If gravity is really a weak force compared to other forces, then how does gravity pull hydrogen atoms in a nebula to from stars. I understand that hydrogen is in atomic or in the molecular state in star forming regions and electromagnetic repulsion does not negate gravity. But still, it's surprising that gravity can actually pull two molecular hydrogens together. I mean even at nanometer distances between molecules, the gravity will be in the order of $10^{-19}$. Can anyone shed light on this?
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To understand star formation you shouldn't view each hydrogen atom individually, but consider the huge ensemble of atoms — i.e. gas clouds — from which stars are born. These molecular clouds are massive collections of atoms with masses of thousands to millions times that of the Sun. Inhomogeneities in the clouds are slowly amplified due to gravitational collapse. A single hydrogen atom in the outskirts of such an overdensity feels the combined gravitational attraction of all the hydrogen in the cloud, just as the atoms in your body feels the combined gravity from all the atoms in Earth, keeping you from flying off the ground.
Bonus info: As the cloud contracts, pressure builds up, preventing further collapse. To make it all the way to a star, the gas must be able to cool and thus get rid of some of its energy. This is done by atoms colliding, which excites their electrons; when the electrons fall back to their ground states, the atoms emit radiation that carries away energy. If the gas is metal-rich, their are many more ways for the atoms to be excited so the gas cools more easily. Once the gas becomes too thick for the photons to escape, the clouds settles in a more-or-less hydrostatic equilibrium, at which point it's called a proto-star.
