0.07%

0.07%


That's a small number.


7 parts in 10,000.


But this small fraction is a significant number in terms of the evolution of our universe. This is the mass fraction difference between one helium nucleus and four hydrogen nuclei. During the proton-proton chain, four protons (essentially, the nucleus of a hydrogen atom is just a proton) are fused into one helium nucleus (two protons and two neutrons). The helium nucleus is smaller than four hydrogen nuclei by only 0.07%. This extra 0.07% mass is converted to energy via E=mc².


This fraction may seem insignificant, but in reality, it is very important. If the mass fraction was only 0.06%, then stars would take too long to evolve and may even be unstable. The energy released by the proton-proton chain would not be enough to hold back gravitational forces from the outer layers of the star, and the star would collapse.*


*The balance between the energy from fusion in the core of a star and the gravity of the star pushing in is called hydrostatic equilibrium. This is why stars are stable. It is also why stars tend to expand and contract. As the star evolves, the fusion in the core increases as it begins to use up its fuel and the star expands as it evolves on the main sequence. When fusion stages end (at the end of the Main Sequence, for example), gravity will overcome the fusion energy and push in. See more about the evolution of a star here.


If the mass fraction was just a tad higher, at 0.08%, the fusion of hydrogen into helium would occur too fast and stars would use up their fuel too quickly (and possible even dissipate the outer layers of the star as the energy would overcome gravity and push out the gases in the outer layers). If the stars were stable, the stars would use up their mass too quickly for planets to form and in turn, life would probably not evolve.