The open cluster NGC 6791 is now considered both the oldest and the most metal-rich known. Its age is 8 -10 Gyrs, twice as old as the canonical solar-metallicity cluster M67 (Garnavich et al. 1994; Demarque, Green, & Guenther 1992; Tripicco et al. 1995). That its metallicity is significantly above solar is suggested from moderate-resolution spectroscopy and from a mismatch of its color-magnitude diagram (CMD) with solar-metallicity isochrones. Tripicco et al. (1995) find [Fe/H] = +0.27 to +0.44. The cluster population is rich. In addition to about a dozen red giants and two dozen red horizontal-branch stars, the cluster has several very hot HB stars (Kaluzny & Udalski 1992). Liebert et al. (1994) have shown that the extremely blue stars are mostly sdB/sdO stars and at least 3 or 4 are likely cluster members, the first ever discovered in an open cluster. These may provide the key to the puzzling upturn in ultraviolet flux below 1500A seen in many high-metallicity systems (Burstein et al. 1988; Ferguson et al. & Liebert 1993).

Observations of distant supernovae have provided startling evidence that the expansion of the Universe may be accelerating, rather than decelerating. If this result is verified by future studies, it has profound implications for cosmology. The reliability of this finding and its implications for both the study of supernovae and cosmology are the subject of this exciting volume. Based on a conference at the University of Chicago, this timely volume, originally published in 2000, presents articles by leading experts on the theory of Type Ia supernovae, observational astronomy, and cosmology. It examines the observational data, the nature of the likely progenitor binary systems, the outburst mechanisms of Type Ia supernovae events, and the cosmological implications. This is a unique and wide-ranging review of one of the most dramatic and controversial results in astronomy in recent decades. It makes fascinating reading for all researchers and graduate students.

The open cluster NGC 6791 is now considered both the oldest and the most metal-rich known. Its age is 8 -10 Gyrs, twice as old as the canonical solar-metallicity cluster M67 (Garnavich et al. 1994; Demarque, Green, & Guenther 1992; Tripicco et al. 1995). That its metallicity is significantly above solar is suggested from moderate-resolution spectroscopy and from a mismatch of its color-magnitude diagram (CMD) with solar-metallicity isochrones. Tripicco et al. (1995) find [Fe/H] = +0.27 to +0.44. The cluster population is rich. In addition to about a dozen red giants and two dozen red horizontal-branch stars, the cluster has several very hot HB stars (Kaluzny & Udalski 1992). Liebert et al. (1994) have shown that the extremely blue stars are mostly sdB/sdO stars and at least 3 or 4 are likely cluster members, the first ever discovered in an open cluster. These may provide the key to the puzzling upturn in ultraviolet flux below 1500A seen in many high-metallicity systems (Burstein et al. 1988; Ferguson et al. & Liebert 1993).