Introduction

The subject of astrophysics involves the application of the laws of physics to large macroscopic systems in order to understand their behaviour and predict new phenomena. This approach is similar in spirit to the application of the laws of physics in the study of, say, condensed-matter phenomena, except for the following three significant differences:

1. (1) We have far less control over the external conditions and parameters in astrophysics than in, say, condensed-matter physics. It is not possible to study systems under controlled conditions so that certain physical processes dominate the behaviour. Identifying the causes of various observed phenomena in astrophysics will require far greater reliance on statistical arguments than in laboratory physics.

2. (2) The astrophysical systems of interest span a wide range of parameter space and require inputs from several different branches of physics. Typically, the densities can vary from 10-25 gm cm-3 (interstellar medium) to 1015 gm cm-3 (neutron stars); temperatures from 2.7 K (microwave background radiation) to 109 K (accreting x-ray sources) or even to 1015 K (early universe); radiation from wavelengths of meters (radio waves) to fractions of angstroms (hard gamma rays); typical speeds of particles can go up to 0.99c (relativistic jets). Clearly we require inputs from quantum-mechanical and relativistic regimes as well as from more familiar classical physics.

3. (3) […]