Life Cycle of Stars
Section: Space Physics | Syllabus: Cambridge IGCSE Physics 0625
Life Cycle of a Star All stars form from clouds of gas and dust in space, then evolve over millions or billions of years. The eventual fate of a star depends primarily on its initial mass . Stage 1: Protostar Stars form from interstellar clouds of gas and dust (molecular clouds) that contain mostly hydrogen The cloud collapses inward due to gravitational attraction , and the collapsing region increases in temperature As molecules fall inward, gravitational potential energy converts to kinetic energy, then thermal energy - the cloud heats up This collapsing, heating sphere of gas is called a protostar The protostar continues growing by pulling in more material from the surrounding cloud Stage 2: Stable Star (Main Sequence) When the core becomes hot enough, nuclear fusion begins : hydrogen nuclei fuse to form helium, releasing energy The outward radiation pressure from the hot core builds up and opposes the inward gravitational force The star becomes stable when the inward gravitational force exactly balances the outward force from the high-temperature core This stable phase is called the main sequence and can last billions of years All stars eventually run out of hydrogen as fuel - what happens next depends on the star's mass Stable Star A star in which the inward force of gravitational attraction is balanced by an outward force due to the high temperature at the star's centre (caused by nuclear fusion).
When balanced, the star maintains a constant size. Life Cycle: Less Massive Stars (like the Sun) Figure: Life Cycle of a Less Massive Star Flow diagram left to right: Interstellar cloud of gas and dust → (gravitational collapse) → Protostar → (nuclear fusion begins, forces balance) → Stable Main Sequence Star → (hydrogen runs out, core contracts, outer layers expand) → Red Giant → (outer layers blown away) → Planetary Nebula with White Dwarf at centre → (cools over time) → Black Dwarf.
Arrows connect each stage. Red Giant: When most of the hydrogen in the star's core is used up, nuclear reactions slow. The core contracts (releasing gravitational PE as thermal energy), causing the outer layers to heat up, expand, and cool → the star becomes a larger, cooler, red-coloured red giant Planetary Nebula: Radiation pressure blows away the outer layers of the red giant, forming a glowing shell of gas called a planetary nebula White Dwarf: The exposed hot core remains as a white dwarf - a dense, Earth-sized remnant.
It is not hot enough inside to fuse heavier elements, so it slowly cools to eventually become a black dwarf The Sun's Fate Our Sun is about 4.6 billion years old and is approximately halfway through its main sequence lifetime.
In about 5 billion years , it will expand into a red giant (swallowing Earth's orbit), then shed its outer layers to form a planetary nebula, leaving a white dwarf at its centre. Life Cycle: More Massive Stars (>8 solar masses) Figure: Life Cycle of a Massive Star Flow diagram: Interstellar cloud → Protostar → Stable Main Sequence Star → Red Supergiant (expands much further than a red giant) → Supernova (sudden violent explosion, shown with starburst) → Nebula rich in hydrogen and new heavier elements.
From the nebula, two branches: upper branch → Neutron Star (for moderately massive cores); lower branch → Black Hole (for very massive cores). A dashed arrow from the nebula points back to "new stars with orbiting planets" indicating the cycle continues.
Red Supergiant: More massive stars expand much further than less massive stars, forming a red supergiant Supernova: The star eventually explodes as a supernova - an extraordinarily violent explosion that outshines entire galaxies briefly.
The explosion creates a nebula containing hydrogen and new heavier elements (heavier than iron) formed during the explosion Neutron Star or Black Hole: The remnant core collapses: A neutron star forms if the remaining core is moderately massive A black hole forms if the core is very massive - gravity is so strong that not even light can escape The nebula from a supernova may eventually form new stars with orbiting planets - continuing the stellar cycle Cosmic Recycling The heavier elements in your body (carbon, oxygen, iron) were forged in the cores of massive stars and scattered through space by supernovae.
The material that formed our Solar System - including Earth and everything on it - came from earlier generations of stars. Summary: Life Cycle Comparison Stage Less Massive Star (like Sun) Massive Star (>8 solar masses) Formation Interstellar cloud → Protostar → Stable Main Sequence Star Expands to become Red Giant Red Supergiant End of life event Outer layers gently blown away Supernova (violent explosion) Ejected material Planetary Nebula Nebula with heavier elements Remnant core White Dwarf → Black Dwarf Neutron Star or Black Hole
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