Learning outcomes
- Explain why stable stars eventually change.
- Describe red-giant formation.
- Describe planetary-nebula formation.
- Identify the white-dwarf remnant.
- Distinguish the low-mass pathway from the massive-star pathway.
9.1 Running out of central hydrogen
During the stable stage, fusion converts hydrogen into helium in the core. Eventually, most hydrogen in the central region has been used. The rate and location of fusion change, disturbing the balance that maintained the star’s size.
The phrase ‘runs out of hydrogen’ is a useful simplification. Hydrogen may remain in outer layers, but the central fuel supply becomes insufficient for the previous stable fusion process.
9.2 Expansion into a red giant
For a less massive star such as the Sun, the outer layers expand greatly and cool at the surface, producing a red giant. The star is redder because the surface temperature is lower, even though its total luminosity may be high because the surface area is enormous.
Expansion occurs as the internal structure adjusts to changing fusion processes. Detailed shell-fusion mechanisms are beyond the syllabus, so answers should focus on hydrogen depletion and expansion.
9.3 Loss of outer layers
The red giant becomes unstable and releases its outer layers into space. The expanding gas forms a planetary nebula. Despite the name, a planetary nebula is not a planet and does not necessarily contain planets.
The nebula may glow because radiation from the hot central remnant excites the surrounding gas. This stage returns material to interstellar space.
9.4 White dwarf
The remaining core becomes a white dwarf. It is very hot initially but has no sustained hydrogen fusion. It is small and dense, containing a substantial fraction of the original star’s mass in a volume comparable with a planet.
Over an extremely long time, a white dwarf cools and becomes dimmer. The later theoretical black-dwarf stage is not required by the syllabus and the Universe is not old enough for such objects to have formed.
9.5 Matter recycling
Gas ejected into the planetary nebula enriches interstellar space. It can mix with other gas and dust and later contribute to new stars and planetary systems.
This recycling shows that stellar life cycles are not isolated sequences. Material from earlier generations of stars becomes part of later astronomical objects.
Worked examples
Choosing the correct sequence
Nebula → protostar → stable less massive star → red giant → planetary nebula → white dwarf.
Correcting a misconception
A white dwarf is not a small main-sequence star. It is the hot, dense remnant core after the outer layers have been ejected.
Practical focus
Investigation or modelling activity
Create a flowchart with arrows and short causal statements, not only stage names. For example, “central hydrogen depleted → balance changes → outer layers expand”. Compare it with the massive-star flowchart and highlight the point where the paths diverge.
Examination guidance
- Use red giant for the less massive pathway, not red supergiant.
- State that a planetary nebula is ejected outer gas.
- Identify the white dwarf at the centre.
- Do not include a supernova in the low-mass sequence.
- Keep the order exact.
Check your understanding
- Why does a stable star eventually leave the stable stage?
- What does a less massive star become after the stable stage?
- What forms a planetary nebula?
- What remains at its centre?
Answers
- Most central hydrogen is eventually converted to helium, changing the balance and fusion conditions.
- A red giant.
- The star’s expelled outer layers.
- A white dwarf.
