  We report high-angular-resolution measurements of polarized dust emission
toward the low-mass protostellar system NGC 1333 IRAS 4A. We show that in this
system the observed magnetic field morphology is in agreement with the standard
theoretical models of the formation of Sun-like stars in magnetized molecular
clouds at scales of a few hundred astronomical units; gravity has overcome
magnetic support, and the magnetic field traces a clear hourglass shape. The
magnetic field is substantially more important than turbulence in the evolution
of the system, and the initial misalignment of the magnetic and spin axes may
have been important in the formation of the binary system.
