Even some of the earliest tentative steps out of our Milky Way galaxy and into the local Universe were guided by Cepheid variables. Indeed, the very first convincing demonstration of the size of our Universe rested on the identification by Edwin Hubble (1929) of Cepheid variables in the nearest galaxies NGC 6822, M31 and M33. Armed with empirical knowledge of the relationship between luminosity and period, discovered for Cepheids by Henrietta Leavitt only a few years earlier, Hubble used the Cepheids to set the scale size for galaxies themselves, and for the Universe in which they are expanding away from each other.
This series of lectures will detail the role played by Cepheids in the calibration of the extragalactic distance scale. In preparation for that application, the physics of Cepheids will be outlined and their empirical calibration will be presented. The properties of individual Cepheids varying through their pulsational cycle will be detailed, and the structure of the instability strip for the time-averaged properties of Cepheids will be discussed. Concerns about the metallicity sensitivity of the Period-Luminosity relation will be dealt with in a number of ways, from theoretical considerations acting as a guide, to direct tests and finally intercomparisons of independent distance determinations to serve as external checks. Means of determining distances freed from the effects of interstellar reddening and extinction will also be outlined.
Secure with the foundations provided by Cepheid variables and an application of their period-luminosity relation, we move on to the recent use of the Hubble Space Telescope in the rapid determination of distances to ``nearby'' galaxies useful for calibrating so-called ``secondary distance indicators'' needed to extend the distance scale out past the perturbing effects of the Virgo cluster and well out to cosmologically significant distances unaffected by peculiar motions and large-scale flows.
We digress slightly en route to highlight a new distance indicator that is fundamentally different, from and yet absolutely complementary to, the Cepeids as a distance indicator, the tip of the red giant branch (TRGB) method. The TRGB is well understood to mark the onset of helium core burning in low-mass Population II stars, and is independently calibrated through the RR Lyrae distance scale.