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The diameter of Betelgeuse (α Orionis) has been measured at a wavelength of 11.15 μm using the Infrared Spatial Interferometer over the past 15 years. During this 1993-2009 time period the star's size has decreased systematically by 15%.  047 at optical wavelengths, but in addition to a measurement uncer- tainty of 10%, they estimated that its actual size was probably 10%-15% larger because of limb darkening. Since then it has been measured from time to time by a number of interferometers at various wavelengths, giving values that vary substantially with wavelength. It has now been measured systematically over a pe- riod of about 15 years with the Infrared Spatial Interferometer (ISI). The ISI uses a narrow frequency range to avoid spectral lines and mid-infrared (mid-IR) wavelengths which penetrate the surrounding dust well. The diameters measured by the ISI are not particularly inconsistent with previous measurements, are rather accurate, and show clearly that the star has systemat- ically decreased in size by about 15% over the past 15 years.

A SYSTEMATIC CHANGE WITH TIME IN THE SIZE OF BETELGEUSE