ABSTRACT
Galaxies are attractive objects to study. The magnetism of their natural beauty adds to the fascinating diversity of physical processes that occur over an enormous range of scales from the global dimension of order 10 kpc12 down to the viscous turbulent scales of 1000 km and less. The visual image of a galaxy (see Figure 7.1) is dominated by the optical light mostly produced by stars that contribute most of the visible galactic mass (2× 1011M for the Milky Way, where M = 2× 1030 kg is the mass of the Sun). A few percent of the galactic mass is due to the interstellar gas that resides in the gravitational field produced by stars and dark matter. Spiral galaxies are flat (Figure 7.1) because the stars and gas rapidly rotate. The gas is ionised by the UV and X-ray radiation and by cosmic rays; the degree of ionization of diffuse gas ranges from 30% to 100% in various phases – see Section 7.2.1. Interstellar gas is involved in turbulent motions that can be detected because the associated Doppler shifts broaden spectral lines emitted by the gas beyond their width expected from thermal motions alone. The effective mean free path of interstellar gas particles is small enough to justify a fluid description under a broad range of conditions. Altogether, interstellar gas can be reasonably described as an electrically conducting, rotating, stratified turbulent fluid – and thus a site of MHD processes discussed elsewhere in this volume, including various types of dynamo action.
