This is a very difficult question to answer. Astronomy requires a mastery and proficiency in the physical sciences, mathematics and statistics. Medicine requires an equal mastery of biology and chemistry, plus a good bed-side manner to deal with patients! I suspect that by the time you enter your first year in college, you have already made the choices that will allow you to consider one or the other of these seriously. If you avoided or disliked mathematics and physics in your pre-college education, virtually all of the astronomers working in the profession that I have chatted with over the years admit that they LIKED math and physics and DID VERY WELL in these subjects in their pre-college years. Such a background will constitute the people you will be competing with to get into graduate school and to secure your first job after your PhD.
Sadly, by the time many students get to college, they have been distracted by either poor counseling, peer pressure, a turbulent home life, or poor roll models, to develop the kind of personality that will serve them well in the physical sciences. I do not mean that they avoided becoming what American school children call 'Nerds' and 'Geaks', I mean that they put aside the ability to ask questions about their world and receive the support at home and at school to pursue answers to their questions.
If the choice is between astronomy and medicine, and you find that you like mathematics ( the currency of physics ) far less than you do helping people, or studying how organisms function, I would surmise that you are far better off in medicine. Both professions are very harsh, demanding and have a long educational learning curve. You must decide within yourself how you FEEL about endless secluded hours of studying how dust grains are heated by starlight in interstellar space versus equally endless hours in the chemistry lab wondering about some equally obscure organic reaction that might be the key to a new vaccine.
Good luck. It is a wonderful, exciting world out there. Whether you enter medicine or astronomy they are both some of the most exciting areas of human endeavor ever created.