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PLASMA ASTROPHYSICS 124 ciations of massive stars. Because mean free paths are long, these shocks must be collisionless. Remote sensing by spectroscopy shows that electrons as well as ions are heated to high temperatures. How is the ion distribution thermalized? How is this energy fed into the electrons? How is a small tail of particles accelerated to high energies, as is observed in the interplanetary medium? What is the back-reaction of the accelerated particles on the shock? These remain outstanding problems, because the Mach numbers are so high that the shocks are probably turbulent. Numerical simulations appear to be the most promising way to attack the problem at this point. Acceleration of Particles to High Energies Spiral galaxies appear to be permeated by a component of energetic particles, cosmic rays. In our galaxy the distribution function can be followed from subrelativistic energies to energies as high as 1021 eV. The most energetic particles cannot be confined by the galactic magnetic field. The energy density of these cosmic rays is similar to both the magnetic and the turbulent energy density in the galactic disk. How are these particles accelerated, and how do they propagate through the galaxy? The prevailing theories have particles at energies less than about 1015 eV accelerated by the Fermi mechanism in shocks and predict that they will be trapped within the galaxy by resonant scattering off AlfvÃ©n waves excited by their own anisotropy. For more energetic particles, the confinement is problematic, and the origin may be extragalactic. Hydromagnetic Turbulence There is abundant evidence for hydromagnetic turbulence in objects as diverse as stellar convection zones, the interstellar gas in galaxies, and the gas in clusters of galaxies. Turbulence can provide hydrodynamic forces (e.g., pressure support in interstellar clouds or acceleration in stellar winds), can lead to transport coefficients such as viscosity or resistivity that are much larger than their molecular values, and can provide significant heating through dissipation. Yet, we do not have a complete theory of hydromagnetic turbulence, and simulations, which are of great educational value, do not yet resolve the full range of relevant scales. Progress in understanding MHD turbulence will probably be made through a combination of direct observation (such as in situ measurements in the solar wind), simulations, and analytical theory. Magnetic Reconnection The magnetic Reynolds number (or Lundquist number) of astrophysical plasmas is typically huge, ranging from 108 in the solar interior to 1021 in the galactic interstellar medium. The naive conclusion is then that magnetic flux is perma