Low-Mass Stars (Like Our Sun)
1. Hydrogen Burning: The star starts by fusing hydrogen into helium in its core, releasing energy. This is the longest stage of a star's life.
2. Red Giant Phase: When hydrogen runs out in the core, the core contracts, becoming hotter. This heats the outer layers, causing them to expand and cool, turning the star into a red giant. The star begins fusing helium into carbon in a shell surrounding the core.
3. Helium Flash: In the core, helium fusion ignites explosively, called the "helium flash." This is a short-lived event that releases a lot of energy but doesn't disrupt the star's structure.
4. Horizontal Branch: After the flash, the star settles onto the horizontal branch, continuing to fuse helium into carbon in its core.
5. Asymptotic Giant Branch (AGB): When helium runs out in the core, the star expands again, becoming even larger, and starts fusing carbon and oxygen in a shell around the core.
6. Planetary Nebula: As the outer layers are ejected, the star becomes a white dwarf, surrounded by a glowing shell of gas called a planetary nebula.
Medium-Mass Stars (Slightly Larger than Our Sun)
The process is similar to low-mass stars, but with some key differences:
1. More Fuel: Medium-mass stars have more fuel, so they live longer.
2. Carbon Fusion: They can fuse carbon into heavier elements like oxygen, neon, and magnesium in their cores.
3. No Helium Flash: The helium ignition is more gradual than in low-mass stars.
4. Multiple Shells: They can have multiple layers where different fusion processes occur.
5. Supernova or White Dwarf: Medium-mass stars eventually stop fusing elements in their cores. They can either shed their outer layers and become a white dwarf or undergo a type Ia supernova if they are in a binary system and accrete mass from a companion star.
Key Differences
* Mass is the key: The mass of a star determines its lifespan and ultimate fate.
* Final State: Low-mass stars end as white dwarfs, while medium-mass stars can become white dwarfs or undergo a supernova.
* No Fusion Beyond Iron: Stars can't fuse iron into heavier elements because it requires more energy than it releases. This leads to the core collapse that triggers a supernova.
Important Notes
* These are simplified descriptions. The actual processes are much more complex and involve a variety of factors, such as stellar rotation, magnetic fields, and binary interactions.
* Our understanding of stellar evolution is constantly being refined by new observations and theoretical models.
