Almost everything.
The main thing is that the size dictates the rate of fuel consumption (larger stars using more fuel). So a larger star will burn hydrogen faster (and hence it will be brighter and...
Stars and Planets are for the most part round because gravity is pulling everything to the center of mass. The force is most balanced when the shape is round. A cube would have imbalanced forces and...
Moons orbit planets. Planets orbit stars. Some stars orbit other stars, or orbit their mutual center of gravity. Stars orbit the center of the galaxy. Galaxies may orbit the center of the "galactic...