A convection zone (or convective region) is a stellar layer in which large-scale mass motions carry a significant fraction of the energy flux, in contrast to radiative (or conductive) zones where photon transport dominates. In many stars, notably the Sun and red giants, convective regions reach the surface and give rise to the observed granulation pattern produced by upwelling hot plasma and returning cooler flows. Convection consists of organized cellular currents: fluid elements heated at depth ascend, expand and cool as they enter lower-pressure layers, then become denser and descend, closing the circulation. The onset of such motion is governed by the Schwarzschild criterion: a small parcel displaced upward will continue to rise only if, after adiabatic expansion, it remains less dense (warmer) than its surroundings; if it becomes denser (cooler) than the environment it will sink back, indicating local stability. Consequently, convective instability arises where the local temperature gradient is steep enough or where the material’s heat capacity is large enough that ascending parcels do not cool sufficiently during expansion, producing the convective layers observed in stellar interiors.
Main-sequence stars exhibit two principal patterns of internal energy transport determined primarily by mass. Stars with masses ≳1.3 M☉ attain central temperatures high enough that hydrogen fusion proceeds mainly via the carbon–nitrogen–oxygen (CNO) catalytic cycle, a reaction chain whose rate is strongly sensitive to temperature compared with the proton–proton chain. The resulting steep central temperature gradient drives a convective core: a region of bulk overturn that continuously mixes fresh hydrogen into, and redistributes helium out of, the burning region, thereby altering fuel consumption and the core composition profile over the main-sequence lifetime. In many such higher-mass stars this convective core is bounded by an overlying radiative zone, where energy is transported predominantly by radiation and the stratification remains close to thermal equilibrium; in the most massive main-sequence objects the convection zone can extend outward to the photosphere, producing a fully convective star without a separate radiative envelope.
Lower-mass main-sequence stars (≲1.3 M☉) develop a contrasting structure in which the outer layers become convectively unstable. Partial ionization of hydrogen and helium in the cool outer envelope raises the local heat capacity, and the increased opacity contributed by heavier elements at these temperatures steepens the temperature gradient. Together these effects drive an outer convection zone; in solar-type stars this convective envelope manifests at the surface as granular convection. At the low-mass extreme (≲0.35 M☉), and for pre-main-sequence objects on the Hayashi track, the entire star is essentially convective and lacks a distinct radiative interior.
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Sun-like stars occupy an intermediate configuration with a radiative core beneath a convective envelope. The boundary between these regimes—the tachocline—is a thin shear and transition layer that separates the well-mixed outer convection zone from the radiatively stratified interior and plays a key role in angular momentum transport and magnetic field generation.
Red giants (asymptotic giant branch)
On the asymptotic giant branch, low- to intermediate-mass stars reach a late evolutionary stage in which nuclear burning proceeds in shells around an inert core rather than in a central furnace. Energy generation alternates between hydrogen- and helium-burning shells, producing time-dependent luminosity and thermal adjustments that drive large-scale structural changes in the envelope.
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A deep convective envelope develops during these shell-burning episodes, but its radial extent is highly variable: the outer convection zone responds to changes in shell luminosity and the resulting thermal stratification, expanding or contracting on relatively short timescales. Occasionally the convective boundary moves inward much more deeply than during quiescent phases, allowing convection to access layers that have recently experienced nucleosynthesis.
These episodic inward motions—dredge-up events—carry nuclear-processed material from the burning shells into the convective envelope and ultimately to the photosphere. The injection of fusion products into the surface layers alters the star’s observable chemical signatures and, when mass loss removes envelope material, enriches the surrounding interstellar medium.
By modifying both stellar spectra and the composition of ejected material, dredge-up on the AGB contributes directly to the chemical evolution of stellar populations, seeding later generations of stars with elements synthesized during prior phases of stellar evolution.