A stellar core is the extremely hot, dense region at the center of a star. For an ordinary main sequence star, the core region is the volume where the temperature and pressure conditions allow for energy production through thermonuclear fusion of hydrogen into helium. This energy in turn counterbalances the mass of the star pressing inward; a process that self-maintains the conditions in thermal and hydrostatic equilibrium. The minimum temperature required for stellar hydrogen fusion exceeds 107 K (10 MK), while the density at the core of the Sun is over 100 g cm−3. The core is surrounded by the stellar envelope, which transports energy from the core to the stellar atmosphere where it is radiated away into space.
Main sequence stars are distinguished by the primary energy generating mechanism in their central region, which joins four hydrogen nuclei to form a single helium atom through thermonuclear fusion. The Sun is an example of this class of star. Once stars with the mass of the Sun form, the core region reaches thermal equilibrium after about 100 million (108) years and becomes radiative. This means the generated energy is transported out of the core via radiation and conduction rather than through mass transport in the form of convection. Above this spherical radiation zone lies a small convection zone just below the outer atmosphere.
With decreasing stellar mass, the outer convection shell expands downward to take up an increasing proportion of the envelope, until at a mass of around 0.35 M☉ or below the entire star is convective, including the core region. These very low mass stars (VLMS) occupy the late range of the M-type main-sequence stars, or red dwarf. The VLMS form the primary stellar component of the Milky Way at over 70% of the total population. The low mass end of the VLMS range reaches about 0.075 M☉, below which ordinary (non-deuterium) hydrogen fusion does not take place and the object is designated a brown dwarf. The temperature of the core region for a VLMS decreases with decreasing mass, while the density increases. For a star with 0.1 M☉, the core temperature is about 5 MK while the density is around 500 g cm−3. Even at the low end of the temperature range, the hydrogen and helium in the core region is fully ionized.
Below about 1.2 M☉, energy production in the stellar core is predominantly through the proton–proton chain reaction, a process requiring only hydrogen. For stars above this mass, the energy generation comes increasingly from the CNO cycle, a hydrogen fusion process that uses intermediary atoms of carbon, nitrogen, and oxygen. In the Sun, only 1.5% of the net energy comes from the CNO cycle. For stars at 1.5 M☉ where the core temperature reaches 18 MK, half the energy production comes from the CNO cycle and half from the pp chain. The CNO process is more temperature sensitive than pp chain, with most of the energy production occurring near the very center of the star. This results in a stronger thermal gradient, which satisfies the criteria for convective instability. Hence, the core region is convective for stars above about 1.2 M☉.
For all masses of stars, as the particle density hydrogen becomes consumed, the temperature increases so as to maintain pressure equilibrium. This results in an increasing rate of energy production, which in turn causes the luminosity of the star to increase.
Once the supply of hydrogen at the core of a non-fully-convective star is depleted, it will leave the main sequence and evolve along the red giant branch of the Hertzsprung–Russell diagram. Low mass stars with up to about 1.2 M☉ will gradually move along the subgiant branch, having an inert helium core surrounded by a hydrogen-rich shell that is generating energy through the pp chain. This process will steadily increase the mass of the helium core, causing the hydrogen fusing shell to increase in temperature until it can generate energy through the CNO cycle. Due to the temperature sensitivity of the CNO process, this hydrogen fusing shell will be thinner than before. The increasing mass and density of the helium core will cause the star to increase in size and luminosity as it evolves up the red giant branch.
In the more massive main-sequence stars with core convection, the helium produced by fusion becomes mixed throughout the convective zone. Once the core hydrogen is consumed, it is thus effectively exhausted across the entire convection region. At this point the helium core starts to contract and hydrogen fusion begins along a shell around the perimeter. As the star ages, the core continues to contract and heat up until a triple alpha process can be maintained at the center, fusing helium into carbon. However, most of the energy generated at this stage continues to come from the hydrogen fusing shell. For stars above 10 M☉, helium fusion at the core begins immediately as the main sequence comes to an end. Two hydrogen fusing shells are formed around the helium core: a thin CNO cycle inner shell and an outer pp chain shell.
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