Life & Death of Stars — Quick Summary

Formation & the main sequence

Both paths start the same way, then diverge based on mass.

• A nebula collapses under gravity → protostar heats up.
• If T reaches ~10⁷ K, fusion ignites (proton-proton chain, H→He, ~25 MeV released) → hydrostatic equilibrium → joins the main sequence (≈90% of a star's life).
• Insufficient mass to ignite fusion → becomes a brown dwarf instead.

Low-mass stars (<6 M☉): the Sun's fate

• H fuel runs out → core collapses & heats to ~10⁸ K → He fusion begins (triple-alpha → carbon) → outer layers expand → red giant.
• He exhausted → outer layers shed as a planetary nebula.
• Core remains as a white dwarf (Earth-sized, ρ≈10⁹ kg m⁻³, held up by electron degeneracy pressure) → slowly cools to a black dwarf over tens of billions of years.

High-mass stars: supernova & remnants

• After the red supergiant phase, successive fusion rounds (C, Ne, Mg, Si...) build an onion-layered star around an iron core — iron can't release net energy by fusing.
• Core collapses catastrophically; infalling gas rebounds off the dense core → Type II supernova (rapid rise in absolute magnitude, briefly rivalling the full moon).
• Remnant becomes either a neutron star or a black hole, depending on the leftover mass.

Neutron stars & pulsars

• Neutron degeneracy pressure halts collapse; density ≈10¹⁷ kg m⁻³ (nuclear density) — a 1.4 M☉ neutron star is only ~10 km across.
• Rapid spin (angular momentum conservation) + intense magnetic field → beamed radio emission. If the beam sweeps past Earth, it's observed as a regular pulsar.

Black holes

The event horizon is a boundary, not a point.

Core mass >~10 M☉ → even neutron degeneracy pressure fails → collapse to a singularity. The event horizon is the boundary where escape velocity = c; its radius is the Schwarzschild radius, R_s≈2GM/c².