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Rotating stars

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Robert Connon Smith
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Rotation has two main qualitative effects on the structure of a star: it distorts the shape and it reduces the total luminosity. The luminosity depends only on the total angular momentum but the shape, and so the observable properties, depends on the internal distribution of angular momentum. The expected distribution, even for main-sequence stars, is an unsolved problem. Redistribution of angular momentum can occur as a result of large-scale circulation currents, instabilities and magnetic stresses and there may be no steady state. If shear instabilities are dominant, then stars may rotate nearly uniformly. Studies of non-radial oscillations might perhaps eventually lead to a direct probing of the internal angular momentum distribution by the methods now being used to study the internal rotation of the Sun. Differential rotation near the surface would probably give rise to turbulent motions, which could generate the mechanical energy flux which seems to be a necessary input to the winds in hot stars.
Be stars are located in or near the main-sequence band for non-rotating stars. Although this stage of evolution is relatively well understood, there are two main effects that make it impossible to say whether all Be stars are in the same stage of evolution and, if so, what that stage is. One effect is the spread in observed magnitude and colour as a result of rotation. The other effect is that there are uncertainties in the theoretical evolutionary tracks because the amount of convective overshooting is unclear. A useful way forward may be to try to understand individual stars in as much detail as possible.