Stars and planets are round primarily due to the force of gravity and the process of gravitational collapse during their formation.
Gravity is a fundamental force of nature that acts to pull matter together. When a cloud of gas and dust collapses under its own gravity, it forms a rotating disk known as a protoplanetary disk. Within this disk, material accumulates at the center, forming a dense core, while the remaining material forms a rotating flattened structure around it.
As the core continues to accrete mass, its gravity becomes stronger, and the pressure at the center increases. This leads to a balance between the gravitational force pulling inward and the pressure pushing outward, resulting in a spherical shape. This balance is known as hydrostatic equilibrium.
The spherical shape is the most stable configuration for a massive object under the influence of gravity. Gravity acts equally in all directions, pulling matter towards the center, and the matter settles into a shape where the forces are balanced.
Additionally, the rotation of a star or planet contributes to its round shape. As the object spins, the centrifugal force tends to flatten it along the equator, creating a slight bulge. This effect is more pronounced for rapidly rotating objects, such as stars with shorter rotation periods.
Other shapes, such as cubes, are not common for celestial bodies because they are not naturally formed through gravitational collapse. The forces involved in the formation process and the subsequent hydrostatic equilibrium tend to favor the formation of spherical or spheroidal shapes.
While there may be small irregularities or variations in shape due to factors like rotation, tidal forces, or external influences, the dominant shape for celestial bodies, governed by the forces of gravity, is a round or spherical shape.