Astrophysics — Stellar Structure and Evolution

Manual: General · Subject: Astrophysics

Stellar equilibrium, nuclear burning, and stellar endpoints

The Life Cycle of Stars

Hydrostatic Equilibrium and the Main Sequence

A star is stable when gravity is balanced by radiation and gas pressure. Hydrostatic equilibrium:<br> $\frac{dP}{dr} = -\frac{G M(r)\rho(r)}{r^2}$ Core temperatures reach $\sim 10^7$ K, allowing hydrogen fusion ( $4p \to {}^4\text{He} + 2e^+ + 2\nu_e + 26.7\ \text{MeV}$ ). Stars spend most of their lives on the main sequence.

Stellar Evolution and Endpoints

After hydrogen exhaustion, stars expand to red giants/supergiants, fusing heavier elements. Stellar endpoints by initial mass:

$M < 8M_{\odot}$ : planetary nebula + white dwarf ( $M < 1.4 M_{\odot}$ Chandrasekhar limit)
$8\text{-}20 M_{\odot}$ : core-collapse supernova + neutron star (pulsars, $10\ \text{km}$ radius)
$M > 20 M_{\odot}$ : black hole ( $r_s = 2GM/c^2 <$ stellar radius)

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Chandrasekhar Limit

The maximum mass of a white dwarf is $\approx 1.4 M_{\odot}$ (Chandrasekhar limit). Above this, electron degeneracy pressure cannot support the star and it collapses to a neutron star or black hole.

The Chandrasekhar limit (~1.4 $M_{\odot}$ ) is the maximum mass for:

A white dwarf supported by electron degeneracy pressure A neutron star The Sun A red giant

What process powers main-sequence stars like the Sun?

Hydrogen fusion (PP chain and CNO cycle) Hydrogen fission Gravitational contraction alone Carbon fusion

Explain why a neutron star is so dense that a teaspoon would weigh $\sim 10^8$ tonnes.

Neuroanatomy Explorer

Memory Deck

Match the Pairs

Concept Constellation

Astrophysics — Stellar Structure and Evolution

The Life Cycle of Stars

Hydrostatic Equilibrium and the Main Sequence

Stellar Evolution and Endpoints