In 1945, when Bidelman left Aberdeen he was hired at Yerkes as an Instructor. Under Otto Struve Yerkes became the leading astrophysics center, when he directed it. In addition to Bidelman, by 1946 the Yerkes astronomy staff included Paul Ledoux, Arne Slettebak, Armin Deutsch, Marshall Wrubel, Arthur D. Code, Carlos Cesco, Víctor M. Blanco, W. W. Morgan, Otto Struve, Jesse L. Greenstein, Gerard P. Kuiper, George Van Biesbroeck, Louis G. Henyey Anne B. Underhill, Guido Münch, Nancy G. Roman, and the two future Nobel Prize winners, Subrahmanyan Chandrasekhar and Gerhard Herzberg. Other astronomers at Yerkes when Bidelman was there were Kaj Strand, W. Albert Hiltner, Aden B. Meinel, and visiting professors Bengt Strömgren from Denmark, and Jan Oort, Hendrik C. van de Hulst and Adriaan Blaauw from the Netherlands. George Herbig, also there, remembered it as an "exciting, stimulating place to work" and a "powerhouse in astronomy" while under Struve's direction.
Bidelman spent long hours observing in remote west Texas at McDonald Observatory because he, like other Yerkes faculty, was also an astronomer at the University of Texas (UT). At the suggestion of Struve, the two universities had cooperated to create McDonald Observatory when the UT system had no astronomy department but W. J. McDonald gave them money in 1926 for an observatory, while in Wisconsin, the Yerkes astronomers needed a larger telescope but lacked the funds to obtain one.
Otto Struve, who directed both Yerkes and McDonald, has been described as dedicated yet demanding. His managerial style included daily inspections of the faculty to see if they were working. Despite reports of tensions there was also "close knit camaraderie" and "boisterous parties" evidenced by Yerkes "spontaneous party songs" including "The Billy Bidelman Song". Sung to the tune of "The Battle Hymn of the Republic", it consisted of repeating three times the line: "Struve, Kuiper, Hiltner, Morgan, Chandrasekhar too," followed by: "And Billy Bidelman".
In 1946, W. W. Morgan and William P. Bidelman published a paper on interstellar reddening using the MK system of spectral classifications and photoelectric photometry. Morgan later said this paper with Bidelman on interstellar reddening was "the principal paper along the way" to the UBV system, which he devised with Harold Johnson.
In 1947, Bidelman became first to note the concentration of type M supergiant stars around χ Per, suggesting they were young Population I objects.
This group, along with the Double Cluster, was later named the Perseus OB1 Association. Based on its radial velocity, Bidelman also 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 Bidelmans "important paper", saying Bidelmans 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 with the head of the CN band, and also an enhanced G band due to CH and possibly CN. There are some supergiant stars with these BA II, strontium lines and CN band, but the G and K-type stars Bidelman and Keenan described did not appear to be supergiants.
G, K, and M-type giants, the most complex area of the Hertzsprung–Russell diagram (HR) diagram, have spectra so complicated "many astronomers have shied away from studying them".
Eventually, Robert McClure discovered that essentially all barium stars began with a companion star that made the s-process elements, and when the companion star aged into a white dwarf, a stellar wind moved what was made by one star to the other star, a shifting of mass from an Asymptotic giant branch(AGB) star that turned into a white dwarf, to its companion star.
Bidelman was the first to find three unusual A- and F-type high-latitude bright stars, HR 6144, 89 Herculis, and HD 161796 in the high galactic plane, an unexpected place for supergiant stars to be found.
Astronomers expect to find massive young stars that are five to twenty times the mass of the Sun, on the galactic plane, a place where stars form, but it is rare to find stars like Her 89, with a spectrum that appears to be a young supergiant so far from the Galactic plane. Regarding these stars with characteristics of Population I supergiants, yet found at high galactic latitude, another astronomer wrote, "If I were a theoretician, I would simply say, They cant be, therefore they arent".
Bidelman's 1951 study also isolated the G- and K-type giant stars with weak G-bands as a class of peculiar giants.