Stellar Nucleosynthesis

Nucleosynthesis is the process by which stars create elements within their cores. The sole exception is hydrogen, the most abundant and lightest element in the universe, which originated in the immediate aftermath of the Big Bang.

• Extreme Conditions in Stellar Cores: The cores of stars experience immense pressures and temperatures. 
  • For instance, the temperature in the Sun’s core reaches approximately 15 million degrees Celsius. Under these extreme conditions, atomic nuclei undergo nuclear fusion.
• Fusion Process in Hydrogen: The hydrogen nucleus consists of a single proton. In the stellar core, these nuclei fuse to create helium nuclei, which contain two protons and two neutrons. This fusion process is known as the proton-proton (p-p) chain. 
• Nucleosynthesis in Massive Stars: However, in more massive stars, stellar nucleosynthesis follows a different route due to the higher energy available. These stars have hotter cores where the carbon-nitrogen-oxygen (CNO) cycle operates. 
  • In this cycle, the nuclei of carbon, nitrogen, and oxygen interact in various ways to form elements starting from helium.Iron production: When a star runs out of nuclei to fuse, its core contracts, leading to an increase in temperature that reignites nuclear fusion. 

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• • This cycle of contraction and fusion continues until the star begins producing iron in its core. Iron is the lightest element for which fusion requires more energy than it releases.
  • Elements heavier than iron can only be synthesized outside a star when it goes supernova.

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