8.2.2Astrophysics
The Life Cycle of Solar Mass Stars — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions
10 min read 12 parts Lesson 7 of 13
Section 1 of 12From gas cloud to protostar
Stars form from a nebula, a huge cloud of gas (mostly hydrogen) and dust.
If the protostar collects enough mass and the core temperature reaches about 107 K, hydrogen nuclei begin to fuse into helium and a star is born. (Objects with too little mass never reach this temperature and become brown dwarfs.)
Section 2 of 12Main sequence: the long stable stage
The star now enters the main sequence, in which it fuses hydrogen to helium in its core. The star is stable because two effects are balanced:
If the core were to shrink, it would heat up, the fusion rate would rise and the extra pressure would push it back out; if it grew, the opposite happens.
- and the life of a star depends on its mass: more massive stars burn their fuel faster,
- so they live shorter lives.
Section 3 of 12Red giant
After billions of years the hydrogen in the core is used up.
Section 4 of 12Planetary nebula and white dwarf
When the helium is used up, the core can not reach the temperature required to fuse carbon.
In theory, after a very long time (longer than the present age of the Universe) it will cool until it no longer emits light, becoming a black dwarf.
Section 5 of 12Exam reminders
In a question about the life cycle of the Sun, state the stages in order and add one fact for each: gravity pulls the gas together in the nebula; the protostar heats as it contracts; on the main sequence hydrogen is fused to helium and forces are balanced; the red giant has a large cool surface; the outer layers are ejected as a planetary nebula; the white dwarf is hot, dense and cooling.
Section 6 of 12Worked examples
Put these stages in order for a star like the Sun: white dwarf, nebula, main sequence star, red giant, protostar.
Nebula, protostar, main sequence star, red giant, white dwarf.
(Between red giant and white dwarf the outer layers form a planetary nebula.)
Explain why a main sequence star stays the same size for billions of years.
The inward pull of gravity is balanced by the outward pressure produced by fusion in the core. If the star were to shrink, the core would heat up, more fusion would occur and the pressure would rise and expand the star again; if the star expanded, the pressure would fall and gravity would shrink it. Hence the forces stay balanced.
The Sun has a main sequence lifetime of about 1.0 × 1010 years and is now about 4.6 × 109 years old. How many more years is it expected to stay on the main sequence?
Remaining time = 1.0 × 1010 − 4.6 × 109
= 5.4 × 109 years, so about 5.4 billion years.
Section 7 of 12Formation and Main Sequence
All stars begin in the same way, before their mass determines their later stages.
Section 8 of 12Late Stages
Later stages depend on the fuel that remains in the core.
Section 9 of 12Why Each Stage Happens
- Hydrogen fuel Fusion of hydrogen into helium powers the main sequence, and the stage ends when the hydrogen in the core is used up.
- Balance of forces During the main sequence, outward pressure from fusion balances the inward pull of gravity, keeping the star stable.
- Expansion The outer layers expand as the core contracts and heats up, which makes the surface cooler and redder.
- White dwarf Gravity compresses the core into a dense, hot object that has no fusion left to support it against gravity.
Section 10 of 12Exam Checklist & Tips
8.2.2 – Key Points for the Exam
- Nebula A giant cloud of gas and dust from which stars form.
- Protostar A hot ball of gas formed by gravitational contraction, before fusion begins in its core.
- Main sequence star A star that is fusing hydrogen into helium in its core and is in stable equilibrium.
- Red giant A large, cool and luminous star formed when a solar-mass star expands.
- White dwarf A small, dense and hot remnant left after a solar-mass star has shed its outer layers.
- Black dwarf
- A cooled white dwarf that no longer emits significant light;
- this stage is only theoretical so far.
- A cooled white dwarf that no longer emits significant light;
Common Mistakes
- Saying the star becomes a red giant because it cools down; it expands, and its surface cools as a result.
- Describing a white dwarf as a star still undergoing fusion; fusion has stopped.
- Giving the wrong end stages for a solar-mass star; the sequence is red giant then white dwarf, not supernova.
- Calling a protostar a main sequence star before fusion has started in its core.
Section 11 of 12Key terms
| Term | Meaning |
|---|---|
| Nebula | Cloud of gas and dust from which stars form. |
| Protostar | Collapsing clump of gas that is heating but not yet fusing hydrogen. |
| Main sequence | Stable stage of hydrogen fusion in the core. |
| Red giant | Expanded, cooler, more luminous star after core hydrogen is used up. |
| Planetary nebula | Shell of gas ejected by a dying Sun-like star. |
| White dwarf | Dense, Earth-sized remnant core of a Sun-like star. |
| Black dwarf | Theoretical cold, dark end state of a white dwarf. |
Section 12 of 12Exam technique
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