|MadSci Network: Astronomy|
Since you're in junior high and asking about graduate school already, I imagine you're pretty serious. So you should also know about the job market in astronomy lately.
Most astronomy PhD programs take between 5 and 7 years, after you get your bachelors degree in college. Most of us got our bachelors degree in physics, but some schools offer astronomy programs as well. Those 5 to 7 years spent in graduate school are not all spent taking classes, though. Most people are finished with classes entirely after 2 years, and then the remaining time is spent doing research.
The cost of getting a PhD in astronomy is currently $0. That's right: it's essentially free. All the graduate programs I know of will only accept as many students as they can hire. So you end up teaching undergraduate classes while you do your research or take classes, and in return your department pays your tuition and a small (very small) salary. Most people teach for their first couple years, and then get what's called a "research assistantship" meaning that instead of teaching for their tuition and salary, now they do some research for their research advisor for their tuition and salary. This is nice, because then you're basically just being paid to do your research and get your PhD. Not all professors can hire graduate students this way, though, so many people who are working for a professor without funds to hire a student do have to teach on and off throughout graduate school. But the point is that, unlike medical school or law school, you don't have to take out loans to get your astronomy PhD. If you want to become a professor, the teaching experience is quite necessary, too, so you're also being trained as a teacher.
But back to what I hinted at about the job market: unlike doctors and lawyers, who can set up their own clinic or company, scientists have fewer job possibilities where they can do pure research. Now, if you want to get a PhD in astronomy and then go work for some kind of aerospace company, you can probably do that. But then you might be better off getting your degree in aerospace engineering or some other related field. If you want to do astronomy -- using telescopes, doing pure research -- then there are a limited number of job openings each year. For the past several years, more people have been going into astronomy than the field can really support, so the job market is not very good, and many new PhDs are having to go work in other types of fields. Not that that's terrible, it's just not what they originally wanted to do, and 6 years is a long time to spend getting trained for something you'll never get to do.
I'm not trying to discourage you, because as an astronomer I can obviously understand why someone would want to enter this field. But everyone, I think, should have a realistic idea of the kinds of opportunities available before they go to graduate school.
Good luck in whatever you decide to do!
Try the links in the MadSci Library for more information on Astronomy.