A star such as the Sun is a gaseous sphere in hydrostatic equilibrium. That is, the inwardly directed gravitational force is balanced by an inwards increase of the pressure. The equilibrium is spherically symmetric if the forces that have one or several preferred axes, such as the forces arising from rotation or from the magnetic field, can be neglected. In a good approximation this is the case for the Sun. Here we shall first assume spherical symmetry, and later treat the rotation as a perturbation. An excursion from the equilibrium leads to oscillations whenever a restoring force is connected to the excursion. In the self-gravitating gaseous sphere, an excursion from the equilibrium causes two types of restoring force: an additional pressure gradient because the stellar matter is compressible, and an additional gravity force because of the star's internal density stratification. In general, both forces contribute, but almost always one of them is predominant. Hence, following Cowling (1941), one speaks of p and g oscillations, or p modes and g modes.
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