In astrophysics, peculiar stars have distinctly unusual metal abundances, at least in their surface layers.

Chemically peculiar stars (CP stars) are common among hot main sequence (hydrogen-burning) stars. These hot peculiar stars have been divided into 4 main classes on the basis of their spectra, although two classification systems are sometimes used: metallic-lined (Am, CP1), Ap (CP2), mercury-manganese (HgMn, CP3) and helium-weak (He-weak, CP4). The class names give a good idea of what peculiarities sets them apart and have been.

The Am stars (CP1 stars) show weak lines of singly ionized Ca and/or Sc, but show enhanced abundances of heavy metals. They also tend to be slow rotators and have an effective temperature between 7000 K and 10 000 K. The Ap stars (CP2 stars) are characterized by strong magnetic fields, enhanced abundances of elements such as Si, Cr, Sr and Eu and are also generally slow rotators. The effective temperature of these stars is stated to be between 8000 K and 15 000 K, but the issue of calculating effective temperatures in such peculiar stars is complicated by atmospheric structure. The HgMn stars (CP3 stars) are also classically placed within the Ap category, but do not show the strong magnetic fields associated with classical Ap stars. As the name implies, these stars show increased abundances of singly ionized Hg and Mn. These stars are also very slow rotators, even by the standards of CP stars. The effective temperature range for these stars is quoted at between 10 000 K and 15 000 K. The He-weak stars (CP4 stars) show weaker He lines than would be expected classically from their observed Johnson UBV colours.