This and the following two chapters present a general introduction to the subject of observing magnetic fields in stars using spectropolarimetry. The three chapters will consider (1) the basic physics of the Zeeman and related effects, direct deductions about stellar magnetic fields that may be made where this effect is detected, and how such measurements have provided much information about the Sun and other magnetic stars; (2) how spectropolarimetric observations of magnetic stars may be modelled, and how such techniques may be used to extract much detailed information about the stellar magnetic field and other characteristics of the magnetized stellar atmosphere; and (3) how other magnetic effects such as the Hanle effect and continuum polarisation may be used to detect fields in a variety of stellar types such as white dwarfs, and how the current body of observational knowledge about magnetic stars may be integrated into a roughly coherent, provisional scenario of field origin and evolution.
In this chapter we survey the physics of an atom in a magnetic field (the Zeeman effect and its relatives), and identify ways in which magnetic effects on the atomic energy level structure may be used to detect and characterise stellar magnetic fields. We survey some of the basic features of magnetism in the Sun. We then describe how the available data may be used to deduce simple averages of various magnetic quantities over the stellar surface (for example, the “mean longitudinal field”) and how such simple field measurements are used to develop and explore the “oblique (dipole) rotator model” of magnetism for a group of middle main sequence stars called the “magnetic Ap stars”.