Question

why are planets and stars sphere in shape? (iam very curious)

Answer 1

Stars are spherical because they are made of hot gas, which behaves as a fluid. The gas atoms are attracted toward the center of the star by gravity, and a sphere is the shape that results when they all try to pack in as closely as possible.

Planets are formed by clouds of debris that coalesce into a big rock, so that process is the same and results in a generally spherical shape as well. Though planets are solid and therefore can't "flow" like stars can, for sufficiently large planets the rock is not strong enough to support large big spikes or points on the surface - there's a reason mountains only grow so tall.

Long story short - gravity acts the same way your hands do when you make a snowball, trying to pack things in as tightly as possible. In 3D, the shape that results is a sphere. In 2D it would be a circular disk.