Question

Why are large astronomical bodies round

Answer 1

I would have to go with gravity, as each object in this universe has some amount of gravity, even humans and creatures and plants.

when a rock is thrown into space, it will have to travels light years away and pull in thousands of other asteroids into yet sometimes this doesn't work and sometimes nothing will happen depending on the speed of how they are going. 

collision of these rocks are also another cause to the round shape, remember these rocks could be thousands of millions of years old. and this would be easy for them to shape because of the amount of asteroids in space.

this shape is also by the gravity on it in of from it. though this rare and it will be called a meteorite.

Answer 2

Matter of all types have gravity, which causes it to attract to each other. The most efficient way for all this matter to congregate is the sphere. As they consolidate, they form the shape. As they compress, temperatures in the center of the mass start to go up and if it hits the proper point, it can ignite and become a star.