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.
Related category• TYPES OF STAR
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