Distribution of calcium on the surface of the Ap star HR 3831.
An Ap star is an A star whose spectrum has unusually strong lines of some ionized metals and rare earth elements, pointing to a vast overabundance (103 to 106 solar values) of these elements in the star's surface layers. More generally, the term Ap star, or peculiar A star, has come to encompass a range of chemically anomalous stars roughly between spectral types B5 (see Bp star) and F5 (see Fp star). The elements in overabundance vary from one Ap star to another and may include manganese, mercury, silicon, chromium, strontium, europium, and others. Ap stars typically have surface temperatures of 8,000 to 15,000 K, strong magnetic fields, and low rotational rates – properties that help explain their observed chemical anomalies.
The separation of elements is enabled by the slow spin and the relatively high temperature, and hence lack of convection. Separation happens because each ion has its own photoabsorption characteristics. If a certain element absorbs photons (light particles) more easily, it will tend to be pushed to the surface and become overabundant. Otherwise it will sink under the force of gravity and appear depleted in the star's spectrum. The strength of the magnetic field also plays a part in determining which elements are overabundant as shown by the fact that manganese stars – similar to Ap stars but without a strong magnetic field – have anomalies of the same order of magnitude but often not for the same elements. Variations in the spectrum of many Ap stars, associated with magnetic variations, can be understood in terms of the oblique rotator model, in which the spin axis and magnetic axis of the star are out of alignment.