stellar-structure-and-evolution

# Stellar Structure and Evolution A star is a self-gravitating ball of plasma held in balance between the inward pull of gravity and the outward push of pressure, radiating away the energy that nuclear fusion liberates in its core. Everything about a star's appearance and destiny — how bright it is, how hot its surface is, how long it lives, and what it leaves behind — follows from a single dominant parameter: its mass. This skill covers the physics that turns mass into a life story. It builds the Hertzsprung-Russell diagram as the organizing map of stellar populations, derives the equilibrium that sets a star's structure, explains why the main sequence exists and why the mass-luminosity relation is so steep, walks the nuclear burning stages that power each phase, follows a star off the main sequence into the giant branches, and ends at the three possible corpses — white dwarf, neutron star, or black hole — with the Chandrasekhar limit as the switch between them. **Agent affinity:** chandrasekhar-astro (structure, evolution, degenerate endpoints), payne-gaposchkin (spectral placement on the HR diagram), burbidge (nucleosynthesis) **Concept IDs:** astro-hr-diagram, astro-nuclear-fusion, astro-stellar-classification ## The Hertzsprung-Russell Diagram The HR diagram plots luminosity (vertically, increasing upward) against surface temperature (horizontally, increasing to the *left* — a historical convention inherited from spectral type OBAFGKM running hot-to-cool). It is the...