Collection Item Summary:
This set of two original photographic plates is of the globular cluster Messier 13 (M13), one of the brightest clusters of its type in the northern skies. These plates were obtained by astronomer Wayne Osborn in July 1968, working with other astronomers as part of an extensive systematic series of photographic and photoelectric observations in the 1960s through 1980s searching for stars that vary in light in the cluster. Known as Cepheid variables, their variation reveals their intrinsic brightnesses, which yields a measurement of distance to the cluster. The photographic observations were made at the 61-inch (1.5 meter) astrometric reflector at the U. S. Naval Observatory Flagstaff Sation.