Allan Sandage obituary

Influential American astronomer who helped calculate the size of the universe
Astronomer Allan Sandage readies the Hale telescope for a viewing
Sandage readying the Hale telescope for a viewing in California in 1974. Photograph: James P. Blair/National Geographic/Getty Images

The astronomer Allan Sandage, who has died aged 84, showed that the universe was at least six times larger than was previously thought. Sandage was active in discovering the properties of the quasar (quasi-stellar radio source) family of objects, whose light originates from material spiralling into black holes, and also showed how type 1A supernovae could be used as standard beacons to measure great distances. He led observational cosmology through the dark nights until the subject was transformed by the interpretation of the spatial fluctuations seen across the sky in the cosmic microwave background.

Unlike Edwin Hubble, who thought that his estimate of the time since all the galaxies overlapped was even less than the age of the Earth, Sandage believed that he was discovering the age of creation. Having studied under Walter Baade, he became Hubble's assistant. When Hubble died suddenly in 1953, it was on Sandage's shoulders that his cloak fell.

Almost at once, Sandage realised that many of Hubble's "brightest stars" in nearby galaxies were tight groups of bright stars, and this led to a doubling of the distances estimated to them. Milton Humason had become Hubble's chief observer of galaxy redshifts (the speeds at which galaxies move away as detected by the shifts of the lines in their spectra toward the red). In 1956, Humason, Nick Mayall and Sandage gathered the 800 known galaxy redshifts and magnitudes. Their preliminary rough estimate of Hubble's constant (the rate for the universe's expansion) was about 200km/sec/Mpc. Sandage's main work on refining distance measurements in astronomy had barely begun.

For his doctoral thesis he studied the stars in the globular cluster Messier 3 and the RR Lyrae variable stars it contains. Refinements in the accuracy of the distances to variable stars became a recurring theme of his work, as these were the first step in determining the ladder of distances through which the true scale of the universe was measured. For a variable star of known period, both its colour and its chemical composition can affect its intrinsic brightness, and the intervening interstellar dust can dim and redden its light. Distance accuracy only comes after detailed studies of how to compensate for such effects.

Sandage found good secondary distance indicators in the brightest galaxies in clusters and in supernovae. He attributed the idea of using supernovae as distance indicators to Fritz Zwicky, and through refinements, to which Sandage and Gustav Tammann contributed, type 1A supernovae have now become the standard candles for measuring the largest cosmological distances.

Sandage had studied the theory of stellar evolution under Martin Schwarzschild at Princeton in 1952, work which gained them the Eddington medal of the Royal Astronomical Society. The age of the globular clusters, determined by theory, provided a lower limit to the age of creation, and for much of Sandage's life this lower limit seemed uncomfortably old (estimates at times reached 18bn years) as compared with that timescale found from the rate of expansion of the universe.

Sandage worked with Tammann to refine distances and redetermine the Hubble constant. Their paper (May 2010) on the linearity of the Hubble flow revised Hubble's original value of his constant 530km/sec/Mpc to 62.3. This paper gives the most accurate relative distances of local structures in the universe, up to about five times the distance to the Virgo cluster of galaxies, but the current consensus, is that Hubble's constant is about 72km/sec/Mpc.

Sandage was born in Iowa City, Iowa. He took part in naval electronics during the second world war and studied physics and philosophy at the University of Illinois, graduating in 1948. He took one of the first PhDs in astronomy at the California Institute of Technology (Caltech) in 1953, and joined the Carnegie Observatories in Pasadena, California, in 1952.

He was elected to the National Academy of Sciences in 1963, but resigned when it failed to elect Olin Eggen, with whom he and I had collaborated on a paper on the formation of the galaxy. Eggen found that stars that lacked the heavier elements did not rotate around in the disc of the Milky Way but had orbits that dived through it. He was so often away observing that it was left to Sandage and me to discuss what this meant. As these were also the oldest stars, we thought that their diving motions were a legacy from the dynamical contraction that formed the galaxy from intergalactic gas. The methods used in our paper gave birth to the subject of galactic archaeology.

Although Sandage was the first to isolate and to take the spectrum of a quasar, 3C48 (number 48 in the third Cambridge catalogue), he was unable to interpret its spectrum. For help, he turned first to Ira Bowen and then to Jesse Greenstein. Meanwhile, he monitored the light of the "radio star" and showed that it varied over a few months. When Maarten Schmidt later showed that 3C273 had a large redshift, and, with Greenstein, finally understood and published the spectrum of 3C48, Sandage felt excluded. Later, Sandage found that there are many more radio-quiet quasi-stellar objects (QSOs) than there are quasars, a discovery that contributed to the current view that most large galaxies contain giant black holes (dead QSOs) in their nuclei.

Sandage's Hubble Atlas of Galaxies (1961) was a guide to many young observers and theorists, and his definitive Revised Shapley-Ames Catalogue (1981), written with Tammann, laid out the basic measurements of the bright galaxies. He was awarded the Crafoord prize in 1991, was the joint recipient of the inaugural Gruber prize in cosmology in 2000, and was a foreign member of the Royal Society. He is survived by his wife, Mary, and their two sons.

Allan Rex Sandage, astronomer, born 18 June 1926; died 13 November 2010