Astronomy

Admission to the PG courses will be done on the basis of GATE/JEST/UGC NET.

For space science, please complete M.Sc in physics and PhD also. After that, you can apply for entrance exam of ISRO.

You can try for Punjab Technical University (PTU). It has courses in aeronautics that might interest you. Moreover, the university has great reputation in case you are not eligible for IITs and NITs.

After class 12th, you have so many opportunities:
Aeronautical Engineering
Aerospace Engineering
Astronautical Engineering
Mechatronics
Aircraft Maintenance Engineering

You would be eligible through GATE score and not CAT.

You can apply in foreign universities based on your GRE score. You need to clear GRE, a score around 320-330 is good enough to get a good university in US.

Hello Aryan,
There are several institutes in India which offer you to pursue a career in astronomy or space research. Below is the list of institutes.
IIAE Uttarakhand
MIT Madras
IIT Mumbai
Indian Institute of Astrophysics, (Bangalore)
Indian Institute of Science, (Bangalore)
Raman Research Institute, (Bangalore)
Inter-University Centre for Astronomy and Astrophysics, (Pune)
National Centre for Radio Astronomy, etc.