11.2 Supernova explosions

Supernovae are cosmic explosions that mark the dramatic deaths of stars. These events release immense energy, creating extreme conditions of temperature and pressure that fascinate high energy density physicists.

Supernovae come in different types, each with unique characteristics. From core collapse in massive stars to thermonuclear explosions in white dwarfs, these cosmic blasts shape the universe by synthesizing heavy elements and triggering star formation.

Types of supernovae

• Supernovae play a crucial role in high energy density physics by creating extreme conditions of temperature and pressure
• Classification of supernovae provides insights into different stellar evolution pathways and energy release mechanisms

Core collapse supernovae

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• Occur in massive stars (>8 solar masses) when iron core can no longer support itself against gravity
• Collapse triggers a shockwave that ejects outer layers of the star
• Leave behind compact remnants (neutron stars or black holes)
• Produce large quantities of heavy elements through r-process nucleosynthesis

Thermonuclear supernovae

• Result from runaway nuclear fusion in white dwarf stars
• Typically occur in binary systems where white dwarf accretes matter from companion
• Complete destruction of progenitor star with no compact remnant
• Major source of iron-peak elements in the universe
• Used as standard candles for measuring cosmic distances (Type Ia supernovae)

Pair-instability supernovae

• Extremely rare events occurring in very massive stars (>140 solar masses)
• Core becomes so hot that photons spontaneously convert into electron-positron pairs
• Loss of photon pressure causes rapid collapse followed by explosive oxygen burning
• Can completely disrupt the star leaving no remnant
• Theoretical mechanism for producing superluminous supernovae

Physical processes

• Supernovae involve complex interplay of nuclear, particle, and plasma physics
• Understanding these processes is crucial for modeling supernova explosions and their effects on the surrounding medium

Nuclear fusion reactions

• Power stellar evolution and supernova explosions
• In core collapse supernovae, silicon burning produces iron-peak elements
• Thermonuclear supernovae driven by carbon and oxygen fusion
• Fusion reactions release enormous amounts of energy ($E = mc^2$)
• Network of reactions determines nucleosynthesis yields

Neutrino production

• Dominant energy transport mechanism in core collapse supernovae
• Produced through electron capture and thermal processes in collapsing core
• Carry away ~99% of the gravitational binding energy released
• Neutrino heating crucial for shock revival and successful explosion
• Detection of neutrino burst provides early warning of supernova events

Shock wave propagation

• Central feature of supernova explosions driving mass ejection
• Initially formed when collapsing core bounces at nuclear densities
• Propagates outward, heating and accelerating stellar material
• Can be revived by neutrino heating in core collapse supernovae
• Drives nucleosynthesis through shock heating and compression

Nucleosynthesis

• Production of new atomic nuclei during supernova explosions
• Responsible for creating majority of elements heavier than iron
• Occurs through various processes
  • r-process (rapid neutron capture) in neutron-rich environments
  • Alpha-process producing elements up to nickel-56
• Yields depend on progenitor composition and explosion dynamics

Energy release mechanisms

• Supernovae are among the most energetic events in the universe
• Understanding energy sources and transport crucial for explosion modeling

Gravitational potential energy

• Primary energy source in core collapse supernovae
• Released as core contracts from ~1500 km to ~10 km radius
• Total energy release ~10^53 ergs
• Majority of energy carried away by neutrinos
• Small fraction (~1%) powers the kinetic energy of the explosion

Nuclear binding energy

• Dominant energy source in thermonuclear supernovae
• Released through fusion of carbon and oxygen into iron-peak elements
• Typical energy release ~10^51 ergs
• Directly powers the kinetic energy of the explosion
• Determines the brightness and duration of the supernova light curve

Neutrino energy transport

• Critical for successful core collapse supernova explosions
• Neutrinos carry energy from hot core to outer layers of the star
• Neutrino heating can revive stalled shock wave
• Challenging to model due to complex neutrino-matter interactions
• Requires sophisticated radiation transport calculations in simulations

Supernova remnants

• Long-lasting aftermath of supernova explosions
• Provide valuable information about explosion mechanisms and progenitors
• Serve as laboratories for studying high energy density physics in space

Expanding shells

• Visible remnants of supernova explosions
• Consist of ejected stellar material and swept-up interstellar medium
• Expand at high velocities (1000-10000 km/s)
• Evolution described by self-similar solutions (Sedov-Taylor phase)
• Emit across electromagnetic spectrum (radio, optical, X-ray)

Neutron stars

• Ultra-dense remnants of core collapse supernovae
• Mass of ~1.4 solar masses compressed into ~10 km radius
• Supported against gravity by neutron degeneracy pressure
• Can manifest as pulsars due to rapid rotation and strong magnetic fields
• Provide unique laboratories for studying matter at extreme densities

Black holes

• Form when core of massive star collapses beyond neutron star limit
• No known mechanism can halt collapse once event horizon forms
• Mass range from ~3 solar masses to supermassive black holes
• Can power energetic phenomena through accretion (quasars)
• Recent detections of gravitational waves from merging black holes

Observational signatures

• Multi-messenger astronomy provides diverse probes of supernova physics
• Combining different observations constrains theoretical models

Light curves

• Plot of supernova brightness over time
• Shape determined by explosion energy, ejecta mass, and composition
• Plateau in Type II supernovae due to hydrogen recombination
• Exponential decay in Type Ia supernovae powered by radioactive nickel-56
• Used to classify supernovae and estimate explosion parameters

Spectra

• Reveal composition and velocity structure of supernova ejecta
• Broad emission and absorption lines due to high expansion velocities
• Evolution of spectral features traces different layers of the star
• Presence or absence of hydrogen distinguishes Type I and Type II supernovae
• Doppler shifts of spectral lines measure ejecta velocities

Gravitational waves

• Produced by asymmetric core collapse or neutron star oscillations
• Detectable by interferometers (LIGO, Virgo) for nearby supernovae
• Provide direct probe of core collapse dynamics
• Complementary to electromagnetic and neutrino observations
• Recent detections from neutron star mergers, not yet from supernovae

Neutrino detection

• Crucial for early detection of core collapse supernovae
• Large underground detectors (Super-Kamiokande, IceCube) sensitive to neutrino burst
• Neutrino signal arrives hours before optical brightening
• Flavor composition and time structure probe neutron star formation
• Only detected so far for SN 1987A in nearby Large Magellanic Cloud

Supernova progenitors

• Understanding progenitor systems crucial for supernova theory
• Diverse range of stellar systems can lead to supernova explosions

Massive stars

• Progenitors of core collapse supernovae
• Main sequence mass >8 solar masses
• Evolve through successive stages of nuclear burning
• Final fate depends on initial mass and mass loss history
• Pre-supernova structure determines explosion dynamics and nucleosynthesis

White dwarfs

• Progenitors of Type Ia supernovae
• Remnants of low and intermediate mass stars
• Composed of carbon and oxygen supported by electron degeneracy
• Accrete matter from companion star in binary system
• Explode when mass approaches Chandrasekhar limit (~1.4 solar masses)

Binary systems

• Play crucial role in many supernova scenarios
• Mass transfer can alter stellar evolution pathways
• Common envelope evolution can lead to mergers
• Provide mechanism for stripping hydrogen envelope in some core collapse supernovae
• Essential for explaining observed supernova rates and properties

Supernova rates

• Important for understanding stellar evolution and galactic chemical enrichment
• Challenging to measure due to observational biases and completeness issues

Galactic supernovae

• Occur in Milky Way galaxy at rate of ~1-3 per century
• Last naked-eye supernova was Kepler's Star in 1604
• Historical records and supernova remnants constrain past rates
• Next galactic supernova eagerly anticipated by astronomers
• Proximity would allow unprecedented multi-messenger observations

Extragalactic supernovae

• Thousands detected annually in other galaxies
• Rate depends on galaxy type and star formation rate
• Core collapse supernovae trace current star formation
• Type Ia supernovae have longer delay times from star formation
• Volumetric rate in local universe ~10^-4 per year per Mpc^3

Astrophysical implications

• Supernovae have far-reaching effects on cosmic evolution
• Influence chemistry, dynamics, and structure of galaxies and intergalactic medium

Chemical enrichment of universe

• Supernovae produce and disperse heavy elements
• Core collapse supernovae main source of oxygen, magnesium, silicon
• Type Ia supernovae dominant source of iron
• Enriched material incorporated into next generation of stars and planets
• Abundance patterns in old stars trace early supernova nucleosynthesis

Cosmic ray acceleration

• Supernova remnants accelerate particles to relativistic energies
• Diffusive shock acceleration primary mechanism
• Can produce cosmic rays up to ~10^15 eV
• Contribute significant fraction of galactic cosmic ray flux
• Accelerated particles can affect supernova remnant dynamics

Triggering star formation

• Supernova shockwaves compress nearby molecular clouds
• Compression can initiate gravitational collapse and star formation
• Evidence for supernova-triggered star formation in some regions (Orion)
• May lead to self-propagating star formation in galaxies
• Contributes to regulation of galactic star formation rates

Numerical simulations

• Essential tools for understanding complex supernova physics
• Require integration of multiple physical processes across vast range of scales

Hydrodynamic models

• Simulate fluid motions in supernova explosions
• Range from 1D spherically symmetric to full 3D simulations
• Must capture wide range of spatial and temporal scales
• Include effects of gravity, nuclear reactions, and neutrino transport
• Recent 3D models crucial for understanding explosion mechanisms

Radiation transport

• Models propagation of photons and neutrinos through supernova ejecta
• Critical for core collapse supernova simulations
• Computationally expensive due to high dimensionality of problem
• Methods include flux-limited diffusion and Monte Carlo techniques
• Accurate treatment essential for modeling supernova light curves and spectra

Nucleosynthesis calculations

• Predict elemental and isotopic yields from supernovae
• Require nuclear reaction networks with hundreds of isotopes
• Must account for changing temperature and density conditions
• Results sensitive to details of explosion dynamics
• Essential for interpreting observed abundance patterns in stars and galaxies

High energy density aspects

• Supernovae create some of the most extreme conditions in the universe
• Study of supernovae closely linked to high energy density physics

Extreme temperatures

• Core temperatures in core collapse can reach ~100 billion Kelvin
• Thermonuclear supernovae involve temperatures of billions of Kelvin
• High temperatures lead to creation of electron-positron pairs
• Thermal neutrino production becomes significant
• Radiation pressure dominates equation of state

Extreme densities

• Core of collapsing star reaches nuclear densities (~10^14 g/cm^3)
• Neutron stars have central densities exceeding nuclear saturation density
• Matter becomes highly degenerate and relativistic
• Exotic phases of matter (quark-gluon plasma) may form in most extreme cases
• Challenges our understanding of nuclear physics and quantum chromodynamics

Plasma physics

• Supernova ejecta rapidly ionized by shock heating
• Magnetic fields can be amplified by turbulence and dynamo processes
• Plasma instabilities (Rayleigh-Taylor, Kelvin-Helmholtz) shape ejecta structure
• Collisionless shocks important for particle acceleration
• Radiative processes in hot plasma determine observed spectra

Supernova diagnostics

• Observational techniques for inferring supernova properties
• Crucial for testing theoretical models and simulations

Elemental abundances

• Measured from spectra of supernova ejecta and remnants
• Reveal nucleosynthesis processes and progenitor composition
• Abundance ratios (Fe/O) distinguish core collapse and thermonuclear origins
• Trace elements (Sr, Y, Zr) provide evidence for r-process nucleosynthesis
• Gamma-ray lines from radioactive decay directly probe nucleosynthesis

Explosion energies

• Estimated from kinetic energy of ejecta and radiated energy
• Typical energies range from 10^51 ergs (Type Ia) to 10^52 ergs (hypernovae)
• Inferred from light curve modeling and ejecta velocities
• Constrain theoretical models of explosion mechanisms
• Hypernova energies may require additional power sources (magnetar, black hole accretion)

Ejecta velocities

• Measured from Doppler shifts of spectral lines
• Reveal dynamics and structure of supernova explosions
• High-velocity features indicate presence of radioactive nickel in outer layers
• Asymmetries in line profiles suggest aspherical explosions
• Evolution of velocities traces shock propagation and ejecta expansion