In the past decade considerable progress has been made in understanding the systematics of the Cepheid distance scale. The amplitudes of individual Cepheids decrease at longer wavelengths. In addition, the width of the instability strip decreases as a function of increasing wavelength. By making use of these two observed properties, it has been possible (1) to reobserve known Cepheids efficiently and optimally, and (2) to optimize the methods by which new Cepheids are being discovered. Moreover, the ability to obtain multiwavelength data has made it possible to undertake corrections for reddening, and to begin to explore the residual effects of metallicity on the PL relation zero point.
Also in the past decade, a number of completely independent external checks of the Cepheid distance scale within the Local Group have been made using RR Lyrae stars, and also using the Pop II TRGB method. These nearby distances now agree to within ± 10% rms. Several programs are currently underway to increase the accuracy of the calibration of the Cepheid period-luminosity relation, as well as to test for potential remaining systematic effects in the zero point.
Cepheids have now been discovered out as far as the Virgo cluster and observations have just been completed for their discovery in the Fornax cluster galaxy NGC 1365; their implications are discussed below. As part of the H0 Key Project, Cepheid distances to approximately 20 galaxies will be obtained which will allow the calibration of 5-6 independent secondary methods, several of which can be applied at velocity distances out to or beyond 10,000 km/sec where the effects of peculiar flows are minimized. Independent, direct Cepheid distances will be measured to three galaxies in each of the Virgo and Fornax clusters. At the end of this program, there will be a solid basis for the intercomparison of secondary distance indicators and therefore for an accurate application to the problem of determining the size scale of the Universe and its absolute expansion rate, the Hubble constant.
We thank the various time allocation committees and observatories that have over the years supported our observational program into the systematic calibration and subsequent application of Cepheids to the extragalactic distance scale; these include Las Campanas, Palomar, KPNO, CTIO and the CFHT, and most recently, the HST. WLF acknowledges support from the NSF in the form of grants AST 87-13889 and 91-16496. Support for this work was also provided by National Aeronuatics and Space Administration (NASA) through grant number GO-2227 from the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. BFM thanks NASA for support through STScI, JPL, LTSA, and the NASA / IPAC Extragalactic Database (NED). We both also wish to thank our collaborators on the distance scale for their contributions over the past decade: R. Bernstein, J. Catanzarite, I. Horowitz, M.G. Lee, D. Murphy, E. Persson, D. Welch, C. Wilson, and the members of the HST Key Project team: R. Kennicutt, J. Mould, F. Bresolin, L. Ferrarese, H. Ford, B. Gibson, J. Graham, M. Han, P. Harding, J. Hoessel, R. Hill, J. Huchra, S. Hughes, G. Illingworth, D. Kelson, L. Macri, R. Phelps, S. Sakai, A. Saha, N. Silbermann, P. Stetson, and A. Turner. We acknowledge having had illuminating correspondence with Drs. M. Feast, A. Gould, N. Reid and F. van Leeuwen in the course of preparing these lecture notes.