- Paper Title: Allan R. Sandage, 18 June 1926 – 13 November 2010
- Authors: Donald Lynden-Bell & François Schweizer
- The Key to Stellar Evolution: As a pre-thesis project while he was a graduate student at Caltech, Sandage and a fellow student, Helmut Arp, worked on detecting the main sequence of stars in a globular cluster, M92. They used the 60-inch telescope at Mount Wilson observatory, and took all of their data on photographic plates. Given the success of this project, Sandage decided to work with Walter Baade on the color-magnitude diagram of M3, another globular cluster. The result of this project was the first instance of theoretical tracks of stellar evolution fitting data. Together with Martin Schwarzschild, who was a friend of Baade, Sandage realized how important his discovery was – with this information, they could use the main-sequence turnoff of clusters to determine stellar ages! (see figure at right)
- The Collapse of the Galaxy: Olin Eggen, a good friend of Sandage, had created a catalog of high transverse-motion stars. Using a new technique, Sandage and Eggen discovered that this population of stars were extremely metal deficient. In addition, unlike the higher metallicity stars in the solar neighborhood, which follow circular orbits around the galaxy, these stars had plunging orbits that took them through the plane of the disk and far into the halo. Together with Eggen and Lynden-Bell, Sandage wrote a paper surmising that these stars must have formed early on, when much of the Galaxy was gaseous. Because the gas clouds on these plunging orbits would have collided, unlike the stars, this indicated that the Galaxy must have formed via dynamical collapse, then setting off a flurry of star formation that would have formed most of the higher-metallicity disk stars. This paperis the single most cited work for each of the authors.
- Quasars and Quasi-Stellar Objects: One of the outstanding questions of the early 1960s was the nature of the strong radio sources being found by several groups. Sandage took the spectrum of a “star” that was coincident with one these sources, 3C 48, and found it to be unlike any spectrum that had ever been seen. It had many strong emission lines, but none of them were at wavelengths that made sense. After further monitoring, Sandage determined that the source must be a star, not a galaxy, because it varied on short timescales. Eventually Maarten Schmidt, a professor at Caltech, realized that the emission lines were the familiar Balmer series, but at a redshift of 0.158. The objects were named “quasars”, and Sandage later made a further discovery – that the radio-loud objects were actually just a small subset of a larger set of “quasi-stellar objects” (QSOs).
- Galaxy Classification and Surveys: While at Caltech, Sandage was a student of Edwin Hubble. When Hubble died in 1953, Sandage inherited his collection of photographic plates of nearby galaxies. Sandage used these plates, along with many others that he took at Palomar observatory over the next decade, to refine Hubble’s galaxy classification scheme of ellipticals, spirals, and irregulars (the source of the famous “tuning-fork diagram”). He decribed his revision in “The Hubble Atlas of Galaxies”, published in 1961. The Hubble Atlas made large-scale photographs of galaxies from the world’s largest telescopes available to all astronomers, enabling them to compare with the prototypes of the Hubble classification, and it remains one of Sandage’s most cited publications.
- The Centennial History of Mount Wilson Observatory: As one of the 20th century’s great observers, Sandage had spent countless hours at the Hale Observatory on Mount Wilson. He met and knew many of Mt. Wilson’s first generation astronomers, such as Hubble and Humason, and many of its second generation were still working there when Sandage joined in 1953. This made Sandage an ideal choice to write the Centennial History of the observatory. His 650-page volume details many of the important discoveries made at Hale, such as Michelson’s measurement of the velocity of light, Babcock’s measurement of the Sun’s magnetic field, and Adams’s detection of gravitational redshift in the spectrum of the white dwarf Sirius B. In addition, the history contains many anecdotes from nights observing on the mountain, the kind of stories that could only be shared by someone with Sandage’s intimate knowledge.
These are just a few of Allan Sandage’s many accomplishments. His determination of the Hubble constant, his role in the discovery of Type Ia supernovae as standard candles, and his work on cosmology are all equally notable. To read more, check out the article on the arXiv!