The use of units for physical quantities in the present volume follows largely the rules given by the International Astronomical Union (IAU), c.f. However, the centimetre-gram-second (CGS) system is used in some sections of the present volume, since it is still dominating in certain areas of astrophysics.

Contributions by: Arnold O. Benz, Eberhard Grün, Alan Harris, Peter Hoppe, Berndt Klecker, Irene Mann, Herbert Palme, Rita Schulz, S.K. Solanki, Tilman Spohn, Matthias Steffen, Michael Stix, Gian-Paolo Tozzi, Mario Trieloff, Klaus Wilhelm