Observations, Analysis, and Orbital Calculation of the Visual Double Star STTA 123 AB

As part of a research workshop at Pine Mountain Observatory, four students from Evergreen State College met with an instructor and an experienced double star ob- server to learn the methods used to measure double stars and to contribute observations to the Washington Double Star (WDS) Catalog. The students then observed and analyzed the visual double star STTA 123 AB with few past observations in the WDS Catalog to deter- mine if it is optical or binary in nature. The separation of this double star was found to be 69.9" and its position angle to be 148.0°. Using the spectral types, stellar parallaxes, and proper motion vectors of these two stars, the students determined that this double star is likely physically bound by gravity in a binary system. Johnson calculated a preliminary cir- cular orbit for the system using Newton's version of Kepler's third law. The masses of the two stars were estimated based on their spectral types (F0) to be 1.4 M?. Their separation was estimated to be 316 AU based on their distance from Earth (about 216.5 light years) and their orbital period was estimated to be 3357 years. Arnold compared the observations made by the students to what would be predicted by the orbit calculation. A discrepancy of 14° was found in the position angle. The authors suggest that the orbit is both eccentric and inclined to our line of sight, making the observed position angle change less than predicted.