Learning outcomes
- Describe a nebula as gas and dust containing hydrogen.
- Explain gravitational collapse into a protostar.
- Explain why temperature rises during collapse.
- Describe the start of fusion and stable-star formation.
- Use force balance accurately.
8.1 Interstellar clouds
Stars form within interstellar clouds of gas and dust, often called nebulae. Hydrogen is the most important component because it later becomes the fuel for fusion. The cloud is initially very diffuse, so star formation requires some region to become denser than its surroundings.
A disturbance such as a shock wave may compress part of a cloud, although the syllabus focuses on the subsequent gravitational collapse rather than the trigger.
8.2 Gravitational collapse
Once a region is dense enough, its own gravitational attraction pulls material inward. As the cloud contracts, gravitational potential energy decreases and energy is transferred into kinetic and internal energy.
Collisions among particles become more frequent, so the central temperature rises. The contracting, heating object is called a protostar. It is not yet a stable star because sustained hydrogen fusion has not fully established balance.
8.3 Accretion and increasing density
Material from the surrounding cloud continues to fall inward, increasing the protostar’s mass. Conservation of angular momentum can produce a rotating disc around it. Some material in such discs may later form planets.
At O Level, the key reasoning is gravity → collapse → higher temperature. Avoid saying the protostar heats because it is already undergoing normal combustion.
8.4 Beginning of fusion
When the core becomes sufficiently hot and dense, hydrogen nuclei fuse into helium and release energy. The energy output increases the outward pressure effect within the star.
The start of sustained fusion marks the transition from protostar to stable star. The exact temperature and detailed reaction chain are not required for this syllabus.
8.5 Stable main-sequence stage
A stable star exists when inward gravitational attraction is balanced by the outward effect associated with the high-temperature core. The star can remain in this state for millions or billions of years, depending mainly on its mass.
Massive stars contain more fuel but use it much more rapidly because their cores are hotter and fusion rates are higher. Therefore, they generally have shorter stable lifetimes than less massive stars.
Worked examples
Energy change during collapse
As gas falls inward, gravitational potential energy decreases. The energy becomes internal energy, raising temperature until fusion can begin.
Why a protostar is not yet stable
Before sustained fusion, there is insufficient outward pressure effect to balance gravity fully, so contraction continues.
Practical focus
Investigation or modelling activity
Model gravitational collapse using a sequence of annotated diagrams or a computer simulation. At each stage, label density, temperature, direction of material motion and the dominant energy transfer. Evaluate which features a two-dimensional model cannot represent.
Examination guidance
- Include hydrogen when describing the original cloud.
- Use “internal gravitational attraction” or “own gravity” for collapse.
- Link collapse to increasing temperature.
- Do not call a protostar a stable star before fusion and force balance.
- Use both inward and outward effects in the final stable stage.
Check your understanding
- What material forms a star?
- Why does a cloud collapse?
- Why does the protostar become hotter?
- When does it become a stable star?
Answers
- An interstellar cloud of gas and dust containing hydrogen.
- Its own gravitational attraction pulls material inward.
- Gravitational potential energy is transferred to kinetic and internal energy during contraction.
- When sustained fusion begins and outward effects balance inward gravity.
