metal abundances.
Chemically peculiar stars are common among hot main sequence (hydrogen-burning) stars. These hot peculiar stars have been divided into 4 main classes: helium-weak (He-weak), mercury-manganese (HgMn), Ap, and metallic lined (Am). The class names give a good idea of what peculiarities sets them apart. He-weak stars appear to have less helium than expected. HgMn stars have strong mercury and manganese absorption lines in their spectra. Am stars have strong heavy metal lines and weak Ca and Sc lines. Ap stars have strong magnetic fields and strong Si, Cr, Sr, Eu and other absorption lines. Some stars show characteristics found in more than one class. It is generally thought that the pecular chemical mixtures observed in these stars have been caused by processes that happened after the star formed, such as diffusion, accretion, magnetic effects, etc. in the outer layer of the stars: it is assumed that the centers of the stars have more normal chemical abundance mixtures which reflect the compositions of the gas clouds from which they formed. (See Annual Review of Astronomy and Astrophysics 1974, vol 12, p 257, by George Preston)
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