W.N. Christiansen and the initial Australian investigation of the 21cm hydrogen line

Abstract

On 25 March 1951 H.I. Ewen was working on his doctoral thesis at Harvard University when he detected the 21cm hydrogen emission-line (H-line). Within four months of the initial detection, small groups working in Australia and in The Netherlands were able to confirm Ewen’s detection, thereby heralding a new chapter in international radio astronomy. This paper examines the Australian efforts that culminated in the confirmation of the H-line detection, and led to an initial survey of the southern Milky Way which produced the first indication of the spiral arm structure of our Galaxy.