Allan Sandage: Astronomer widely acknowledged as among the most outstanding of the 20th century

Allan Sandage, widely regarded as one of the greatest and most influential astro-nomers of the 20th century, played an integral part in increasing our understanding of the scale of the universe and determining the Hubble Constant, which describes the universe's expansion.

Sandage, a self-described "hick who fell off the turnip truck", was a prolific writer and researcher whose work covered the evolution and behaviour of stars, the birth of the Milky Way, the age of the universe and the discovery of the first quasar, in addition to his crucial work on the Hubble constant.

He pursued the latter with his long-standing Swiss collaborator, Gustav Tammann of the University of Basel. This lifelong quest brought him into conflict with many in the space community who believed the pair had overestimated the distances to other galaxies – a crucial part of the equation for the constant – making the universe appear bigger and older than it really was. The universe, they said, was really about 10 billion years old.

An only child, Allan Rex Sandage was born in 1926, in Iowa City, Iowa. His father was a professor of business studies while his mother ran the home. As a boy Sandage enjoyed watching the Ohio skies through a friend's telescope. When his father gave him his own telescope he began keeping a journal of sunspot activity.

He enrolled at Miami University but after two years was drafted into the US Navy, serving during theSecond World War as an electronics specialist. After the war he resumed his studies at the University of Illinois, earning a physics degree in 1948and a doctorate from California Institute of Technology in 1953, where he studied stellar evolution for his PhD under the celebrated astronomer Walter Baade.

Sandage joined Pasadena's Carnegie Observatories in 1949 while still a graduate student, assisting Edwin Hubble at the Mount Wilson and Palomar observatories. It was there in the 1920s that Hubble had established that the universe is larger than the Milky Way and is expanding. In 1952, Sandage joined the Carnegie staff and at the age of only 27, following Hubble's death in 1953, he inherited Hubble's work, heading the development of the cosmology programme. This involved determining the Hubble Constant, which indicates rate at which the universe is expanding and thus lets us estimate the age of the universe.

As Hubble's protégé, Sandage never came close to achieving his mentor's celebrity status, nor did he try to, but while Hubble set the ball rolling it was Sandage who corrected his mistakes, which arose from the limitations of the telescopes of the time, and introduced unprecedented accuracy into the measurements of the universe. When Sandage began his work the best estimates for the age of the universe were about 1.8 billion years. In 1958, he published the first good estimate for the Hubble constant, revising his mentor's value of 250 kilometres per second per megaparsec down to 75. In 2001, using the Hubble Space Telescope, a team led by Dr Wendy Freedman showed that the Hubble Constant is about 71 km/s/mpc, underlining how close Sandage was. This allowed him to estimate the age of the universe at 15 billion years, not too far away from today's established age of 13.7 billion years.

Later, however, also using the Hubble Telescope, Sandage became the chief advocate of an even lower value, of around 50 km/s/mpc, which would put the age of the universe closer to 20 billion years. His disputes with the scientific community over the age of the universe were often so heated that they were sometimes known as the "Hubble Wars." Sandage regularly noted that the community at large had, until recently, used much larger numbers than 71 and had gradually been coming closer to his own preferred value. His assumption up until his death was that eventually everyone else would agree with him.

He was known for his very strong opinions, one science journalist noting, "You weren't anybody in astronomy if Sandage hadn't stopped talking to you at one time or another." We may never know the fate of the universe or the Hubble Constant, he once said, but the quest and discoveries made along the way were more important and rewarding than the answer anyway.

Sandage was also successful in a number of other areas. In the 1950s, radio surveys of the sky by astronomers at Cambridge University found a number of compact radio sources rather than the broad sources previously observed. In 1960, Sandage and Thomas Matthews used the 200-inch Hale Telescope to identify one of those sources, a faint star-like object whose spectrum was found to be unlike that of any other star. Other astronomers eventually showed that these quasi-stellar objects, or quasars, are not in our own galaxy, but are located at the farthest reaches of the universe.

Quasars are the brightest and most distant objects in the known universe. Sandage found a way to discover them by searching for objects that emit an excessive amount of ultraviolet or blue radiation, the so-called blue stellar objects. Most of these are not radio emitters, but can be classified as quasars because of their very large red shift, which places them very far away.

He also led the first major survey of the distances of other galaxies from the Milky Way, which he used to create a three-dimensional map to explore galaxy distribution and the dynamics of the nearby universe.

Although he officially retired in 1997, Sandage was still working until July this year, when a paper on variable stars was published in the Astrophysical Journal. His tireless and prolific work yielded honorary degrees from Yale University, University of Illinois, University of Chicago, University of Southern California, Miami University, Graceland College and the University of Chile.

He also received numerous awards, including the Eddington Medal of the Royal Astronomical Society in 1963; the Pope Pius IX Gold Medal in 1966; the Elliot Cresson Medal of The Franklin Institute in 1973; the US's highest scientific award, the National Medal of Science, in 1971; the Crafoord Prize of the Swedish Royal Academy of Sciences in 1991, the closest thing to a Nobel Prize for astronomy; and the Cosmology Prize of the Peter Gruber Foundation in 2000.

Allan Rex Sandage, cosmologist: born Iowa City, Iowa 18 June 1926; married 1959 Mary Connelly (two sons); died San Gabriel, California 13 November 2010.

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