Large bodies are round because of gravity, one of the four basic forces in the universe. It is the large scale organizer of the universe itself. It is gravity that will determine gross shapes of objects (as well as their motions). Matter attracts matter (gravity), and when matter in the massive quantities necessary to make a star accumulates (accretes), it forms up in the "best" shape for the 3D space. Stars have massive gravity, and gravity on that scale won't let objects line up. Stars begin as finely divided material or as (mostly) gas and dust. It can't help but ball up once things get started. Gravity is so massive it "flattens" anything that would stick out. Even the planets are mostly round owing to their formation in a gravimetric field. (But they "flatten" slightly due to their spin.)
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Thanks to their mass, planets generate enough gravity to pull all the material in themselves toward the central point of the planet. All this material piles up in all directions, resulting in a sphere. The rotation of the planet can actually augment the sphere somewhat, so the planets aren't perfectly round, but close enough for many concerns.
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.
Every shape that is round does not have edges. A shape that has sharp points and has straight lines can have edges.
A sphere.
The shape of a circle is round. Circles are plane shapes that have all points that are the same distance from the center.
10cm is a length. You can only find perimeter (length all the way round) if there is a shape!
It all depends. If it is 2-D, then yes. But, if it is 3-D, then no, because a sphere is round, not flat.