1. Fusion of Hydrogen into Helium
* The Main Sequence: A star spends the majority of its life in a stable state known as the main sequence. During this stage, the star's core fuses hydrogen atoms into helium atoms, releasing enormous amounts of energy in the process. This energy creates outward pressure that balances the inward force of gravity.
* Hydrogen Depletion: As the star fuses hydrogen, the amount of hydrogen in the core decreases. Eventually, the core is primarily composed of helium.
2. The Helium Core Contracts and Heats Up
* Gravitational Collapse: Without hydrogen fusion to provide outward pressure, the helium core begins to contract under its own gravity.
* Increased Temperature: This contraction increases the temperature and density of the core, making it even hotter.
3. The Star Expands and Becomes a Red Giant
* Hydrogen Fusion in Shell: The increasing core temperature allows hydrogen fusion to begin in a shell surrounding the helium core.
* Expansion: The energy released by this shell fusion causes the star's outer layers to expand dramatically, transforming the star into a red giant.
4. Helium Fusion Begins
* Triple-Alpha Process: If the core becomes hot enough (around 100 million degrees Celsius), helium nuclei (alpha particles) can fuse together to form carbon in a process called the triple-alpha process.
* Further Expansion: Helium fusion releases even more energy than hydrogen fusion, causing the star to expand further.
5. The Star's Future Depends on Its Mass
* Low-Mass Stars: For stars smaller than about 0.8 times the mass of our Sun, the core will never become hot enough to fuse helium. They will eventually shed their outer layers, leaving behind a white dwarf composed primarily of carbon and oxygen.
* Intermediate-Mass Stars: Stars like our Sun will fuse helium into carbon and oxygen, and may even fuse carbon into heavier elements. They will then shed their outer layers, forming a planetary nebula, and leave behind a white dwarf.
* High-Mass Stars: Stars more massive than the Sun will fuse helium and heavier elements, eventually reaching iron in their cores. They will explode as supernovae, leaving behind a neutron star or black hole.
Key Takeaways:
* Hydrogen fusion is the primary energy source for a star's main sequence life.
* When hydrogen is depleted in the core, the star enters a new phase, expanding and becoming a red giant.
* Helium fusion begins in the core, releasing even more energy and further expanding the star.
* The star's fate after helium fusion depends on its initial mass.
