Hello aspirant,

To become Astronomer you should have Ph.D degree. By writing JEE Mains or Advanced itself you can't directly become Astronomer. First you have to complete your 10+2 exams under science stream with Mathematics & Physics subjects compulsory. After that you have to give exams to join in undergraduate course in Astronomy. Later you can join postgraduate degree in Physics or Astronomy. In this way you can become Astronomer.

To get more information go through below link. https://www.careers360.com/courses/astronomy-course

Hope this helps you.

Good Luck!!