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This lesson covers the life cycle of a star, from its formation in a nebula to its eventual death, as required by the AQA GCSE Physics specification (4.8.1). This is a Physics-only topic. You need to understand the different stages a star goes through, how the life cycle depends on the mass of the star, and how heavier elements are formed during the life and death of stars.
All stars begin life in a nebula — a vast cloud of hydrogen gas and dust in space. A nebula can be many light-years across.
The process of star formation:
Exam Tip: The force that causes a nebula to collapse is gravity (gravitational attraction between particles of gas and dust). Do not say "the gas is pulled inward" without stating that it is gravitational force doing the pulling.
A main sequence star is a stable star in which the inward force of gravity is balanced by the outward force of radiation pressure (caused by the energy released during fusion). This balance is called hydrostatic equilibrium.
Key features of main sequence stars:
| Star Mass | Main Sequence Lifetime | Colour | Surface Temperature |
|---|---|---|---|
| Low mass (less than 0.5 solar masses) | Tens of billions of years | Red | Cooler |
| Sun-like (about 1 solar mass) | About 10 billion years | Yellow | Moderate |
| High mass (more than 8 solar masses) | Millions of years | Blue / blue-white | Very hot |
Exam Tip: The main sequence is the longest phase because hydrogen fusion provides energy steadily over billions of years. More massive stars have more fuel but use it up much faster, so they have shorter main sequence lifetimes. This is a common 4-mark question.
A star with a mass similar to the Sun (up to about 8 solar masses) follows this path:
A cloud of gas and dust begins to collapse under gravity.
The collapsing material heats up. Not yet a true star — fusion has not begun.
Hydrogen fusion begins. The star is stable for billions of years.
When the hydrogen in the core runs out, the core contracts and heats up. The outer layers expand and cool, turning the star red. Helium fusion begins in the core, forming heavier elements such as carbon and oxygen.
The outer layers of the red giant are ejected into space, forming a glowing shell of gas called a planetary nebula (despite the name, this has nothing to do with planets).
The remaining core is left behind as a white dwarf — a small, very dense, hot object that gradually cools over billions of years. A white dwarf is roughly the size of Earth but has a mass similar to the Sun. No fusion occurs — it simply radiates stored thermal energy.
graph LR
A["Nebula"] --> B["Protostar"]
B --> C["Main Sequence Star"]
C --> D["Red Giant"]
D --> E["Planetary Nebula"]
E --> F["White Dwarf"]
style A fill:#8e44ad,color:#fff
style B fill:#e67e22,color:#fff
style C fill:#f1c40f,color:#000
style D fill:#e74c3c,color:#fff
style E fill:#1abc9c,color:#fff
style F fill:#ecf0f1,color:#000
A star with a mass much greater than the Sun (more than about 8 solar masses) follows a different path after the main sequence:
Same as for a sun-like star — collapse of gas and dust under gravity.
Collapses more rapidly due to greater gravitational force.
Burns hotter and brighter, but for a much shorter time (millions of years instead of billions).
When the hydrogen in the core is exhausted, the star expands enormously to become a red supergiant. The core fuses heavier elements in successive layers — helium to carbon, carbon to neon, neon to oxygen, oxygen to silicon, and silicon to iron. Iron is the heaviest element that can be produced by fusion in a star's core because fusing iron does not release energy — it requires energy.
When the core becomes iron, fusion stops. The core collapses catastrophically under gravity in a fraction of a second. The outer layers rebound off the dense core in a colossal explosion called a supernova. A supernova can briefly outshine an entire galaxy. During the supernova, temperatures and pressures are so extreme that elements heavier than iron are formed (such as gold, silver, uranium, and lead). These elements are scattered into space.
If the remaining core has a mass between about 1.4 and 3 solar masses, it forms a neutron star — an incredibly dense object composed almost entirely of neutrons. A neutron star may be only about 20 km across but have a mass greater than the Sun.
If the remaining core has a mass greater than about 3 solar masses, gravity is so strong that nothing can prevent further collapse. The core becomes a black hole — a region of space where the gravitational field is so strong that not even light can escape.
graph LR
A["Nebula"] --> B["Protostar"]
B --> C["Main Sequence Star"]
C --> D["Red Supergiant"]
D --> E["Supernova"]
E --> F["Neutron Star"]
E --> G["Black Hole"]
style A fill:#8e44ad,color:#fff
style B fill:#e67e22,color:#fff
style C fill:#3498db,color:#fff
style D fill:#c0392b,color:#fff
style E fill:#f39c12,color:#fff
style F fill:#2c3e50,color:#fff
style G fill:#000000,color:#fff
| Stage | Sun-like Star | Massive Star |
|---|---|---|
| Formation | Nebula to protostar | Nebula to protostar |
| Main sequence | Billions of years | Millions of years |
| After main sequence | Red giant | Red supergiant |
| End stage | Planetary nebula then white dwarf | Supernova then neutron star or black hole |
| Elements produced | Up to carbon and oxygen | Up to iron (heavier elements in supernova) |
One of the most important concepts in this topic is the origin of elements.
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