Question

Why are large astronomical bodies such as planets and stars round?

Answer 1

Large astronomical bodies such as planets and stars are round as a result of their own gravity. Gravity pulls astronomical bodies mass inwards towards their core causing it to even out and form a sphere. Although not a perfect sphere, more astronomical bodies more or less resemble a sphere. 

Answer 2

Gravity. A gravitational field creates a circle around the central point, in this case the core, typically a large amount of iron with other atoms following the gravitational waves. Here's an image that may help. If you don't understand then I will try my best to simplify.