This thesis by Cole Johnston brings novel insights into the inner workings of young massive stars. By bridging the observational fields of binary stars and asteroseismology this thesis uses state of the art statistical techniques to scrutinise theories of modern stellar astrophysics. Developing upon the commonly used isochrone fitting methodology, the author introduces the idea of isochrone cloud fitting in order to account for the full breadth of physics observed in stars. The author combines this methodology with gravity mode asteroseismic analysis to asses the level of chemical mixing deep within the stellar core in order to determine the star's age and core mass. Wrapped into a robust statistical framework to account for correlations, this methodology is employed to analyse individual stars, multiple systems, and clusters alike to demonstrate that chemical mixing has dramatic impact on stellar structure and evolution.

Cole Johnston began his academic career at the Department of Astrophysics and Planetary Science at Villanova University, where he engaged in undergraduate research projects with Profs. Ed Guinan and Andrej Prsa. Before graduating, he traveled to the University of Central Lancashire to work with Prof. Don Kurtz for a summer and to KU Leuven to study under Prof. Conny Aerts. Upon graduating from Villanova, Cole returned to Belgium with the support of the Belgian American Educational Foundation to enroll in the Master of Astronomy and Astrophysics. Upon completion of his MSc, Cole began his PhD under the supervision of Prof. Conny Aerts and Drs. Steven Bloemen and Andrew Tkachenko.