Star Properties
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Life Cycle Stages
Main Sequence Star
Main sequence stars like our Sun are in a stable period of hydrogen fusion in their cores. This stage typically lasts for billions of years depending on the mass of the star.
The Sun is about halfway through its expected 10-billion-year main sequence lifetime.
Nuclear Fusion Processes
Stars generate energy through nuclear fusion processes that convert lighter elements into heavier ones.
Hydrogen Fusion (Proton-Proton Chain)
4 ¹H → ⁴He + 2e⁺ + 2νₑ + Energy
Core temperature: ~15 million K
Main energy source for Sun-like and lower mass stars.
Hydrogen Fusion (CNO Cycle)
4 ¹H + ¹²C → ⁴He + ¹²C + 2e⁺ + 2νₑ + Energy
Core temperature: >17 million K
Dominant in higher mass stars.
Helium Fusion (Triple-Alpha Process)
3 ⁴He → ¹²C + Energy
Core temperature: ~100 million K
Occurs in red giants and more massive stars.
Carbon Fusion
¹²C + ¹²C → ²⁰Ne/²³Na + n/p + Energy
Core temperature: ~500 million K
Only in stars >8 solar masses.
Oxygen, Silicon and Other Heavy Element Fusion
Increasingly hotter temperatures required
Core temperature: >1 billion K
Only in massive stars just before supernova.
Iron: Fusion Endpoint
Iron-56 has the highest binding energy per nucleon
Fusion beyond iron absorbs energy rather than releasing it
Leads to core collapse in massive stars