A doctoral degree is the usual educational requirement to become an astronomer .To opt for this field, a pass in plus two examination under the science stream, specially in physics and math's is essential. Since universities offering undergraduate programme in Astronomy are rarely found, you can join for a major/ honours in Physics with Mathematics as a subsidiary subject. Thereafter, take a master's degree in Physics or Astronomy. After the successful completion of MSc, specialization in astronomy leading to Ph. D. which enables to become an astronomer/ astronaut/ astrophysicist or a scientist/ research officer etc. in the field of space research. There are many institutes offers masters and doctoral courses in Astronomy.
For details about astronomy click here