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If gravity is the weakest of the four forces of nature, how does the gravity in stars cause fusion of nuclei of atoms, which are held together by a stronger force, the weak nuclear force? Is it a matter of strength over distance?

When scientists compare fundamental forces, they are referring to the magnitude of the forces in a comparable situation. The strong force is much stronger than gravity in the context of the quarks and gluons that make up a proton. However, when you assemble a large amount of matter, the impact of the strong force does not accumulate the way it does for gravity. With a large amount of matter, gravitational forces add on each other, and certainly can generate forces larger than the strong force in a proton. So, yes, it is partially a length issue, but it is also an additive effect.

Actually, this explanation is a an (over-)simplification and is not rigorously correct -- to get the technically correct story, the topic to study is quantum chromodynamics (QCD), which remains an active field of research today.

Hope this helps,
Burr Zimmerman

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