For both SL and HL: A star's initial mass is the main factor determining its core temperature, fusion rate, lifetime, evolutionary path, and final remnant. Massive stars have more fuel, but consume it much faster, so they generally have shorter lives and more violent endings.
The Reasoning and Mechanism
A star forms when gravity causes gas and dust to contract. Compression raises the core's temperature and pressure until nuclear fusion begins. On the main sequence, outward thermal and radiation pressure balance inward gravitational effects in hydrostatic equilibrium.
Greater mass creates stronger gravitational compression, producing a hotter core. This causes a much higher fusion rate and luminosity. Therefore, although a massive star starts with more hydrogen, it exhausts that fuel sooner than a lower-mass star.
| Initial mass | Evolutionary path | Final outcome |
|---|---|---|
| Low mass | Slow fusion and very long main-sequence lifetime | White dwarf; smallest stars outlive the universe's present age |
| Sun-like or intermediate mass | Main sequence, red giant, planetary nebula | White dwarf |
| High mass | Main sequence, red supergiant, fusion to iron | Supernova, then neutron star or black hole |
In a high-mass star, fusion can continue until an iron-rich core forms. Fusion beyond iron does not release energy, so pressure support decreases and the core collapses. A core-collapse supernova ejects the outer layers; the remnant becomes a neutron star or, if its core is sufficiently massive, a black hole.
The common misconception is that massive stars live longer because they contain more fuel. In fact, fusion rate increases so strongly with mass that the additional fuel is consumed much sooner.
Exam Technique
For syllabus topic E.5 Fusion and stars (SL and HL), an response should give a linked chain: greater mass → stronger compression → higher core temperature → faster fusion → shorter lifetime. Then connect initial mass to the relevant final remnant rather than merely listing stages.