The spectral variability of these neutron stars was seen to correspond to changes in the QPOs. Typical QPO frequencies were found to be between about 1 and 60 Hz. The fastest oscillations were found in a spectral state called the Horizontal Branch, and were thought to be a result of the combined rotation of the matter in the disk and the rotation of the collapsed star (the "beat frequency model"). During the Normal Branch and Flaring Branch, the star was thought to approach its Eddington luminosity at which the force of the radiation could repel the accreting gas. This could give rise to a completely different kind of oscillation.