Graduate schools recognize that most students eventually choose their Ph.D. thesis topic based on a very wide variety of influences. Your letter should sound enthusiastic, and draw attention to your strengths. Avoid making excuses for your shortcomings unless you have good corroborating evidence or compensating strengths. If you have a particular area within astronomy you are interested in, say so and target graduate programs strong in that area. A personal statement should clearly state your professional and career goals.
Graduate schools are reluctant to accept students who appear to be fishing for something to do.
Never apply only to top schools. A good recommendation is to apply to one top school you are really interested in and at the same time always have one school that is very likely to accept you on your list as well. Talk to the faculty and the DUGS about your plans for graduate school. They are in the best position to give you advise about what schools to aim for. Check out University of Minnesota's Department of Astronomy's Graduate Program for further information and and idea of what to expect when applying to our department and other graduate programs.