Astronomy & Celestial Mechanics
the sky, computed
Concept

Nuclear fusion — where it appears

The joining of light nuclei into heavier ones, releasing the difference in binding energy. In a star it is regulated by a thermostat — expansion cools the core when the rate rises — which fails only where the pressure has stopped depending on the temperature.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Energy generation against core temperature. The proton–proton chain and the CNO cycle, in solar units, against core temperature on logarithmic axes. The CNO curve is far steeper, so the two cross at 18.8 million kelvin — above that temperature a star runs mostly on CNO, and below it mostly on pp.

The furnace that runs cooler than a compost heap

The Sun's core produces 276 watts per cubic metre. A human body produces more. The Sun is bright because it is enormous, and its fusion is astonishingly slow — which is exactly why it lasts.

stars · Fusion Inside a star that is holding itself up (polytrope n = 3). Temperature, density and pressure through a star, as fractions of their central values, against fractional radius. All three come from one numerical integration of the Lane–Emden equation at index 3, which is the hydrostatic balance written for a gas whose pressure is a power of its density. The inner half of the radius holds 90% of the mass, and the outer half is nearly weightless — which is why the load, and so the temperature, is concentrated where the burning is.

A star is held up by its own weight

A star has a central temperature because it has a central pressure, and it has a central pressure because everything above is pressing down. The nuclear reactions do not set that temperature — they obey it.

stars · Hydrostatic equilibrium

Named alongside it

The objects these essays reach for when they reach for this one.

Binding energyCNO cycleEddington limitEffective temperatureGravitational potentialHydrostatic equilibriumMain sequenceMetallicityProton proton chainQuantum tunnellingStellar radius

All concepts