Formally, all Ph.D. students in our program select both a major and a minor subject. For most, both are astronomy. However, it is possible for the minor to be in one of the other science or engineering disciplines. The course requirement is set by the minor department, but usually, at least three of the required elective courses are graduate courses in the minor department.
The astronomy department allows graduate students whose Ph.D. major is in another department to minor in astronomy. We require that they pass nine units of coursework that the department considers acceptable for this purpose, preferably "core" courses, and that one astronomy faculty member participates in the preliminary and final oral exams (thesis defense).