The process, known as Hydrogen Burning, is due to a fusion reaction: hydrogen nuclei fuse together to make helium nuclei.
In fusion, light atomic nuclei collide with such violence and frequency in the high-temperature, high-density stellar interior that they fuse into heavier nuclei and release tremendous quantities of energy (such as in a hydrogen bomb).
This reaction requires very high temperature and pressure found mainly in the core of a star and cannot be easily duplicated on Earth, unless using a thermonuclear device (H-bomb).
The great abundance of hydrogen makes it the key constituent in stellar nuclear reactions. The next stable nucleus is helium, #"^4He#, with atomic weight 4. Since the hydrogen nucleus (one proton) only has atomic weight 1, four protons are required to make one helium nucleus. The atomic weights do not exactly match because the more exact atomic weight of a proton is 1.0078, and four of them add to 4.0312, while the weight of #"^4He# is 4.0026, leaving a mass defect of 0.0286. This mass is converted to an amount of energy given by Einstein’s equation for the equivalence of mass and energy,
#E = mc^2#
where c is the speed of light. Because a unit atomic weight is #1.66 xx 10^(-27)# kg, the energy released by the conversion of four #"^1H# nuclei to one #"^4He# nucleus is:
#E=0.0286(1.66xx10^(-27))(9xx10^16)=4.3xx10^(-12)J#
Additional Info:
In atomic nuclei, the strong nuclear force overcomes the electrostatic repulsion of the positively charged protons and binds from one to 260 nucleons (protons and neutrons) in a region about #10^(-15)# m in diameter. Two nuclei will fuse to form one larger nucleus if they approach within #10^(-15)# m of one another, but their mutual electrostatic repulsion — all nuclei have a positive charge — amounts to a #1# MeV potential barrier. In contrast, at temperature of #10^7# K the average thermal energy of a proton is only #1# KeV. Classically, protons cannot fuse because of the strong coulombic barrier. Fusion does happen, however, because quantum physics allows the protons to tunnel through the barrier rather than go over it. The easiest fusion reaction involves two protons (hydrogen nuclei); such reactions become significant at temperatures around 10 million K.
(Reference for supporting data and figures: Introductory Astronomy and Astrophysics - M. Zeilik, S. A. Gregory, E. v. P. Smith)
