Some early theories of stellar evolution

At the end of the nineteenth century the two main branches of stellar spectroscopy were spectral classification and radial-velocity measurements. The latter department was still in its relative infancy, but classification, thanks mainly to the energy of Pickering at Harvard, was a major activity. Classification had become closely related to theories of stellar evolution and these two aspects could hardly be disentangled in, for example, the classification devised by Lockyer [1], which involved first rising and then falling temperatures of stars over their life cycles, and which had some theoretical support from the work of Jonathan Homer Lane (1819–80) and August Ritter on the gravitational collapse of gaseous spheres.

The classification schemes used by Antonia Maury and Annie Cannon were also implicitly evolutionary theories, but with the direction of evolution being from the ‘earlier’ to ‘later’ spectral types. Rival theories to Lockyer's were then proposed for stellar evolution in the early years of the nineteenth century, with the Harvard spectral types as their basis, notably by Sir Arthur Schuster (1851–1934) in Great Britain [2] and by George Ellery Hale (1868–1938) in the United States [3]. Schuster's scheme involved gravitational collapse and cooling of gaseous masses on the socalled Kelvin–Helmholtz timescale.