Barry Madore's long-term research activities have centered on the distances to nearby galaxies. However, after some 25 years of working with Cepheid variables as primary distance indicators (and most recently with HST and the Key Project on the extragalactic distance scale), his interest has refocused on a relatively new method: using the luminosity of the tip of the red giant branch (TRGB) as a distance indicator.

Unlike Cepheids, which are high-mass, high-metallicity, young (Population I) stars, the TRGB method uses some of the oldest stars of the low-metallicity Population II as its distance-determination candles. The most evolved members of the Population II giant branch derive their energy from an interior shell burning hydrogen, which surrounds an ever-growing degenerate helium core. The temperature of that isothermal core depends only on its mass, built up from the "helium ash" raining down upon it from the hydrogen-burning shell above. Very basic physics, however, predicts that the core mass has a hard upper limit because at a certain core mass the core temperature reaches a point at which helium will ignite. Because of the isothermal nature of the core, ignition occurs throughout the interior once the critical temperature (mass) is reached. At that moment, all pressure support for the core is derived from electron degeneracy, not temperature. The core therefore undergoes a thermonuclear runaway sufficiently intense to raise the temperature to where the electron degeneracy is lifted. And controlled helium core burning supplants the shell hydrogen burning as the primary support mechanism for the entire star. At this moment the red giant star abruptly terminates its upward evolution in luminosity and rapidly evolves to the helium-core-burning main sequence. This is observationally identified with the horizontal branch seen on globular cluster color-magnitude diagrams.

This abrupt transition leaves its mark. The so-called "tip" of the red giant branch is the observed result: the brightest red giant stars in the old Population II have a well-defined upper limit to their bolometric luminosities. This natural truncation in the total luminosity is closely tracked by constancy in the optical/infrared I-band luminosity. This wavelength regime is easily observed with CCD detectors available both on ground-based telescopes at Las Campanas and Palomar, for instance, and in orbit aboard the Hubble Space Telescope. With these facilities Madore, Wendy Freedman, and several postdocs have refined, calibrated, and applied the TRGB distance-determination method to galaxies as close as the Large Magellanic Cloud (50 Kpc) (using only a few minutes of observing time from the ground), and as distant as 10 Mpc with the HST, requiring a dozen orbits of integration.

Madore and his collaborators are mapping the detailed structure of the local universe (as probed by luminous matter) by combining data from Las Campanas out to 1-2 Mpc, and a more selected group of HST targets out to five Mpc. When combined with radial velocity information, this will lead to a better understanding of the local "geography" and the history and the dynamical evolution of the Local Group of galaxies and its immediate surroundings.