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SESK: Stuff Everyone Should Know · sesk.info/astro-hydrostatic-equilibrium

What balance keeps a stable star like the Sun from collapsing or flying apart?

Hydrostatic equilibrium: inside pressure balances the weight of the layers above.

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  1. Magnetic fields against spin
  2. Gas pressure against gravity ✔
  3. Solar wind against starlight

Why: Gravity pulls every layer inward. Pressure from hot gas pushes outward. In a stable star these match at every point, a balance called hydrostatic equilibrium.

Huh, didn't know that: Earth's oceans and atmosphere are also in hydrostatic equilibrium.

The dad joke
Stars are great at balance. They never lose their cool under pressure.
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References

Answer: Gas pressure against gravity. Gravity pulls every layer inward. Pressure from hot gas pushes outward. In a stable star these match at every point, a balance called hydrostatic equilibrium.

  1. Astronomy 2e, 16.3 The Solar Interior: Theory openstax.org/books/astronomy-2e/pages/16-3-the-solar-interior-theory
    equilibrium would again be reached when the pressure at every internal point equaled the weight of the stellar layers above that point.
    Checked 2026-10-10.

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