Upper limit of 3.3 astronomical units to the diameter of the galactic center radio source sgr a*.

Sagittarius (Sgr) A() is a unique radio source located at the center of our galaxy. The radiation from Sgr A() may be generated in matter accreting onto a massive black hole. In observations at long wavelengths, the apparent angular size of Sgr A() decreases in the manner expected for emission from a point source scattered by electron density fluctuations along the line of sight. Measurements at a wavelength of 7 millimeters with the nearly completed Very Long Baseline Array indicate a size of 0.7 milliarc seconds, which is consistent with an extrapolation from results at longer wavelengths. The true size of Sgr A() must be less than 0.4 milliarc seconds, or 3.3 astronomical units. The inferred black hole mass is less than 1.5 x 10(6) solar masses according to a recent model for the emission.