Abstract
The author reviews the importance of magnetic fields for the support of molecular clouds against self-gravity and the process of bimodal star formation. For the case of subcritical clouds that evolve through the process of ambipolar diffusion forming small dense cores, the author discusses the two regimes for the evolution of the cores according to their mass. For a mass greater than an umbral mass, the process of ambipolar diffusion produces a centrally condensed core, that ultimately collapses, from inside out, to form a star. This core looks like the ammonia cores, sites of low mass star formation, observed in the Taurus dark clouds. On the other side, a subumbral region evolves towards a stable final configuration where the mean magnetic field asymptotically becomes uniform and straight with thermal and turbulent pressure providing support against gravity. This region will not be either dense enough or big enough to excite measurable ammonia emission. Only if turbulence decays, this type of regions will be able to form stars.
