@RhiJaneJoyce,
we hve advanced enrollment for students who can do the work, and have shown a sense of scholarship in past courses. AS max says, go talk to the department chairs or instructors in any area you are interested in. In the US, due to the pressures in "legacy enrollments " in the elite universities, many mart students are enrolling in programs of nursing or EMT and then, transferring into med school. This is a good strategy for 10 or so years for those who wish to go to med school and also want a PhD in some research area like genomics or applied engineering for all sorts of med devices.
You are interested in Astronomy so an undergrad program that focuses on chemistry (P chem and spectroscopy) are good areas to build up to do grad work in cosmology or astronomy.
Good grades in your past and a way to demonstrate that you CAN do the work, is what colleges look for today , along with wider interests beside just the area of concentration. The SAT test used to be of greater importance, but lately weve found that SAT's don't equate with success at the highest degree of correlation , The Graduate Record Exam (GRE's) are more indicative of success AFTER the baccalaureate degree.
you'll be the first astronaut flunkie.
