Hello aspirant,
IISER only takes BS-MS for astrophysics on the basis of JEE advanced, KVPY,SCB only.it takes admission for PhD through GATE/JEST examination.
If you want to pursue Ms astrophysics you can checkout indian institute of astrophysics.
IISER only gives direct admission to those students who have done BS-MS from their institute.
I hope it helps,
Best of luck.
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