Einstein’s equation states that if m amount of mass was completely converted to energy, the total amount of energy released would be E = mc²

• In fusion reactions, the final products have a lighter total mass than the total mass of original products.
• This is called the mass defect , denoted by
• The energy released can be calculated as

• The earliest understanding of what provides energy in the stars came when Cecilia Payne , using spectroscopy discovered that Sun is composed mostly of Hydrogen .
• The extreme heat in the sun due to change of gravitational potential energy to thermal energy, releases the electrons from the atoms and creates a ball of plasma , ionised hydrogen and helium nuclei.
• One possible nuclear reaction could provide energy to the stars was the proton-neutron fusion.
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  • The deuteron formed has less mass than the original proton and neutron .
  • This mass defect is converted to energy, according to the equation .
  • The energy released with every fusion is about 2.2 MeV
  • But this reaction cannot be the major source of energy as there aren’t enough neutrons in the stars to sustain the reaction.
• Since majority of the element in the stars are Hydrogen atoms, proton-proton fusion is likely the major cause of energy release.
  • Although proton-proton electrostatic repulsion is very strong, the extreme conditions in stars help the protons overcome this force and get close enough for strong nuclear force to act.
  • As they fuse the to form deuterium, the electrostatic repulsion doesn’t play a part anymore as one of the protons change into a neutron.
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    • The energy released comes from the difference in mass of the original products from the final products, according to the equation :
    • The positron gets annihilated by an electron :
  • The deuterium then further fuses with another proton to form Helium
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