ASTR 330 · Lecture 12

Electron Degeneracy and the Chandrasekhar Mass

A maximum stellar mass from quantum mechanics alone

Learning Goals

Derive the electron degeneracy pressure’s scaling with density from the Pauli exclusion principle and the Heisenberg uncertainty principle.
Derive the Chandrasekhar mass from the n=3 polytrope (Lecture 08) applied to the relativistic-degenerate equation of state, and evaluate it numerically.
Apply the result to Sirius B and to the more massive PSR J0348+0432 neutron star, and correctly classify each relative to the limit.

Reading anchor: OpenStax Astronomy 2e, Chapter 22.1-22.2 (the death of a high-mass star, white dwarf limits) states the Chandrasekhar mass’s existence and approximate value; this course derives its numerical value directly from the n=3 polytrope and fundamental constants.

Why This Matters

This is the single most important quantitative result connecting stellar evolution to compact-object astrophysics: it explains why white dwarfs have a maximum possible mass, why exceeding that mass produces a different kind of compact object entirely (Lecture 13), and it is derived here using exactly the n=3 polytrope machinery built in Lecture 08 for a completely different-looking physical system.

Opening Phenomenon

PSR J0348+0432’s neutron star, whose mass (2.01 ± 0.04 M☉) was live-verified this session via a direct fetch of its measured orbital and relativistic parameters, is far too massive to be a white dwarf -- exactly the physical distinction this lecture’s Chandrasekhar-mass derivation makes precise.

First question: what here is directly observed, and what follows only once a physical law is derived and applied?

Terms for This Lecture

electron degeneracy pressure

Definition: Electron degeneracy pressure denotes the physical quantity, process, or model component captured by this lecture-specific statement: The Pauli exclusion principle forbids two electrons from occupying the same quantum state; compressing a gas of electrons to high density therefore forces them to occupy increasingly high-momentum states, producing a pressure (degeneracy pressure) that exists even at zero temperature and...

Why it matters here: This is the single most important quantitative result connecting stellar evolution to compact-object astrophysics: it explains why white dwarfs have a maximum possible mass, why exceeding that mass produces a different kind of compact object entirely (Lecture 13), and it is derived here using exactly the n=3 polytrope machinery...

Relationships: It is used with Pauli exclusion principle to interpret the lecture model. The quantitative connection is the working relation \(M_{\rm Ch} = \dfrac{\omega_3^{0}\sqrt{3\pi}}{2}\left(\dfrac{\hbar c}{G}\right)^{3/2}\dfrac{1}{(\mu_e m_p)^{2}}, \qquad \omega_3^{0} = 2.018236\ (\text{Lane--Emden } n=3)\).

Boundary or common confusion: Assuming the Chandrasekhar mass is exactly 1.4 M☉ for every white dwarf. The exact value depends on the composition-dependent mean molecular weight per electron μ_e (2.0 for carbon-oxygen, close to 2.0 but not identical for other compositions); this lecture’s numerical result of {:.3f} M☉ for μ_e=2.0 is close to, but not required to be pixel-identical with,...

Evidence We Need to Explain

No white dwarf has ever been observed with a mass exceeding approximately 1.4 M☉, despite thousands of white dwarfs having been precisely characterized, directly consistent with the Chandrasekhar mass acting as a genuine, universal upper limit rather than a typical or average value.
Type Ia supernovae, understood to result from a white dwarf being pushed to (or very near) the Chandrasekhar mass by mass transfer or merger, show a strikingly uniform peak luminosity across a very wide range of host galaxies -- direct observational evidence that a single, nearly universal mass scale governs their explosion, exactly as the Chandrasekhar-mass derivation predicts.
PSR J0348+0432’s neutron star, measured via a combination of radio pulsar timing and white-dwarf-companion spectroscopy (Antoniadis et al. 2013), has a mass well above any observed white dwarf, directly confirming that objects above the Chandrasekhar mass are supported by a physically distinct mechanism (neutron degeneracy pressure, Lecture 13) rather than simply being unusually massive white dwarfs.

Derivation and Model

The Pauli exclusion principle forbids two electrons from occupying the same quantum state; compressing a gas of electrons to high density therefore forces them to occupy increasingly high-momentum states, producing a pressure (degeneracy pressure) that exists even at zero temperature and depends only on density, not on temperature at all.
For non-relativistic degenerate electrons, this gives P ∝ ρ^(5/3) (an n=3/2 polytrope, Lecture 08); but as density rises further, the electrons’ Fermi momentum approaches m_e c, and the equation of state softens to the relativistic-degenerate limit, P ∝ ρ^(4/3) -- exactly the n=3 polytrope of Lecture 08.
Because the n=3 polytrope’s mass is independent of central density (Lecture 08), applying it to the relativistic-degenerate equation of state produces a single, fixed maximum mass -- the Chandrasekhar mass -- above which no static, cold, electron-degenerate configuration exists at all, regardless of how much further the star is compressed.

Quantitative Tool

\[ M_{\rm Ch} = \dfrac{\omega_3^{0}\sqrt{3\pi}}{2}\left(\dfrac{\hbar c}{G}\right)^{3/2}\dfrac{1}{(\mu_e m_p)^{2}}, \qquad \omega_3^{0} = 2.018236\ (\text{Lane--Emden } n=3) \]

Worked Example

Evaluating the Chandrasekhar-mass formula numerically for a carbon-oxygen white dwarf (μ_e = 2.0, one electron per two nucleons): M_Ch = 1.4351 M☉ -- in close agreement with the commonly cited value of about 1.4 M☉ (this course’s own first-principles derivation, not simply quoting the standard number).
Sirius B: measured mass 1.018 M☉ = 70.9% of the Chandrasekhar mass derived above -- comfortably below the limit, consistent with its stable, observed existence as a white dwarf for a Sirius-system age of a few hundred million years.
PSR J0348+0432’s neutron star: measured mass 2.01 M☉ = 140% of the Chandrasekhar mass -- well above the limit for any electron-degenerate configuration, direct confirmation that this object cannot be, and is not, a white dwarf; its companion white dwarf, at 0.172 M☉ (12.0% of M_Ch), is safely below the limit, as real, stable white dwarfs must be.

Visual Reasoning

Trace the reasoning chain from raw observation to derived physical law to inferred quantity in this lecture’s figure.

Which stage is directly observed?
Which stage is the physical law or derivation step?
What would change if an assumption in the derivation failed?

Electron Degeneracy and the Chandrasekhar Mass: from observation to inferred quantity.

Common Misconception

Assuming the Chandrasekhar mass is exactly 1.4 M☉ for every white dwarf. The exact value depends on the composition-dependent mean molecular weight per electron μ_e (2.0 for carbon-oxygen, close to 2.0 but not identical for other compositions); this lecture’s numerical result of {:.3f} M☉ for μ_e=2.0 is close to, but not required to be pixel-identical with, the frequently quoted "1.4 M☉" figure.

Active Learning Segment

Using M_Ch ∝ μ_e⁻², predict qualitatively (without recomputing) whether a white dwarf composed of pure helium (μ_e ≈ 2.0, essentially the same as carbon-oxygen) or one with a larger fraction of heavier, more neutron-rich nuclei (slightly higher μ_e) would have the larger Chandrasekhar mass.

Lab Connection

Lab 06 computes the Chandrasekhar mass exactly as derived here and applies it to Sirius B; Lab 07’s capstone applies the same limit to PSR J0348+0432 to confirm it must be a neutron star, not a white dwarf.

Synthesis

The Chandrasekhar mass, derived here from the same n=3 polytrope machinery built in Lecture 08 but now applied to relativistic electron degeneracy pressure, is a genuine maximum mass for any cold, static, electron-degenerate star -- a limit this lecture confirms quantitatively using both a real white dwarf safely below it (Sirius B) and a real neutron star safely above it (PSR J0348+0432).

References

OpenStax Astronomy 2e, Chapter 22.1-22.2 (the death of a high-mass star, white dwarf limits) states the Chandrasekhar mass’s existence and approximate value; this course derives its numerical value directly from the n=3 polytrope and fundamental constants.
Course dataset and derivations used in this lecture’s worked example: materials/ASTR330/data/ and materials/ASTR330/src/generate_astr330_content.py.