Allan Sandage
Allan Sandage was an American astronomer born in 1926 who died in 2010. Sandage worked with Carnegie Observatories in Pasadena, and is distinguished for approximating the Hubble constant and the age of the universe.
Sandage's influence is pervasive. After graduating from the University of Illinois, he obtained his Ph.D. from Caltech, advised by Walter Baade, a German astronomer. He actually was a grad student assistant of Edwin Hubble, perhaps influencing his work related to Hubble's constant. He continued in Hubble's footsteps, carrying on his research program after Hubble's death. Baade then discovered two populations of Cepheid variables in the Andromeda Galaxy. Since Hubble had only considered one of the populations, the estimate of the age of the universe doubled from 1.8 to 3.6 billion years. Sandage demonstrated that H II regions do not comprise stars and are inherently brighter than stars in distant galaxies, contrary to the assumption that the brightest stars in galaxies are of equal intensity. This increased the age of the universe yet again to 5.5 billion years.
From the 1950s to the 1980s, Sandage was the premier observation cosmologist (study of the universe). He used a variety of methods for measurement, from local indicators in the Milky Way to extremely distant galaxies. In 1958 Sandage started to work at the Palomar Observatory. It was from there that he published the first reasonable estimate of the Hubble constant, the ratio of the recession velocity of a distant object to its "proper distance." Sandage's value was 75 km/s/Mpc, much lower than Hubble's 250 km/s/Mpc. This is quite impressive, considering one 2011 study estimated it at 73.8 km/s/Mpc. However, he later decreased his estimate even further to 50 km/s/Mpc, corresponding to a Universe age of 20 billion years!
Sandage utilized photometry on globular clusters, coming to the conclusion that they were about 25 billion years old. Accordingly, he concluded that the Universe expanded and contracted with a period of 80 billion years. However, most estimates place the Universe as 14 billion years old. He co-wrote an extremely influential paper with Eggen and Lynden-Bell on the collapse of a proto-galactic gas cloud into the Milky Way. Sandage wrote a paper called "The Ability of the 200-inch Telescope to Discriminate Between Selected World Models" wherein he described observational cosmology as essentially a search for the Hubble constant and the deceleration parameter q0. He basically created a blueprint for how to use a large telescope to perform observational tests. He published an atlas of galaxies that employed the Hubble classification scheme in 1961 and again in 1981.
He devised a way to measure the temporal variation of extra-galactic redshift, which became known as the Sandage-Loeb test. He also discovered jets erupting from the core of the M82 galaxy, which had been occurring for 1.5 million years. He stayed at Carnegie Observatories up until his death, having published over 500 papers in his life.
