Astronomy

If you want to join ISRO, then every year ISRO conduct their own exams which you have to clear it.

You can check the this year's category wise opening and closing ranks of various IITs at
http://jam.iitb.ac.in/round_3.pdf
and for more information about IIT JAM, visit:
http://jam.iitb.ac.in
Best of luck.

Hi Sarjeet, Candidates must have a Bachelor's degree in Physics or Mathematics or CSE or EE or ME. Minimum Eligibility to Apply: 55% graduation. For further details, refer below
https://www.shiksha.com/science/course/m-sc-in-astronomy-indian-institute-of-technology-Indore-291586.

Basic qualification is 10+2 with Physics, Chemistry, Mathematics subjects. You can choose B.Tech in Aerospace engineering. There are entrance exams, which will decide the seat allotment. Another option is B.Sc. Physics with Astronomy or astrophysics specialisation and then M.Sc. and go on to do Ph.D. with astronomy or astrophysics specialisation.

Yes, elective subjects are not considered to be that much of important so you can choose any branch.