All the iron in your blood, the calcium in your bones, the carbon in every molecule of your body — except the hydrogen — was manufactured inside a star and expelled when that star died. The hydrogen is older still: made in the first three minutes after the Big Bang. Stellar nucleosynthesis is the origin of the periodic table beyond H and He. Carl Sagan's formulation — we are made of star-stuff — is not metaphor; it is a load-bearing claim of nuclear astrophysics. Cecilia Payne's 1925 Harvard PhD thesis established that the Sun is mostly hydrogen and helium, a result initially rejected by senior figures who insisted the Sun must match Earth in composition, later universally accepted. By 1939 Hans Bethe had worked out the proton-proton chain and CNO cycle.
The single most useful diagram in stellar physics is the Hertzsprung-Russell diagram: plot luminosity against surface temperature, and the structure of stellar life appears. Most stars cluster on a main sequence running from cool red dwarfs at the bottom right to hot blue giants at the top left, where they fuse hydrogen to helium for most of their lifetime. Off the main sequence sit red giants, white dwarfs, and supergiants. Where a star sits is set by its mass, and mass also determines speed of evolution: L ∝ M^3.5, so a star twice the Sun's mass burns ~10× faster and lives ~5× shorter. A 0.5 M_⊙ red dwarf lives ~100 billion years — longer than the current age of the universe, so no red dwarf has ever died of old age. The endpoint depends on mass. Low-mass stars (~0.08 to ~8 M_⊙, including the Sun) burn hydrogen on the main sequence, swell into a red giant when the core hydrogen exhausts, burn helium to carbon via the triple-alpha process, shed the envelope as a planetary nebula, and end as a white dwarf supported by electron degeneracy pressure. Higher-mass stars (~8 to ~25 M_⊙) push fusion past carbon through neon, oxygen, silicon, until the core is iron and fusion no longer releases energy. The core collapses in seconds; the outer layers blow off in a core-collapse supernova; the remnant is a neutron star, ~1.4 M_⊙ in a ~12 km radius. Stars above ~25 M_⊙ collapse further to stellar-mass black holes. The 2015 LIGO detection of GW150914 was the first direct observation of objects produced this way.
JWST's deep observations are reaching the era of the first stars — Population III, hypothetical extremely-metal-poor stars formed from pristine post-Big-Bang gas; as of 2024, suggestive evidence exists in galaxy spectra at z > 6 but no individual Pop III star has been resolved. Multi-messenger astronomy — gravitational-wave detectors coupled with electromagnetic follow-up — has moved stellar-collapse physics from inference to direct observation. The 2017 GW170817 binary-neutron-star merger, detected first as gravitational waves and then across the electromagnetic spectrum over two months, is the most comprehensively observed astrophysical event in history. Asteroseismology — surface oscillations used to map stellar interiors — has been transformed by Kepler and TESS data, now profiling thousands of nearby stars at unprecedented precision. Stellar physics is the bridge from particle physics to chemistry: without stellar nucleosynthesis, there is no carbon, oxygen, calcium, or iron.