Ken Freeman (born August 27, 1940, Perth, Western Australia, Australia) is an Australian astronomer known for his work on dark matter and the structure and evolution of the Milky Way Galaxy.
Freeman received a bachelor’s degree in mathematics (1962) from the University of Western Australia in Perth and a doctorate (1965) in applied mathematics and theoretical physics from the University of Cambridge. He was a research fellow at Cambridge from 1965 to 1969 and held a postdoctoral fellowship at the University of Texas (1966). In 1967 he became a research fellow at the Mount Stromlo and Siding Spring Observatories of the Australian National University. He spent the rest of his career there, becoming Duffield Professor of Astronomy in 2000.
Freeman’s most cited work, “On the Disks of Spiral and S0 Galaxies,” appeared in 1970. He studied the rotation of 36 spiral galaxies and found that 2 seemed to have more mass than could be explained by the amount of starlight observed. Thus, those galaxies consisted of stars and some unseen component of “dark” matter. (Similar results were found at the same time by American astronomers Vera Rubin and W. Kent Ford.) Prior to the work of Freeman, Rubin, and Ford, the existence of such dark matter had been an open question ever since it was first postulated in 1933. Dark matter makes up 26.5 percent of the matter-energy content of the universe. (Ordinary visible matter constitutes only 0.5 percent.)
Freeman’s later work centred on a collaboration that began in 1988 with Australian astronomer Joss Bland-Hawthorn. In their paper “The New Galaxy: Signatures of Its Formation” (2002), they described the field of “galactic archaeology,” in which obtaining accurate velocities, positions, and chemical compositions of many individual stars in the Milky Way would lead to a better understanding of how the galaxy formed. In that paper Freeman and Bland-Hawthorn predicted that the combination of data from Gaia—a European Space Agency satellite designed to provide highly accurate position and velocity measurements for one billion stars that was launched in 2013—and from a project such as the Galactic Archaeology with HERMES (GALAH) survey would answer many of the questions in galactic archaeology. Freeman and Bland-Hawthorn became principal investigators on GALAH—which began in 2014 and would use the HERMES spectrograph on the Anglo-Australian Telescope at Siding Spring to collect high-resolution spectra of one million stars.
Freeman was honoured with the Australian Prime Minister’s Prize for Science (2012) and the Henry Norris Russell Lectureship of the American Astronomical Society (2013). He was made a fellow of the Royal Society in 1998.