A wide variety of stars lose mass during their lifetimes; in the more extreme examples this affects their evolution. In hot stars (spectral types O and B) the strong radiation field exerts an outwards pressure that leads to high velocity (~ 103 km/s) winds and a large mass-loss rate. In cool stars (spectral types G, K and M) the physics underlying mass-loss is related to the processes which heat the outer layers (see below). Mass-loss from the Sun (the solar wind) is a result of the high kinetic temperature (~ 106 K) in the corona and the influence of wave motions, but the mass-loss rate is relatively small. In cool giant stars the mass-loss rate is largest for the evolved stars with the lowest surface gravities. These stars do not appear to have hot coronae, although heated regions exist above their photospheres. The pressure associated with waves passing through the atmosphere is thought to contribute to the mass-loss. These stars have low velocity winds and moderate mass-loss rates which when summed over a large number of stars give a significant contribution to the return of material to the interstellar medium. In very cool supergiants, stellar pulsations, waves and radiation pressure on dust grains which form around these stars, can drive substantial mass-loss.
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