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Allan Sandage A Staff Member since 1952, Allan Sandage has received numerous awards, including the Peter Gruber Cosmology Prize, the Tomalla Prize from the Swiss Physical Society, the Crafoord Prize from the Swedish Academy of Sciences, and gold medals from the Royal Astronomical Society, the Astronomical Society of the Pacific, and the Pontifical Academy of Sciences. He has also received the Russell Prize from the American Astronomical Society, the Presidential National Medal of Science, and the Eddington Medal from the Royal Astronomical Society. In 1903, George Ellery Hale told the first trustees of the Carnegie Institution why an astrophysical observatory on Mount Wilson was necessary: it could advance the new sciences of origins and evolution, and "solve the problem of stellar evolution." Hale argued that by comparing the physical properties of the stars with those of the Sun, the solution to stellar evolution would "simply fall out." Although it did not quite work that way, the solution did emerge beginning in the 1940s, from data obtained principally at Mount Wilson. The contemporary ideas of how stars age, how our galaxy formed, and how the universe is arranged have now largely been solved through many long-range programs started at the Observatories decades ago. Hubble's discoveries in observational cosmology in the 10s and Baade's concept of stellar populations in the 1950s, for instance, have been integrated into a fabric that has become the present paradigm of astronomical origins. Sandage has been involved in astronomical origins and evolution since he joined Carnegie in 1952. His first result that year was the discovery made with Arp and Baum that the main sequence termination luminosity of the globular clusters M92 and M3 were about the same as the intrinsic luminosity of the Sun. The faintness of this termination point showed that stars of Baade's population II, which include the globular clusters, were extremely old. Determining the actual age has been a master problem that has occupied Sandage and others for the better part of the last half-century. Globular clusters, which date to the formation of our galaxy and are close to the age of the universe, are now estimated to be about 12 billion years old. This classic color-magnitude diagram reveals the evolution of stars in 10 clusters of different ages. Magnitudes of stars are plotted against their colors (or, luminosities against temperatures). The position of a star on this diagram depends on age as well as mass and chemical composition. Most stars lie along the "main sequence," which extends from the upper left (hot, bright, blue stars) to the lower right (cool, faint, red stars). Depending on their ages, stars "peel off" from the main sequence at different points (the higher the turnoff point, the younger the cluster). When these relationships were discovered by Sandage and others it revolutionized our ideas about stellar evolution. [Full size image.] The other means to date events in the universe is to determine the rate at which the universe expands and the galaxies move away from each other. Since the early 1950s, Sandage has investigated this problem and has been involved in the various aspects of establishing the distance scale to galaxies. With Dr. Gustav Tammann, at the University of Basel, and Dr. Abijit Saha, of Kitt Peak National Observatory, the scientists determined that the universe is expanding at a rate of 58 kilometers/second/megaparsec. Using this result, we can estimate the age of the universe to be about 14 billion years. The fact that these two timescales are so close to each other is one of the strongest proofs of the standard model of observational cosmology. SELECTED PUBLICATIONS Sandage, A. 1999. Bias properties of extragalactic distance indicators VIII: H0 From distance-limited luminosity class and morphological type-specific luminosity functions for Sb, Sbc, and Sc galaxies calibrated using Cepheids, Astrophys. J. 527, 479. Sandage, A. 1975. The redshift-distance relation VIII: magnitudes and redshifts of southern galaxies in groups: a further mapping of the local velocity field and an estimate of the deceleration parameter, Astrophys. J. 202, 563. Eggen, O. J., D. Lynden-Bell, and A. Sandage. 1962. Evidence from the motions of old stars that the galaxy collapsed, Astrophys. J. 136, 748. Sandage, A. 1961. The ability of the 200-inch telescope to discriminate between selected world models, Astrophys. J. 133, 355. Sandage, A. 1957. Observational approach to evolution III: semi-empirical evolution tracks for M67 and M3, Astrophys. J. 126, 326. PREVIOUS STAFF NEXT