The Saha Equation Turns Hundred

Four years after joining the University College of Science in Calcutta as a lecturer in 1916, Meghnad Saha submitted a paper entitled ‘Ionization in the solar chromosphere’ to the Philosophical Magazine. This was quickly followed by three more, the last of which, ‘A physical theory of stellar spectra’ appeared in 1921 in the Proceedings of the Royal Society of London, forwarded by A. Fowler, Saha’s host at Imperial College, London. Fowler suggested many improvements on Saha’s original draft made in Calcutta. These papers were based on the concept of thermal ionization. At the high temperatures found in stars, some electrons can be removed from atoms. The precise ratio between the numbers of neutral atoms and the various ions is given by an equation which, ever since, has borne Saha’s name. This centenary provides an excuse to revisit the scientific background to this work, its immediate impact, and the aftermath.