Supernova

A powerful stellar explosion marking the death of a massive star or the thermonuclear destruction of a white dwarf. The most energetic events in the observable universe after the big-bang. Supernovae are the primary mechanism for distributing heavy elements through the interstellar-medium, the confirmed source of galactic cosmic-rays, the origin of the local-bubble, and the distance rulers that discovered dark-energy.

Two Fundamental Types

Type Ia — Thermonuclear (White Dwarf)

Occurs when a carbon-oxygen white dwarf in a binary system accretes enough mass to approach the Chandrasekhar limit (~1.44 M☉). Before true collapse, carbon fusion ignites and propagates as a runaway thermonuclear explosion — completely unbinding the star.

• Energy: ~1.5×10⁴⁴ J, split mostly into kinetic energy of ejecta and heavy element synthesis; ~½ M☉ of ⁵⁶Ni produced, which radioactively decays through ⁵⁶Co → ⁵⁶Fe, powering the light curve
• Peak luminosity: absolute magnitude ~−19.3 (5 billion times brighter than the Sun) — extremely consistent across events
• Standard candles: this consistency makes Type Ia the most accurate cosmological distance rulers. The 1998–1999 discovery that distant Type Ia supernovae were fainter than expected (i.e., farther away) revealed the accelerating expansion of the universe — the key evidence for dark-energy (2011 Nobel Prize in Physics: Perlmutter, Schmidt, Riess)
• Nucleosynthesis: silicon and iron-peak elements (Fe, Ni)

Core Collapse (Massive Stars ≥9 M☉)

When a massive star exhausts its nuclear fuel, the iron core collapses at 70,000 km/s (0.23c). The result depends on core mass:

• Low-mass degenerate cores → neutron star; collapse halted by neutron degeneracy pressure at ~30 km diameter, ~10¹⁴ g/cm³
• High-mass cores (>15 M☉) → black hole forms directly with little emitted energy; no visible supernova
• Non-degenerate cores → runaway fusion (pair-instability supernovae for 60–130 M☉ cores)

Energy budget: ~10⁴⁶ J total — 99% emitted as neutrinos in a 10-second burst (approximately 10% of the star's rest mass). The visible explosion is powered by ~1% of this neutrino energy being reabsorbed by the shock wave. Core collapse supernovae are visually fainter than Type Ia but release 100× more total energy.

Shock wave: The suddenly halted collapse rebounds, producing a shock that stalls within milliseconds until neutrino reabsorption (mechanism not fully understood) re-energizes it, expelling the outer layers.

Subtypes

Nucleosynthesis and ISM Enrichment

Supernovae are the dominant mechanism for distributing heavy elements through the galaxy:

• Type Ia: silicon, iron-peak elements (Fe, Ni, Co)
• Core collapse: oxygen, neon, and elements beyond zinc; r-process (rapid neutron capture) produces ~half of all isotopes heavier than iron — including gold, platinum, uranium
• Each stellar generation has higher metallicity; heavy element abundances influence planet formation (more giant planets around metal-rich stars)
• Remnant shock waves trigger star formation by compressing molecular clouds — and may inhibit it if turbulent pressure exceeds the cloud's binding energy
• Evidence from short-lived radioactive isotopes (⁶⁰Fe, ²⁶Al) in Solar System meteorites suggests a nearby supernova helped shape the Solar System's composition 4.5 billion years ago — and may have triggered its formation

Supernova Remnants

The remnant consists of a compact object (neutron star or black hole) surrounded by an expanding shock wave. The shock:

1. Free expansion phase (~200 years) — sweeps up surrounding ISM
2. Adiabatic expansion (~10,000 years) — gradually cools
3. Mixing — material disperses into the ISM

Cosmic Rays and Gravitational Waves

Cosmic rays — Supernova remnants confirmed as accelerators of galactic cosmic-rays (2013, Fermi data; gamma-ray pion decay observed in remnants IC 443 and W44). Shock front acceleration bounces stripped nuclei between shock fronts until they achieve cosmic ray energies.

Gravitational waves — Supernovae are theoretically strong GW sources but none detected yet; all GW detections to date come from compact object mergers — the probable remnants of prior supernovae.

Fast radio bursts — Magnetars formed by core-collapse supernovae are the leading candidate explanation.