Theme E — Nuclear and Quantum Physics
E.5.4 Life Cycle of a Star — IB Physics HL Revision Notes & Topical Questions
5 min read 8 parts Lesson 41 of 45 1 quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
E.5.4 Life Cycle of a Star — IB PHYSICS SL notes: Describe the stages of stellar evolution for low-mass and high-mass stars and their end states.Every star follows nebula, protostar, then main sequence — the path afterwards depends on its initial mass.Main sequence — hydrogen fus…
- Lifetime depends strongly on mass
- Core hydrogen runs out
- Fusion continues in a shell around the core
- The outer layers swell and cool
- Helium fusion starts when the core is hot enough, about 108 K
- A white dwarf is about the size of the Earth
Part 1 of 7Stages common to all stars
1Every star follows nebula, protostar, then main sequence
- Every star follows nebula, protostar, then main sequence the path afterwards depends on its initial mass.
2Main sequence
- Main sequence hydrogen fuses to helium in the core at steady luminosity in equilibrium.
3Lifetime depends strongly on mass
- Lifetime depends strongly on mass luminosity rises roughly as M3.5, so lifetime goes as M-2.5.
- The Sun spends about 10 billion years there.
- A 10 solar-mass star lasts only about 30 million years;
- a 0.5 solar-mass red dwarf lasts over 50 billion years.
Part 2 of 7Leaving the main sequence
Core hydrogen runs out
- Core hydrogen runs out most nuclei in the core are now helium and fusion slows.
Fusion continues in a shell around the core
- Fusion continues in a shell around the core the core contracts and gets hotter.
The outer layers swell and cool
- The outer layers swell and cool forming a red giant (low mass) or red supergiant (high mass).
Helium fusion starts when the core is hot enough, about 108 K
- Helium fusion starts when the core is hot enough, about 108 K releasing energy that stabilises the star for a time.
- Helium fuses to carbon and oxygen.
Part 3 of 7Low-mass stars
Low-mass stars
Red giant — hydrogen shell burning and helium core burning.
Planetary nebula — helium runs out, the core contracts but is not hot enough for further fusion, and the outer layers are released.
White dwarf — the hot dense solid core remains and slowly cools.
Low-mass stars
A white dwarf is about the size of the Earth
- A white dwarf is about the size of the Earth with a mass of order the Sun's.
Electron degeneracy pressure supports it
- Electron degeneracy pressure supports it not fusion.
The Chandrasekhar limit is about 1.4 solar masses
- The Chandrasekhar limit is about 1.4 solar masses above it a white dwarf cannot be held up.
Part 4 of 7High-mass stars
1
Red supergiant — shell and core burning continue past helium, forming carbon, neon, oxygen and silicon.
2
Fusion ends at iron — an onion-like structure forms with an iron core.
3
Supernova — the iron core collapses and the outer shell is blown out in an explosion.
4
Remnant — a neutron star or a black hole.
1Iron fusion releases no energy
- Iron fusion releases no energy so the iron core cannot support the star.
2Supernovae create elements heavier than iron
- Supernovae create elements heavier than iron and briefly shine as bright as a galaxy.
Part 5 of 7Neutron stars and black holes
Neutron stars and black holes
Remnant core between 1.4 and about 3 solar masses
- Remnant core between 1.4 and about 3 solar masses a neutron star.
- It is a ball of neutrons 10–20 km across, with density about 1017 kg m-3.
- Neutron degeneracy pressure supports it.
Remnant core above about 3 solar masses
- Remnant core above about 3 solar masses the collapse continues and a black hole forms.
- Not even light escapes from within the event horizon.
The remnant type depends on the mass of the core left
- The remnant type depends on the mass of the core left not on the original mass, since much mass is lost in winds and the explosion.
Part 6 of 7Summary of paths
| Initial mass | Evolution | End state |
|---|---|---|
| Below about 8 solar masses | Main sequence, red giant, planetary nebula | White dwarf |
| About 8 to 20 solar masses | Main sequence, red supergiant, supernova | Neutron star |
| Above about 20 solar masses | Main sequence, red supergiant, supernova | Black hole |
1Boundaries are approximate
- IB applies the 1.4 and 3 solar mass thresholds to the remnant core.
2Stars are the source of nearly all elements
- Stars are the source of nearly all elements hydrogen and helium come from the Big Bang, carbon to iron from stellar fusion and heavier elements from supernovae.
Part 7 of 7More from the specification
Stellar degenerate energy levels
- Stellar degenerate energy levels core collapse in aging stars forces matter into dense states governed by closely packed quantum energy levels.
Core collapse
- Core collapse when fusion slows, radiation pressure falls, no longer balances gravity, and the core contracts and heats up.
Core collapse
- Core collapse when fusion slows, radiation pressure falls, no longer balances gravity, and the core contracts and heats up.
Wavelength
- Distance between a point on one wave and the same point on the next wave
Surface temperature
- Effective temperature of the star's radiating surface.
Binding energy
- The minimum energy required to separate the nucleons of a nucleus to infinity.
Emission spectrum
- The set of discrete wavelengths emitted by excited atoms of an element.
Energy level
- A discrete energy an electron in an atom can have
Temperature
- Measure of the average kinetic energy of the particles.
Spectrum
- Range of colours in white light.
Absorption
- The conversion of wave energy to internal energy of the medium.
Reaction
- The partner force acting on the other object.
Parallax
- Apparent shift between objects caused by viewing from different positions
Distance
- Total length of path travelled, a scalar.
Photon
- A quantum of electromagnetic energy, E = hfWhat the symbols mean: E = energy of one photon (J) · h = Planck constant (6.63 × 10−34 J s) · f = frequency (Hz)
Radius
- Distance from the centre to the circular path.
Core collapse
- Core collapse when fusion slows, radiation pressure falls, no longer balances gravity, and the core contracts and heats up.
Key terms
| Term | Meaning |
|---|---|
| Red giant | Large, cool, luminous star after core hydrogen exhaustion. |
| Planetary nebula | Shell of gas expelled by a dying low-mass star. |
| White dwarf | Dense remnant of a low-mass star supported by electron degeneracy pressure. |
| Chandrasekhar limit | Maximum mass of a white dwarf, about 1.4 solar masses. |
| Supernova | Explosion of a massive star at the end of its life. |
| Neutron star | Dense remnant of neutrons supported by neutron degeneracy pressure. |
| Black hole | Remnant so dense that not even light escapes from its event horizon. |
Avoid these mistakes
1
Saying a red giant is hot — it is cool at the surface but luminous because of its size.
2
Thinking the Sun will become a supernova — it will become a white dwarf.
3
Applying the Chandrasekhar limit to the star's original mass — it applies to the remnant core.
4
Forgetting that electron degeneracy pressure, not fusion, supports white dwarfs.
5
Thinking a black hole is a hole in space — it is a very dense region with an event horizon.
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