What should you do if you want to get a bachelor's degree in both physics and mathematics?
Make sure you attend a college with both majors and research the requirements for both. Talk to a counselor at the school about how to work out the double-major and declare both. Check and see if any prerequisites apply to both majors and take those classes first.
You must list both as your majors. It will take you about a year, maybe two to do both. Try to get as high a GPA as you can. Apply then to grad school and work towards a PhD. By then you will have to decide which of the two you want the doctorate in. Some very good students are actually recruited by some grad schools. Perhaps you will, also. Also work towards doing well in writing and composition as you will need those as well. Schools are looking for well rounded students.
Modern astronomy is a very mathematical branch of science. To become a professional astronomer you would need a degree from a university in astronomy, mathematics or physics. With a first degree you would then go on to take either a masters degree followed by a PhD or perhaps go straight on to a PhD. In preparation for an astronomy degree you should be studying (and doing well in) mathematics, physics, chemistry and possibly biology.
To become MRI Technologist you should be certified with a bachelors degree or the associate degree in any field provided by the college but as you have opted to become MRI Tech it is helpful if the degree is on the subjects which helps in understand the mechanism of the MRI Instrument, so the topics which are advisable are biology, anatomy, health, health physics, pathology etc.