In 1913, Niels Bohr published his "trilogy" on atomic structure that included a reconsideration of the Pickering–Fowler series as central evidence in support of his model of the atom.</ref> This series is named for Edward Charles Pickering, who in 1896 published observations of previously unknown lines in the spectrum of the star ζ Puppis (these are now known to occur with Wolf–Rayet and other hot stars). Pickering attributed the observation (lines at 4551, 5411, and 10123 Å) to a new form of hydrogen with half-integer transition levels. In 1912, Alfred Fowler managed to produce similar lines from a hydrogen-helium mixture, and supported Pickering's conclusion as to their origin. Bohr's model does not allow for half-integer transitions (nor does quantum mechanics) and Bohr concluded that Pickering and Fowler were wrong, and instead assigned these spectral lines to ionised helium, He+. Fowler was initially skeptical but was ultimately convinced that Bohr was correct, and by 1915 "spectroscopists had transferred Pickering–Fowler series definitively hydrogen to helium." Bohr's theoretical work on the Pickering series had demonstrated the need for "a re-examination of problems that seemed already to have been solved within classical theories" and provided important confirmation for his atomic theory.