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Published online by Cambridge University Press: 07 August 2017
In 1976 Georgelin et al.(1976) combined optical and radio observational data of HII regions to study the structure of our Galaxy. The distance of each HII region can be obtained from its radio continuum flux caused by free-free emission and the flux of the Balmer lines with high accuracy. The results clearly show three arms and are used to draw the spiral structure of our Galaxy. It is commonly accepted now.