8.1.3Space Physics
The Life Cycle of Stars — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions
5 min read 11 parts Lesson 4 of 10
Part 1 of 10Common start
1Early stages
- Early stages every star goes through nebula, protostar and stable star.
2Main sequence
- Main sequence a long stable stage in which hydrogen fuses into helium and gravity is balanced by thermal pressure.
3Sun
- Sun it will spend about 10 billion years on the main sequence and has used about 4.6 billion so far.
4Mass
- Mass the final stages depend on the mass of the star.
Part 2 of 10Low-mass stars
Definition
- Definition a low-mass star has up to about eight times the mass of the Sun.
Final path
- Final path red giant, planetary nebula, then white dwarf.
Rule
- Rule any star that will become a white dwarf is a low-mass star.
Part 3 of 10Low-mass star: red giant
Hydrogen runs out
- Hydrogen runs out after several billion years the fusion rate falls.
Core
- Core it shrinks and heats up because gravity becomes greater than the thermal pressure.
Helium fusion
- Helium fusion the core becomes hot enough for helium to fuse into carbon.
Outer layers
- Outer layers the energy released makes them expand and cool, forming a red giant.
Future of the Sun
- Future of the Sun in about 5 billion years it will probably swallow Mercury and Venus.
Part 4 of 10Low-mass star: planetary nebula and white dwarf
Helium runs out
- Helium runs out fusion cannot continue and the star becomes unstable.
Planetary nebula
- Planetary nebula the core collapses under gravity and the outer layers are ejected into space as a shell of gas.
- The name has nothing to do with planets.
White dwarf
- White dwarf the collapsed core, about the size of the Earth with about the mass of the Sun.
Cooling
- Cooling a white dwarf gives out less energy as it cools over time.
Part 5 of 10High-mass stars
Definition
- Definition a high-mass star has more than about eight times the mass of the Sun.
Final path
- Final path red supergiant, supernova, then neutron star or black hole.
Rule
- Rule a star that will not become a white dwarf is a high-mass star.
Part 6 of 10High-mass star: red supergiant and supernova
Red supergiant
- Red supergiant after several million years hydrogen runs out and the outer layers expand and cool.
Heavier elements
- Heavier elements helium fuses to carbon, followed by nitrogen, oxygen and successively heavier elements.
Core
- Core it collapses and expands repeatedly as fusion starts and stops.
Iron
- Iron fusion cannot continue once iron forms.
Supernova
- Supernova the core collapses rapidly and causes a gigantic explosion.
Part 7 of 10Remnants of a supernova
Neutron star
- Neutron star a dense body, about 20 km across, formed at the centre of the explosion.
Black hole
- Black hole in the most massive stars the neutron star keeps collapsing until it forms a black hole.
- Not even light can escape from a black hole.
Outer layers
- Outer layers they are ejected into space and form new clouds of gas and dust (nebulae).
Most massive stars
- Most massive stars only the highest-mass stars become black holes, and most high-mass stars become neutron stars.
Part 8 of 10Recycling of matter
Supernova nebula
- Supernova nebula it may form new stars with orbiting planets.
Heaviest elements
- Heaviest elements elements heavier than iron are formed in a supernova and are ejected into space.
Planetary nebulae
- Planetary nebulae they also return enriched gas to space.
Our origin
- Our origin the atoms in the Earth and our bodies were made inside earlier stars.
Part 9 of 10Summary of the two routes
| Stage | Low-mass star | High-mass star |
|---|---|---|
| Start | Nebula, protostar, stable star | Nebula, protostar, stable star |
| Expanded stage | Red giant | Red supergiant |
| End of fusion | Planetary nebula | Supernova |
| Remnant | White dwarf | Neutron star or black hole |
Part 10 of 10More from the specification
1
Stellar nucleosynthesis tracking
- Stellar nucleosynthesis tracking tracking core helium burning and subsequent alpha-process fusion provides a clear means to understand stellar evolution.
2
Mass ranges
- Mass ranges some sources split low-mass stars from intermediate-mass stars (2 to 8), but both follow the same stages.
- (up to 2 solar masses)
3
Mass ranges
- Mass ranges some sources split low-mass stars from intermediate-mass stars (2 to 8), but both follow the same stages.
- (up to 2 solar masses)
4
Light-year
- The distance light travels in one year, 9.5 × 10^{15} m.
5
Spectrum
- Range of colours in white light.
6
Galactic redshift
- Shift of galaxy spectral lines to longer wavelength due to recession.
7
Nuclear fusion
- Joining of light nuclei to form a heavier nucleus, releasing energy.
8
Background radiation
- Radiation that is always present in the environment.
9
Universe
- All of space and everything in it.
10
Galaxy
- Huge group of stars, gas, dust and solar systems held by gravity.
11
Redshift
- Increase in the wavelength of light from a source moving away from the observer.
12
Distance
- Total length of path travelled, a scalar.
13
Radiation
- Transfer by electromagnetic waves, requiring no medium
14
Wavelength
- Distance between a point on one wave and the same point on the next wave
15
Reaction
- The partner force acting on the other object.
16
Constant
- Not changing.
17
v = H0d
What the symbols mean: v = recession speed of the galaxy (km s−1) · H0 = Hubble constant (km s−1 Mpc−1) · d = distance (Mpc)
18
Mass ranges
- Mass ranges some sources split low-mass stars from intermediate-mass stars (2 to 8), but both follow the same stages.
- (up to 2 solar masses)
Key terms
| Term | Meaning |
|---|---|
| Red giant | A large, cool star formed when a low-mass star runs out of hydrogen in its core. |
| Planetary nebula | The shell of gas ejected by a red giant. |
| White dwarf | The small, hot, dense core left after a low-mass star sheds its outer layers. |
| Red supergiant | A very large, cool star formed from a high-mass star. |
| Supernova | The explosion of a high-mass star at the end of its life. |
| Neutron star | A tiny, extremely dense star made mainly of neutrons. |
| Black hole | An extremely dense point in space from which not even light can escape. |
Avoid these mistakes
1
Saying the Sun will explode as a supernova — it is not massive enough and will become a white dwarf.
2
Thinking a planetary nebula has to do with planets — it is the outer layers thrown off by a red giant.
3
Mixing up red giant and red supergiant — a red giant comes from a low-mass star and a red supergiant from a high-mass star.
4
Saying all massive stars become black holes — most become neutron stars.
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