  We establish a localization phase diagram for light in a random
three-dimensional (3D) ensemble of motionless two-level atoms with a three-fold
degenerate upper level, in a strong static magnetic field. Localized modes
appear in a narrow spectral band when the number density of atoms $\rho$
exceeds a critical value $\rho_c \simeq 0.1 k_0^3$, where $k_0$ is the wave
number of light in the free space. A critical exponent of the localization
transition taking place upon varying the frequency of light at a constant $\rho
> \rho_c$ is estimated to be $\nu = 1.57 \pm 0.07$. This classifies the
transition as an Anderson localization transition of 3D orthogonal universality
class.
