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A rarity among supernova, SN 1993J in M81 can be studied with high spatialresolution. Its radio power and distance permit VLBI observations to monitorthe expansion of its angular structure. This radio structure was previouslyrevealed to be shell-like and to be undergoing a self-similar expansion at aconstant rate. From VLBI observations at the wavelengths of 3.6 and 6 cm in theperiod 6 to 42 months after explosion, we have discovered that the expansion isdecelerating. Our measurement of this deceleration yields estimates of thedensity profiles of the supernova ejecta and circumstellar material in standardsupernova explosion models.Comment: to appear in Astrophysical Journal Letters 10 pages of text, three  figures (two of them in postscript format, one in gif format

Deceleration in the Expansion of SN 1993J