Stellar core

A star’s central region, where gravity compresses matter and nuclear fusion may generate energy. Its temperature, pressure, and composition shape the star’s structure and evolution.

Hydrostatic equilibrium: A balance in which inward gravity is offset by outward pressure in a stable astronomical object. It describes how pressure in a core can support the overlying star against gravity.
Gravitational pressure: Pressure produced within an astronomical object by the weight of matter under gravity. The weight of the star’s outer layers compresses its central region.
Main sequence: The long-lived phase when a star fuses hydrogen into helium in its core. A main-sequence core’s hydrogen-burning rate largely sets the star’s luminosity and lifetime.
Asteroseismology: The study of stellar oscillations to infer the internal structure and properties of stars. Core-sensitive oscillation patterns provide indirect measurements of stellar interiors.
Nuclear fusion: The joining of light atomic nuclei into heavier nuclei, releasing energy when the products have lower total mass. Fusion supplies energy in the cores of main-sequence stars and many evolved stars.
Plasma: A state of matter made of free electrons and ions, often responsive to electromagnetic fields. Stellar cores consist largely of ionized matter rather than ordinary neutral gas.
Helium flash: A rapid onset of helium fusion in a degenerate core of a low-mass red giant. It marks a dramatic change in the core after central hydrogen is exhausted.
Convective overshoot: The extension of convective motions beyond the boundary predicted by a simple stellar model. Overshoot can mix fuel into or out of a core and change its effective size.
Proton–proton chain: A sequence of nuclear reactions that converts hydrogen into helium, dominant in stars like the Sun. This is the Sun’s principal core hydrogen-fusion pathway.
Equation of state: A relation connecting thermodynamic quantities such as pressure, density, and temperature for a material. It links core temperature and density to the pressure that supports the star.

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