B.Sc Astronomy

In my opinion you should take Physics in graduation and Masters in astrophysics. (Astrophysics is a broader term covering other planets, galaxies, black holes, etc. ). .

Edx offers both individual courses and advanced programs designed to help you learn about astrophysics in an engaging and effective online learning environment complete with video tutorials, quizzes and more. Additionally, edX offers the option to pursue verified certificates in astrophysics courses.

The colleges which offer B.Sc. astronomy courses are:
Indian institute of Astrophysics Bangalore,
JNTU college of Engineering Hyderabad,
Tata institute of fundamental research, Pune.
Osmania University.

Yes, B.Sc. Physics Mathematics and Astronomy is a full-time three years duration course offered by University of Lucknow.

Some of the other institutes which offer B.Sc. In Astrophysics:
Indian Institute of Astrophysics, Bangalore. Tata Institute of Fundamental Research, Pune. Osmania University. JNTU College of Engineering, Hyderabad. Indian Institute of Astrophysics, Bengaluru
National Centre for Radio Astrophysics, Pune
Physical Research Laboratory
Raman Research Institute, Bengaluru.

Top Colleges for Bachelor of Science [B.Sc] (Astrophysics)
B.Sc in Delhi-NCR
B.Sc in Maharashtra
B.Sc in Chennai
B.Sc in Uttar Pradesh
B.Sc in Telangana
Hope this helps you. Be free to share any problem you have. All the best for your bright future!

The cut-off marks for Saint Xavier's every year changes but it is very high, always. I would request you to please mention your subject wise marks.