Theoretical appearance of a hot spot in orbit around the black hole in the center ofthe Milky-Way galaxy. The frame on the left shows the image of the spot as it moves around the black hole in a circular orbit of a radius that is only three times larger than the horizon size for a non-spinning black hole. The scales of this frame are in units of micro-arcseconds (10,000 times sharper than Hubble's vision). The accretion disk in which the hot spot is embedded is viewed at an angle of 45 degrees. The tick marks reflect the polarization orientation of the radiation. The frame on the right shows the polarized flux (upper panel) and the fractional flux (lower panel) as they evolve during the orbit of the hot spot.
The difference between the two movies is that the top movie shows the case of a non-spinning black hole and the bottom movie shows the case of a maximally spinning black hole. At present we do not know by how much the black hole in the Galactic center is spinning. Future observations will enable astronomers to infer the spin of the black hole, and to test the validity of Einstein's theory of gravity.