Introduction

Although no two novae show exactly the same properties or development during eruption, there are systematics in the overall population of novae that assist in the interpretation of the physics underlying their behaviour and their relationship to other systems of stars. Furthermore, some of these properties are of value in other areas, such as distance indicators for extragalactic research. Here we give a general introductory overview of the general properties of novae, derived largely from ground-based observations; later chapters add more detail and extend the wavelength range.

Frequency and Galactic distribution of novae

The frequency of classical nova discoveries over the past century, corrected for non-uniformity of coverage in time, is shown in Table 2.1 (Duerbeck, 1990). In the range 4 ≤ mv ≤ 6 the values do not increase as fast as expected, showing that many bright novae go undetected. At fainter magnitudes the increase is due to the contribution of novae in the Galactic bulge. The total mean detected nova rate is ∼3 yr-1. There are so many factors that lead to incompleteness in nova searches (seasonal, weather, sky coverage biased towards the Galactic bulge, missed fast novae) that it is possible to conclude that Table 2.1 shows the few novae that were actually detected from an observable number of ∼12 yr-1 (Liller & Mayer, 1987).