8 · IGCSE Physics
Life cycle of a star
Pick a star's starting mass and step through its life, from gas cloud to final remnant, watching how inward gravity and outward pressure compete at each stage.
Stable star: gravity (in) = pressure (out)
Fusion: 4 ¹H → ⁴He + energy, E = Δm c²
Lifetime t ≈ 1.00 × 10¹⁰ yr × (M/M☉)^−2.5
Black hole: r = 2GM/c²
Try this
- At 1.00 M☉ go to the main sequence and note the lifetime; then set 10.0 M☉. How many times shorter is the massive star's life?
- Compare the arrows for the protostar and the main-sequence star. Which force is bigger in each, and what does the star do as a result?
- Slowly raise the mass and find where the final stage changes from white dwarf to neutron star, and then to black hole.
Exam tip: for a stable main-sequence star, name both forces and their directions — gravity pulling inwards balanced by outward pressure from fusion energy — rather than just saying the forces are balanced.