Allan Sandage, one of the greatest astronomers of the twentieth century, has died at the age of 84. Sandage, who worked with Edwin Hubble as an assistant at Mount Wilson Observatory, 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.

In the 1920s and 1930s, Edwin Hubble had become famous for discovering that the spiral nebulae were in fact galaxies beyond ours, and that the Universe was expanding. As Hubble’s protegé, 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 and introduced unprecedented accuracy into the measurements of the Universe, continuing the tradition of showing how what we think we know is merely the tip of the iceberg.

He graduated from the University of Illinois in 1948, and achieved a PhD from Caltech in 1953 under the tutelage of Walter Baade, while at the same time working as an assistant to Hubble at Mount Wilson. In 1950 Baade had discovered that Hubble’s estimate of the distance to the Andromeda Galaxy, 700,000 light years, was half too small, based on the identification of Cepheid variables in Andromeda. After Hubble’s passing in 1953, Sandage continued his mentor’s research.

In 1958 Sandage won the American Astronomical Society’s Warner Prize for outstanding achievement by a young astronomer, based on his work on the physics and evolution of stars, allowing him to develop a method of calculating the ages of stars that have left the main sequence of the Hertzsprung–Russell diagram. At the awards ceremony, in front of the great and the good of American astronomy, he presented revelatory new results: using the Hale Telescope, he had re-worked Hubble’s distance measurements to the galaxies having found several miscalculations in them (not of any fault of Hubble’s – he was limited by the telescopes of the time). Hubble had misidentified various objects in several distant galaxies, such as globular clusters and bright knots of star-forming gas, as supergiant stars with which he had estimated distances based on their brightness. Hubble had known this might prove to be a problem – from a distance a globular cluster of hundreds of thousands of stars can look like one massive star. Consequently Hubble’s distance ladder was scaled incorrectly, explained Sandage. Sandage showed that the Andromeda Galaxy was two million light years away. Hubble had estimated that the Virgo Cluster was seven million light years distant; Sandage proved that it was more like 50 million light years.

And so on. Literally overnight Sandage had expanded the Universe to unimaginable scales, just as Hubble has done before him, and this resulted in a more accurate assessment of Hubble’s Constant, which is the measure of the expansion of the Universe, of 75 kilometres per second per megaparsec (today’s best measurements give the Hubble Constant as 71 kilometres per second per megaparsec, showing how close Sandage was). This allowed him to estimate the age of the Universe as 15 billion years, not too far away from today’s established age of 13.7 billion years.

Sandage, working at the Carnegie Observatories in California, continued his efforts to hone the Hubble Constant, in his latter years using the Hubble Space Telescope to observe extragalactic Type Ia supernovae as standard candles. He also discovered the starburst activity and black hole jets in the galaxy M82, and published the comprehensive Hubble Atlas of Galaxies, in 1961. He continued to research and publish papers right up until his death, his last paper on RR Lyrae variable stars (another kind of standard candle) appearing in the Astrophysical Journal in June. A true giant of astronomy, he published more than 500 scientific papers in total throughout his career.