Betelgeuse and Its Variations

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Abstract
Observations of Betelgeuse show that (1) the photospheric spectral lines are very broad with a 1/e width of 11 km s-1, but they have stable shapes with time as the star varies in other ways; (2) all the spectral lines within the observed wavelength region change their depths by essentially the same factor and in phase with the photometric brightness of the star on timescales of months; (3) small deviations from perfect scaling of the line depths imply temperature variations that to first order are in phase with the line depths, i.e., cooler temperatures go with weaker lines; and (4) to second order, the temperature variations lag behind the changes in line depths by ~5 days. The hypothesis of changing continuous opacity is put forward as a coherent explanation of all of these spectroscopic and photometric effects. In addition, the bright spot observed by others is explained as a region where the opacity does not change; it is not a giant convection cell. The physical reason for the opacity change has yet to be identified. Structure in the cores of spectral lines varies on the timescale of a day, and this may be the signature of giant convection cells, but other interpretations are also possible, including hot spots, prominences, or nonradial oscillations.