That is a great question and a lot more complex than most people think. Most people think that astronauts all did the same thing to get where they are and in most cases that is not true. Astronauts come from a multitude of backgrounds and multiple different disciplines. You don't have to study astronomy or even space to become an astronaut. A lot of astronauts have degrees in subjects like geology, engineering, biology, and multiple different areas of science and math. I would definitely recommend focusing on fine tuning your math and science skills because that will definitely be useful to you down the line. Then I would recommend looking at the requirements to become an astronaut. You will probably find those requirements on NASA's website. Lastly, just do your best and put in max effort and you will achieve your goal. Good luck!
to start a career in astronut, make sure you have good grades in school, especially in the math and sciences spectrum. Going to college, while in college, Doing interships/ and volenteer work from NASA a large company or space X another company of space explorers there are also other space companies. Make sure you are in good shape and healthy. Also, getting a mentor that specializes in that field. After that, apply to various space companies. Good Luck !!!!!
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I've always thought being an astronaut is such a cool career! As you may know, NASA is the largest space exploration group in the US, and they have a great page on their website about what it takes to become an astronaut that you should check out: https://www.nasa.gov/audience/forstudents/postsecondary/features/F_Astronaut_Requirements.html
I hope this is helpful!