These lectures will first concentrate on our developing understanding of the very basic principles characterizing the variability seen in individual Cepheids, as well as the systematics and trends observed for Cepheids as an ensemble. The emphasis will be on Cepheids as accurate extragalactic distance indicators, and will only touch upon the debate concerning the exact value of the zero point later, when the recent results from the Hipparcos satellite are discussed. Previous work on the zero point of the Cepheid period-luminosity relation is adequately covered in recent commentaries by Feast & Walker (1987), Walker (1988) and Schmidt (1991), all of which suggest that there is a convergence of opinion at the level of about ± 0.10 mag. As optimistic as these reports are, readers are still referred to the cautioning remarks by Turner (1990).
We begin by presenting a physical basis from which to view Cepheids as distance indicators. (However, for dissenting views on this whole process, interested readers are referred to Clube & Dawe 1980, and to Stift 1982, 1990). We then go on to discuss the difficulties and uncertainties caused by reddening and metallicity effects. Following sections put into perspective the explicit determination of reddening made possible by employing panoramic and long-wavelength detectors. We then review the status of the Cepheid-based distances to Local Group galaxies and then link it to those beyond the Local Group being observed by the Hubble Space Telescope. The impact of these distances on a determination of the Hubble constant concludes the lectures. Later sections look at the differences between the PLC and the PL relation, discuss an implicit method for dealing with reddening, and finally, examine the prospects for determining reddenings to individual Cepheids.
For historical notes on Cepheids as distance indicators the interested reader is referred to the monographs by Fernie (1969), Sandage (1972, 1988a), Stothers (1983), Walker (1988), Madore (1986) and Tanvir (1997) and references therein. Other, more recent reviews of Cepheids in the context of the extragalactic distance scale can be found in Madore (1985), Freedman (1988), Feast & Walker (1987), Madore & Freedman (1991), Jacoby et al. (1992), and Freedman & Madore (1996). A review by Hodge (1981) on the extragalactic distance scale is itself especially relevant in as much as many of the major topics of concern raised by him regarding the Cepheid PL relation have now largely been addressed at least by direct observations. The evolutionary status of Cepheids is most recently reviewed by Chiosi (1990), while Simon (1990) gives a detailed look at the convergence of techniques used to calibrate the Galactic PL relation, and the confrontation of these observations with pulsation theory. And finally, Chiosi et al. (1993) have published theoretical PL relations which they then map to both optical and near-infrared wavelengths in a modern attempt to illucidate the effects of metallicity on those zero points.