  Recent observations of Sgr A* give strong constraints for possible models of
the physical nature of Sgr A* and suggest the presence of a massive black~hole
with M<2 10^6 M_sun surrounded by an accretion disk which we estimate to
radiate at a luminosity of <7 10^5 L_sun. We therefore calculate the appearance
of a standard accretion disk around a Kerr hole in Sgr A* following from
general relativity and a few fundamental assumptions. Effective temperature and
luminosity of the disk spectra do not depend on the unknown viscosity mechanism
but instead are quite sensitive to variations of intrinsic parameters: the
mass, the accretion rate, the angular momentum of the accreting hole and the
inclination angle. A radiation field of L~7 10^4 - 7 10^5 L_sun and T_eff ~ 2-4
10^4 K can be ascribed to a rapidly rotating Kerr~hole (a>0.9) accreting
10^-8.5 - 10^-7 M_sun/yr at a black~hole mass of M=2 10^6 M_sunseen almost edge
on. A low mass black hole of M<10^3 M_sun seems to be very unlikely. We provide
a ``Hertzsprung-Russell diagram for black holes'' together with simple scaling
laws to provide an easy-to-handle test for the black hole model.
