For historical reasons, Physics 5600 (Statistical Mechanics) is listed as a prerequisite to Astronomy 5681 (Principles of Stellar Evolution and Nucleosynthesis).
After the change to semesters, Stat. Mech. is no longer a prerequisite, but we have not formally changed the rules in the registration system.
So if you want to take Astronomy 5681 but can’t enroll on our own, please send me an email (firstname.lastname@example.org) and I will enroll you in that course.