Allan Sandage

Allan Rex Sandage (June 18, 1926 – November 13, 2010) was an American astronomer who spent his career as Staff Member Emeritus at the Carnegie Observatories in Pasadena, California. He is credited with producing the first reasonably reliable measurements of the Hubble constant and the age of the universe.

Born in Iowa City, Iowa, Sandage earned his undergraduate degree from the University of Illinois in 1948 and a PhD from the California Institute of Technology in 1953, where his advisor was Walter Baade. As a graduate student, he worked as an assistant to Edwin Hubble and took over Hubble’s research program after Hubble’s death in 1953.

In 1952, Baade announced that the Andromeda Galaxy contained two distinct populations of Cepheid variable stars, which doubled the estimated age of the universe from 1.8 to 3.6 billion years. Sandage then demonstrated that earlier assumptions about the brightness of the brightest stars in galaxies were wrong—specifically, that H II regions, previously mistaken for stars, are actually much brighter. This correction pushed the universe’s estimated age up to about 5.5 billion years.

From the 1950s through the 1980s, Sandage was considered the leading observational cosmologist, working on every part of the cosmological distance scale—from nearby calibrators in the Milky Way to very distant galaxies. Based at Palomar Observatory, he published the first solid estimate of the Hubble constant in 1958, revising Hubble’s value of 250 down to 75 km/s/Mpc, close to today’s accepted figure. Later, he argued for an even lower value of around 50, which implied a Hubble age of roughly 20 billion years. At the time, Sandage and many others believed the cosmological constant was zero, so a low Hubble constant was needed to keep the universe’s age at least as old as its oldest objects—about 14 billion years.

Sandage also studied globular clusters photometrically, calculating their age at a minimum of 25 billion years. This led him to propose that the universe might not just expand but instead expand and contract in cycles lasting 80 billion years. (Current estimates put the universe’s age at around 14 billion years.) Along with Olin J. Eggen and Donald Lynden-Bell, he co-authored the influential ELS paper describing how a proto-galactic gas cloud collapsed to form the Milky Way; he defended that paper again in 1990.

In 1961, his paper “The Ability of the 200-inch Telescope to Discriminate Between Selected World Models” argued that observational cosmology would boil down to measuring two parameters: the Hubble constant H0 and the deceleration parameter q0. That work shaped the field for decades. He also published two galaxy atlases, in 1961 and 1981, based on Hubble’s classification scheme. In 1962, he explored whether the redshift of extra-galactic sources could be measured over time—an idea now known as the Sandage–Loeb test. He discovered jets erupting from the core of the Cigar Galaxy, likely caused by massive explosions that have been ongoing for at least 1.5 million years.

Sandage was extraordinarily productive, publishing over 500 papers. He remained active at the Carnegie Observatories until his death, still releasing several papers each year.

In 1959, he married fellow astronomer Mary Connelley; they had two sons, David and John. In 1983, Sandage announced he had become a Christian and began writing about religion and science. He died of pancreatic cancer on November 13, 2010, at his home in San Gabriel, California, at age 84.

His honors include the Helen B. Warner Prize for Astronomy (1957), election to the American Philosophical Society (1962) and the National Academy of Sciences (1963), the Eddington Medal (1963), the Gold Medal of the Royal Astronomical Society (1967), the National Medal of Science (1970), the Henry Norris Russell Lectureship (1972), the Elliott Cresson Medal (1973), a fellowship in the Royal Society, the Bruce Medal (1975), the Golden Plate Award of the American Academy of Achievement (1976), the Crafoord Prize (1991), and the Gruber Prize in Cosmology (2000). The main-belt asteroid 9963 Sandage (1992 AN) is named after him.