Earlier today I learned of the passing of Allan Sandage. Allan was a tremendously broad astronomer, who had a lasting impact on fields of astronomy stretching from stellar evolution to the largest cosmological issues. He is perhaps best known for his work on the distance scale, and measurements of the Hubble Constant, but he had equally significant contributions to our understanding of stars.
The prominence of his work on the Hubble Constant is in part due to the rather contentious history of this subject over much of the 90’s and early 2000’s. Allan was at heart a stellar astronomer, but one who found himself tied to Hubble’s legacy by virtue of being Hubble’s telescope assistant in the years leading up to Hubble’s unexpected death in 1953. As one of the earliest pioneers (with Martin Schwarzschild) of the technique of using main sequence turnoffs to assign ages to globular clusters, Allan was deeply (and understandably) bothered by experiments that returned large values of the Hubble Constant — these values implied ages for the universe that were younger than the oldest globular clusters, which was clearly an implausible contradiction. Instead, Allan and his collaborators published a long series of papers attempting to deal will every uncertainty and bias in the distance scale, and found a consistently smaller value of the Hubble Constant than the other competing team (the “Hubble Key Project”, led by Wendy Freedman with many collaborators). In time, Allan’s group’s on-going evaluations of the distance calibration gradually pushed their value of the Hubble Constant up somewhat, while the Key Project’s values were being nudged down a bit (although they never did actually meet, particularly as error bars shrank in more recent years). Simultaneously, the discovery of dark energy changed the age estimates for the universe, allowing old globular clusters to co-exist harmoniously with a moderate value of the Hubble Constant.
During this time, Allan developed a reputation for being, well, difficult. His scientific disagreements on this issue unfortunately veered occasionally into the personal. That said, I had the pleasure of being a postdoc at the Carnegie Observatories during this time, and had an office a few doors down the hall from him. Allan was invariably gracious and kind to the postdocs. He was scientifically engaged, and always willing to share his knowledge, which was both deep and wide. I enjoyed having him for a colleague for 4 years, during a very scientifically vibrant stage of my astronomical training, and I am very sorry to hear of his passing.