Novae possess a number of attributes that make them potentially valuable standard candles. They are luminous (approaching MV = -10) and easy to recognize. Because they belong to an old stellar population, they are found predominantly in ellipticals and the bulges of spirals (Ciardullo et al. 1987); such environments are relatively dust-free and photometrically smooth, so that observations of novae beyond the Local Group are simpler and easier to interpret than observations of Cepheids. The available evidence suggests that observations of novae are not strongly affected by metallicity effects (van den Bergh and Pritchet 1986). Finally, the calibration of novae as standard candles possesses relatively low intrinsic scatter (van den Bergh and Pritchet 1986), and is well understood theoretically (Shara 1981a, b).
The first known observations of novae in external galaxies were made in the early years of the twentieth century (Ritchey 1917, Shapley 1917). As noted by van den Bergh (1988), these observations held the potential for resolving the fundamental distance scale problems of that era, but failed to do so because of the confusion between novae and supernovae. The first systematic study of novae in an external galaxy was Hubble's (1929) survey of M31, a study that was greatly extended by Arp (1956); recent work on M31 has been summarized by Capaccioli et al. (1989). Other early observations of novae in Local Group galaxies are summarized by Pfau (1976). The first detection of novae beyond the Local Group was apparently by Hubble in the Virgo cluster elliptical M87 (Bowen 1952); Sandage (1987) has confirmed that Hubble's M87 nova candidates were probably real. More recent studies of novae in Virgo cluster ellipticals are by Pritchet and van den Bergh (1985a, 1987a). Sandage (1986) has also reported on observations of novae in the Sb spiral M81 made in the early 1950's.
All of the above observations of extragalactic novae were made in continuum blue light. More recently Ciardullo and collaborators have reported on H observations of novae in both M31 (Ciardullo et al. 1983, 1987, 1990a) and NGC 5128 (Ciardullo et al. 1990b). The relevance of H observations of novae to the extragalactic distance scale is discussed further in Sec. 5.6.