to pile up until radiation and gravitational forces almost cancelled each other, then its isotopes would separate, sorting themselves. Michaud suggested that element segregation would proceed naturally due to gravitational settling and radiation pressure if the stellar atmosphere was steady. In 1975, Bidelman and San-Gak Lee reported spectral classifications for 601 proper-motion stars that had been listed in both a Lowell Observatory survey under Henry L. Giclas, and the Luyten Two-Tenths Catalogue and supplement, and their report included data from Gerard Kuiper. Kuiper and Bidelman had been at Yerkes Observatory at the same time. For six years Kuiper had worked to classify ~3,200 high proper motion stars using ~9,000 spectra taken at Yerkes and McDonald, and with an added 300 spectra from Luyten, Kuiper had planned to publish the data with Luyten. Bidelman called it "very important spectroscopic work and a "large-scale assault" on the problem of unclassified proper motion stars. When Kuiper ran out of stars of large proper motion and parallax in a region of sky, he observed the planets and their moons "nicely lined up" in that region. When Kuiper found evidence of an atmosphere on Saturn's moon, Titan, his research changed focus and much of his data on proper motion stars remained unpublished. Kuiper died in 1973. Following his paper with Lee, Bidelman had "renewed interest" in proper motion stars and asked whether Kuiper's unpublished proper motion material could be found. With help from the Kuiper Memorial Committee at the University of Arizona, Ewen A. Whitaker, Elizabeth Roemer, and Helmut Abt, Bidelman obtained copies of five of Kuiper's notebooks which had stars noted by name and right ascension without declination, and many with multiple spectra and more than one classification. Bidelman established a card file for every star or binary pair, and sought to find the exact star Kuiper observed. His goal was not to create definitive spectral types or change Kuiper's classifications, but to set out clearly "the enormous amount of useful spectral data relating to these objects gathered by an energetic and most talented astronomer to whom many, including the writer, owe much. This work represents a partial repayment of that debt". He estimated there could be ~1,000 stars with better spectral types than otherwise known. Bidelman worked on Kuiper's data. and published it in 1985. In 1975, Peter Pesch replaced Bidelman as Director of the Observatory and Chairman of the Astronomy department. While