OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(d) characteristics of a white dwarf; electron degeneracy pressure;…
Full specification wording
Stars: (d) characteristics of a white dwarf; electron degeneracy pressure; Chandrasekhar limit
A white dwarf is the dense, hot remnant left after a low-mass star ejects its outer layers.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 4 of 7
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A white dwarf is the dense, hot remnant left after a low-mass star ejects its outer layers. It is roughly the size of a planet but contains about the mass of the Sun, so its density is extremely high, around 10⁹ kg m⁻³. It no longer fuses nuclei; instead, it is supported against gravitational collapse by electron degeneracy pressure, which arises because electrons cannot occupy the same quantum state. A white dwarf gradually cools and dims. The Chandrasekhar limit is the maximum mass a white dwarf can have, approximately 1.4 solar masses; above this limit, degeneracy pressure cannot support the star. In this specification, the limit is used to explain why more massive remnants collapse further.
What earns the marks
- A white dwarf is a small, dense, hot stellar remnant.
- Electron degeneracy pressure arises from the exclusion principle limiting electron states.
- The Chandrasekhar limit is about 1.4 solar masses.
- Above the Chandrasekhar limit, electron degeneracy pressure cannot prevent further collapse.
Common mistakes and how to put them right
Thinking a white dwarf shines by fusion.
Put it right: Fusion has stopped, and it shines by stored thermal energy as it cools.
Confusing electron degeneracy pressure with gas pressure.
Put it right: Degeneracy pressure does not depend on temperature in the same way and arises from quantum restrictions on electrons.
Treating the Chandrasekhar limit as a minimum mass.
Put it right: It is a maximum mass, approximately 1.4 solar masses.
Exam tips
- Quote the Chandrasekhar limit as approximately 1.4 solar masses, not an exact value.
- Link electron degeneracy pressure to the exclusion principle for a precise explanation.
- In multiple-choice questions, reject options that give a white dwarf an active fusion core.
Try the lesson
The start of the Stars: (d) characteristics of a white dwarf; electron degeneracy pressure; Chandrasekhar limit lesson, made from checked teaching notes for this specification point.
Listen
A white dwarf is the dense, hot remnant left after a low-mass star ejects its outer layers. It is roughly the size of a planet but contains about the mass of the Sun, so its density is extremely high, around 10⁹ kg m⁻³. It no longer fuses nuclei; instead, it is supported against gravitational collapse by electron degeneracy pressure, which arises because electrons cannot occupy the same quantum state. A white dwarf gradually cools and dims. The Chandrasekhar limit is the maximum mass a white dwarf can have, approximately 1.4 solar masses; above this limit, degeneracy pressure cannot support the star. In this specification, the limit is used to explain why more massive remnants collapse further. A strong answer earns marks for these points. A white dwarf is a small, dense, hot stellar remnant. Electron degeneracy pressure arises from the exclusion principle limiting electron states. The Chandrasekhar limit is about 1.4 solar masses. Above the Chandrasekhar limit, electron degeneracy pressure cannot prevent further collapse. Exam tip: Quote the Chandrasekhar limit as approximately 1.4 solar masses, not an exact value.
See it
Key words
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White dwarf
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Electron degeneracy pressure
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Chandrasekhar limit
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See every definition
- White dwarf
- A small, dense, hot stellar remnant left after a low-mass star ejects its outer layers, with a mass comparable to the Sun but a radius comparable to a planet.
- Electron degeneracy pressure
- Pressure arising because electrons cannot occupy the same quantum state, which supports a white dwarf against gravitational collapse instead of fusion.
- Chandrasekhar limit
- The maximum mass a white dwarf can have, approximately 1.4 solar masses, above which degeneracy pressure cannot support the star.
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