A main-sequence star is in hydrostatic equilibrium: outward gas pressure balances inward gravity. When the core runs out of hydrogen, this balance breaks and evolution begins. The path a star takes depends almost entirely on its mass.
Sun-like (low-mass) stars evolve through identifiable stages. (7) Main sequence → (8) Subgiant (H-shell burning starts) → (9) Red giant (envelope expands to ~100 R☉, cools, brightens) → (10) Horizontal branch (after the helium flash ignites core He → C at ~100 million K) → (11) Asymptotic giant branch (second red giant, H- and He-burning shells) → (12) Planetary nebula (outer layers ejected) → (13) White dwarf (hot, dense, Earth-sized carbon-oxygen remnant) → (14) Black dwarf (cooled, non-luminous — theoretical only, the universe isn't old enough yet).
A nova is a sudden brightening on a white dwarf that has accreted hydrogen from a binary companion; the white dwarf survives and can repeat. A supernova is a one-time, catastrophic explosion roughly 10⁶ times brighter than a nova. Two types: Type Ia — a white dwarf in a binary exceeds the Chandrasekhar limit (1.4 M☉) and carbon-detonates. All Type Ia have similar peak brightness, making them precious standard candles for cosmology. Type II — the iron core of a high-mass star collapses and rebounds.
High-mass stars (> 8 M☉) fuse heavier and heavier elements (He → C → O → Si → Fe) in concentric shells, ending in core collapse and a Type II supernova. The remnant is a neutron star (8–25 M☉ progenitor) or a black hole (> 25 M☉).
Star clusters are natural laboratories: all stars formed simultaneously but with different masses. The main-sequence turnoff point on the cluster's H-R diagram migrates to lower-mass (cooler) stars as the cluster ages, giving a direct age measurement.
FRQ 01From stage 7, list all stages of a sun-like star's life cycle.
FRQ 02Nova vs. supernova? Two types of supernova?
FRQ 03Evolution of stars of the same age in a cluster on the H-R diagram?