stellar evolution theory. In addition to stellar rotation, he also studied Stark effect in stellar spectra, that is broadening of the spectral lines by the electric field in the stellar atmosphere. He also worked on the turbulence of stellar atmosphere and expanding shells around stars. This topic required a larger telescope than those available to him. Therefore, between 1933 and 1939, he built an 82-inch telescope at the McDonald Observatory, which was then the second largest telescope in the world (after the Mt. Wilson telescope).