Stellar nucleosynthesis creates most elements in the periodic table. The Big Bang created 80% hydrogen and 20% helium. Proton-proton chain and CNO cycle are types of hydrogen burning. Heavier elements are formed through alpha, carbon, neon, oxygen, and silicon burning. A star can collapse into a neutron star or black hole.
Stellar nucleosynthesis is the process by which most of the elements in the periodic table are created. Nucleosynthesis is the synthesis of new elements from the nucleons (protons and neutrons) of lighter elements. The result of the Big Bang at the beginning of the universe was a cosmic composition of approximately 80% hydrogen and 20% helium. This process is called Big Bang nucleosynthesis, or occasionally primordial nucleosynthesis, and it took about three minutes. Other types of nucleosynthesis include stellar nucleosynthesis, which occurs in stars over billions of years, and supernova nucleosynthesis, which occurs within seconds.
The gas coalesced into stars, which fuse atomic nuclei, producing huge amounts of light and heat and producing heavier elements in the process. In stars of the mass of the Sun or smaller, energy is mainly produced using the proton-proton chain reaction. The proton-proton chain reaction occurs at temperatures between 10 and 30 megaKelvin and at pressures found at the centers of stars of the Sun’s mass or less. During the reaction, hydrogen atoms are fused into deuterium, which are then fused into helium-3. Then the atoms follow one of three possible paths to produce helium and the reaction is over. The reaction can take up to 109 years, which explains why our Sun is still around.
The proton-proton chain is a type of hydrogen burning, the nucleosynthetic process in which stellar hydrogen is converted into helium. Another hydrogen burning process, important in the most massive stars of the Sun, is the CNO (carbon-nitrogen-oxygen) cycle. The CNO cycle uses carbon, nitrogen and oxygen as catalysts for the star to fuse four protons in a helium nucleus. After the carbon is initially formed, it is converted to nitrogen, then carbon again, then nitrogen again, then oxygen, then nitrogen, then carbon again, and the cycle continues.
Eventually most of the hydrogen in the star is consumed and helium burning begins. This is done through the alpha process or triple alpha process. If a star is massive enough, it will continue to fuse heavier and heavier elements through the carbon burning process, the neon burning process, the oxygen burning process, and the silicon burning process, eventually forming a core of iron weighing 1.44 solar masses. Then, as the molten iron consumes more energy than it produces, the star loses its ability to support its own weight and collapses, sometimes catastrophically as a supernova, an explosion that can take days or months. The rest is a neutron star or a black hole.
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