stellar model
We can never drill into a star to see what is inside, so how do astronomers claim to know the temperature and density at the Sun's core? They build a stellar model: a detailed, physics-based calculation of a star's hidden interior, layer by layer, from center to surface. It is the theoretical 'cross-section' of a star you could never cut open — a virtual star living inside a computer, made to obey the same laws the real one does.
A model is built by solving the equations of stellar structure together with the supporting physics — how the gas's pressure depends on temperature and density, how opaque it is, and how fast it fuses. You feed in just two main ingredients, the star's total mass and its chemical composition, and the equations grind out a self-consistent profile: the temperature, pressure, density, and energy flow at every radius. Let the composition slowly change as fusion converts hydrogen to helium, and the model evolves, tracing the star's brightness and size through time.
Stellar models are how we know almost everything about stellar interiors and life cycles. They predict that the Sun's center is 15 million kelvin, that a star ten times heavier lives only a few tens of millions of years, that a star will swell into a red giant and end as a white dwarf — predictions tested against starlight, star clusters, helioseismology (the Sun's vibrations), and neutrinos straight from the core. Models are powerful but not perfect: convection, mixing, and mass loss are handled with approximations, so the best models are continually checked and refined against observations.
When astronomers say the Sun's core is 15 million kelvin and 150 times denser than water, no one measured that directly — a stellar model computed it. The model's success is confirmed because the neutrinos and the surface vibrations it predicts match what detectors and telescopes actually see.
Core conditions are computed by models, then validated by neutrinos and helioseismology.
A stellar model is a calculation, not a direct measurement; its reliability rests on the physics fed in and on tests like neutrinos and helioseismology. Treating model numbers as if they were observed facts overstates how directly we 'see' inside stars.