became first to see that S Persei is part of the Per OB1 association, which was later confirmed. Among the first stars that were studied at far-infrared wavelengths, M-type supergiants may be used to find the spiral arms of the Milky Way galaxy. Bidelman found four red supergiant stars in 1947, bringing the total then known to 13. How red supergiant stars evolved was considered an "astronomical puzzle", so the Double Cluster was used to test ideas about the evolution of red supergiant stars during the 1960s. The M-type supergiants of h and χ Per became the prototypes of this class of stars, and the major source of data for their properties. Unlike most the usual relatively young star clusters including few supergiant stars, 18 were found in the Double Cluster of Perseus by 2007, which Robert F. Wing noted as the 60th anniversary of Bidelman's "important paper", saying Bidelman's 1947 two-dimensional classifications of the then-known supergiants in h and χ Per had "served as the benchmark" for later studies of the red supergiant stars. Hydrogen is the most abundant element in the Universe, so stars displaying very little hydrogen in their atmospheres are chemically peculiar stars. There are many kinds of hydrogen-deficient stars. Upsilon Sagittarii is a hydrogen-deficient star. It is a very luminous, variable, and unusual eclipsing binary with a spectrum quite difficult to classify. In 1949, Bidelman was possibly the first to suggest that Upsilon (υ) Sagittarii's violet-shifted absorption lines, which apparently takes place during some conjunctions of this binary pair when star 2 advances in front of star 1, could be caused by gas streaming from the primary star. Bidelman suggested when the displaced H-alpha(Hα) absorption line was present, it happened at regular intervals when the primary star was furthest away from Earth. Upsilon Sag was the only example of a star of its type until Bidelman discovered another star similar to it, HD 30353. This star became known as "Bidelman's star". Bidelman and Keenan were the first to regard the barium star red giants as different from other red giants and to describe them as a spectroscopic class. Barium is a heavy metal element made by certain advanced stars with a helium-burning shell surrounding a spent carbon core In addition to the λ4554 barium line, some other characteristics of the group were two enhanced strontium (SR II) lines, at λ4077 and another at λ4215 blended