Early Stages (Main Sequence):
* Dominant Elements: Primarily hydrogen (H) and helium (He), with trace amounts of other elements.
* Fusion Process: The star fuses hydrogen into helium, releasing energy in the form of light and heat.
Later Stages (Red Giant):
* Hydrogen Depletion: The core's hydrogen fuel runs out, and fusion ceases in the core.
* Core Contraction: The core collapses under its own gravity, increasing its temperature and density.
* Shell Fusion: Hydrogen fusion ignites in a shell surrounding the core, causing the star to expand dramatically into a red giant.
* Helium Burning: At a high enough temperature, helium fusion begins in the core, forming carbon (C) and oxygen (O).
Further Evolution (Beyond Red Giant):
* Fusion of Heavier Elements: Depending on the star's mass, it may continue fusing heavier elements like carbon, oxygen, neon (Ne), silicon (Si), and eventually iron (Fe).
* Iron Accumulation: Iron is the most stable element and cannot be fused to release energy. The core becomes dominated by iron.
* Core Collapse and Supernova: Once the core becomes mostly iron, it collapses catastrophically, leading to a supernova explosion. This explosion releases immense energy and synthesizes even heavier elements.
* White Dwarf, Neutron Star, or Black Hole: The remnant of a supernova can be a white dwarf (for less massive stars), a neutron star (for more massive stars), or a black hole (for the most massive stars).
Key Changes in Composition:
* Decreasing Hydrogen: The star's hydrogen content decreases as it is converted into helium.
* Increasing Helium: The helium abundance increases due to hydrogen fusion.
* Formation of Heavier Elements: The star's core and outer layers acquire heavier elements as fusion proceeds.
* Final Composition: The final composition of the star's remnant depends on its initial mass and the stages of fusion it undergoes.
In summary, a star's aging process involves a continuous transformation of its chemical composition, driven by nuclear fusion reactions. This leads to the formation of heavier elements and ultimately determines the star's fate and the composition of its remnant.
