I am in maths field if I want to join astronomy to be an astronaut so which field I would have to selet after Bsc in maths?

I can't think of a single astronaut whose primary training is in astronomy. There simply is no astronomical work available for astronauts. They tend to have a combination of backgrounds, usually in aeronautical engineering and some specialty which makes them valuable as mission specialists. Often this is in biology or medicine. Mathematics is always an essential grounding for an astronaut.

You should take some Englishes classes too.

PHD Biology, Chemistry and Physics.
Aeronautics and Engineering .is essential too.
Physically fit and not easily put off.
Basicaly a willingness to give your life to it.