Describe the life cycle of a star: (a) a star is formed from interstellar clouds of gas and dust that contain hydrogen (b) a protostar is an interstellar cloud collapsing and increasing in temperature as a result of its internal gravitational attract

Published by Patrick Mutisya · 8 days ago

IGCSE Physics 0625 – Stars: Life Cycle

Life Cycle of a Star

This note outlines the main stages in the evolution of a typical star, from its birth in an interstellar cloud to its ultimate fate. The description follows the points (a)–(h) required for the Cambridge IGCSE Physics syllabus.

Key Stages

  1. Formation from interstellar clouds (a)

    Stars begin in giant clouds of gas and dust, called nebulae. These clouds are primarily composed of hydrogen (\$\sim 90\%\$ by number) with smaller amounts of helium and heavier elements.

  2. Protostar formation (b)

    Gravitational attraction causes a region of the cloud to collapse. As the material contracts, potential energy is converted into thermal energy, raising the temperature of the core.

  3. Hydrostatic equilibrium – stable star (c)

    When the core temperature becomes high enough (\$\approx 10^7\ \text{K}\$) nuclear fusion of hydrogen into helium starts. The outward pressure from the hot gas balances the inward pull of gravity, establishing hydrostatic equilibrium.

  4. Hydrogen exhaustion (d)

    Fusion consumes hydrogen in the core. Eventually the core runs out of hydrogen fuel, and the balance of forces is disturbed.

  5. Expansion to red giant or red supergiant (e)

    With hydrogen depleted, the core contracts and heats further, while the outer layers expand and cool, giving the star a reddish appearance.

    • Stars with initial mass \$< 8\,M_{\odot}\$ become red giants.

    • Stars with initial mass \$\ge 8\,M_{\odot}\$ become red supergiants.

  6. End of a low‑mass star – planetary nebula & white dwarf (f)

    The red giant sheds its outer layers, creating a glowing planetary nebula. The remaining core, no longer undergoing fusion, becomes a dense white dwarf.

  7. End of a massive star – supernova, neutron star or black hole (g)

    When the core of a red supergiant reaches iron, fusion can no longer release energy. The core collapses catastrophically, producing a supernova. The explosion ejects a nebula enriched with heavy elements. The remnant core becomes:

    • A neutron star if the core mass is \$< 3\,M_{\odot}\$.

    • A black hole if the core mass exceeds this limit.

  8. Re‑formation of new stars (h)

    The supernova nebula, rich in heavy elements, can cool and fragment, forming new interstellar clouds that may eventually collapse to form new stars and planetary systems.

Summary Table

StageTypical Mass RangeKey ProcessesFinal Remnant
Protostar0.1 – 100 \$M_{\odot}\$Gravitational collapse, heating— (becomes main‑sequence star)
Main‑sequence star0.1 – 100 \$M_{\odot}\$Hydrogen fusion (\$4p \rightarrow \,^{4}\!He + 2\gamma + 26.7\ \text{MeV}\$)— (evolves to giant phase)
Red giant (low‑mass)0.1 – 8 \$M_{\odot}\$Helium fusion, outer envelope expansionPlanetary nebula + white dwarf
Red supergiant (high‑mass)8 – 30 \$M_{\odot}\$Fusion of heavier elements up to ironSupernova → neutron star
Very massive star\$>30\,M_{\odot}\$Rapid fusion, strong stellar windsSupernova → black hole

Suggested diagram: Flowchart of the stellar life cycle showing the progression from nebula → protostar → main‑sequence star → giant phases → end states (white dwarf, neutron star, black hole) and the recycling of material into new nebulae.

Key Equations

Energy released in hydrogen fusion (main‑sequence phase):

\$4\,^{1}\!H \;\rightarrow\; ^{4}\!He \;+\; 2\,\gamma \;+\; 26.7\ \text{MeV}\$

Hydrostatic equilibrium condition (balance of forces):

\$\frac{dP}{dr} = -\frac{G M(r) \rho(r)}{r^{2}}\$

Schwarzschild radius (relevant for black‑hole formation):

\$R_{s} = \frac{2 G M}{c^{2}}\$