Magnetohydrodynamics problems in the interstellar medium

Many defining features of galaxies and their evolution are controlled by th eir interstellar gas. From 20% to 50% of the interstellar gas in the Milky Way is in molecular clouds, which are also the sites of star formation. Mol ecular clouds are turbulent, and magnetic fields are strong enough that the turbulence is believed to be magnetohydrodynamic in character. Analytical and numerical estimates suggest that the turbulence would be dissipated in less than the lifetime of the cloud if it were not driven. A small fraction of the mass in molecular clouds fails to be supported against self-gravity and collapses to form stars; star formation also is strongly influenced by magnetohydrodynamic effects. (C) 1999 American Institute of Physics. [S107 0-664X(99)90805-4].