Since the sun is one giant ball of gas, what force holds its consistent shape and size? Why doesn't it expand and burn up quickly?

Asked by: J.J. True

Answer

The Sun maintains its size and shape against the outward pressure of fusion energy by the force of gravity. In other words, its own weight keeps the Sun from growing larger.

It is the stable balance of outward gas pressure vs. the inward pull of gravity that determines the size of any star.

The predominantly spherical shape of all but the smallest astronomical bodies (asteroids, for example) is due to the radially symmetric nature of the gravitational force.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor

'Physics is mathematical not because we know so much about the physical world, but because we know so little; it is only its mathematical properties that we can discover.'