Astronomy

Stellar death

Red giants, supernovae, and sketches of white dwarfs, neutron stars, and black holes.

Basics

After the main sequence

When core hydrogen runs out the core contracts, shell burning swells the star into a red giant. Sun-like stars flash helium then walk the horizontal and asymptotic giant branches. They shed envelopes as planetary nebulae and leave cooling white dwarfs. Much heavier stars take shorter, fiercer paths. On the HR diagram they move right, then down.

White dwarfs and neutron stars

White dwarfs are held by electron degeneracy. Past the Chandrasekhar limit (~1.4 M_☉) they are unstable. Heavier cores collapse to neutron stars, held by neutron degeneracy and nuclear forces. Pulsars are beams that sweep Earth as pulses. Densities approach nuclear matter. Mass–radius tracks the equation of state.

Supernovae

Type Ia often pictures a white dwarf accreting toward the limit and thermally runawaying. Similar peak light makes them distance markers. Core-collapse supernovae (Type II and kin) follow iron-core collapse in massive stars. Neutrinos and a shock unbind the envelope. Hydrogen in the spectrum sorts types. Remnants enrich the ISM.

Black-hole sketch

Outside the event horizon even light cannot escape. The Schwarzschild radius is 2GM/c². Stellar-mass holes are remnants of heavy stars; supermassive ones sit in galaxy centres. We see accretion disks, jets, and lensing — not a solid horizon. Hawking radiation is a micro/theory story and still tiny for observed objects.