Question

why the shape of sun is circle?

Answer 1

If it is that big and had any other shape, gravity would squeeze it round in a jiffy.

Basically, there’s a center of gravity, being the mean point of all mass of a big thing in space. Everything is pulled there.

Sadly, there’s not space for all matter, so matter is trying to get as close as possible. Anything sticking up is pulled down, any valleys are filled up.

There can be local deviations, like mountains on Earth where the composition of rock is stronger than gravity, or like protuberances on the Sun