3. CLASSICAL CEPHEID VARIABLE STARS

Classical Cepheid variable stars are potentially the most accurate distance indicators available for the near-field (0.5 kpc - 10 Mpc). Their suitability as distance indicators arises from a number of factors including luminosity (M_V = -2 to -7 mag), ease of detection through variability, precision, permanence, and understanding of the pulsation phenomenon. The classical (or Type I) Cepheids are young, disk objects and as such are only found in galaxies with recent star formation (primarily spiral or irregular galaxies).

There is a vast literature on Cepheid variables as distance indicators and a full historical review is beyond the scope of this paper. Fernie (1969) gives a thorough and entertaining discussion of the early years. Other useful reviews of Cepheids as distance indicators are found in Feast and Walker (1987), Freedman (1988a), Walker (1988), and Madore and Freedman (1991).

The advantages of modern detectors for Cepheid work are legion and a number of known extragalactic Cepheids have been re-observed with CCDs and IR arrays. However, few discovery surveys have made it into print at this writing. Rather than discuss such surveys on a case by case basis, we summarize the observational papers on extragalactic Cepheids in Table 1 (excluding LMC and SMC work). Future surveys are likely to be tied to specific calibration needs such as the luminosity-line width relations or supernovae.

The sequence of operations required to find and use Cepheids for estimating distances is the same for any region of the visible spectrum, and we describe this process in the next section. We then discuss the tradeoffs between optical and near-infrared photometry.