  The cores of most galaxies are thought to harbour supermassive black holes,
which power galactic nuclei by converting the gravitational energy of accreting
matter into radiation (ref 1). Sagittarius A*, the compact source of radio,
infrared and X-ray emission at the centre of the Milky Way, is the closest
example of this phenomenon, with an estimated black hole mass that is 4 million
times that of the Sun (refs. 2,3). A long-standing astronomical goal is to
resolve structures in the innermost accretion flow surrounding Sgr A* where
strong gravitational fields will distort the appearance of radiation emitted
near the black hole. Radio observations at wavelengths of 3.5 mm and 7 mm have
detected intrinsic structure in Sgr A*, but the spatial resolution of
observations at these wavelengths is limited by interstellar scattering (refs.
4-7). Here we report observations at a wavelength of 1.3 mm that set a size of
37 (+16, -10; 3-sigma) microarcseconds on the intrinsic diameter of Sgr A*.
This is less than the expected apparent size of the event horizon of the
presumed black hole, suggesting that the bulk of SgrA* emission may not be not
centred on the black hole, but arises in the surrounding accretion flow.
