Hi aspirant,
To become an Astronomer, a Ph.D degree is compulsory.
1. After doing 12th( mathematics and physics) you may join an undergraduate course in Astronomy.Following are some courses, such as Bsc in astrophysics,  bsc in astronomy or btech in relevant engineering.
2 .Similarly you have to do master in msc astronomy or mtech( astronomy and space engineering).
3. You have to further continue with you doctral programme- PhD. in Astronomy, PhD. in Astronomy and Astrophysics, PhD. in Astrophysics, PhD. in Atmospheric Science and Astrophysics.
4. You can also opt for integrated programmes such as- Integrated M.Sc Ph.D Programme in Physics and Astrophysics, Integrated M.Tech Ph.D (Tech.) programme in Astronomical Instrumentation
Hope this helps.