Core Collapse and Supernovae

The death of a massive star, or the explosive ignition of a white dwarf, releases in seconds more energy than the Sun emits in its entire life, briefly outshining a galaxy and forging and dispersing new elements.

Definition

Core collapse is the catastrophic implosion of a massive star's iron core once it exceeds the mass its degeneracy pressure can support, and a supernova is the resulting luminous explosion, whether from such a collapse or from the thermonuclear disruption of a white dwarf.

Scope

The topic covers the collapse of a massive star's iron core when degeneracy support fails, the bounce and neutrino-driven mechanism that may revive the stalled shock to produce a core-collapse supernova, the contrasting thermonuclear explosion of a white dwarf as a type Ia supernova, the spectral classification of supernovae, and the remnants and enrichment they leave behind.

Core questions

• What triggers the collapse of a massive star's core?
• How does the collapse turn into an explosion?
• How do thermonuclear and core-collapse supernovae differ?
• What do supernovae leave behind?

Key concepts

• iron-core collapse
• core bounce
• neutrino-driven mechanism
• type Ia supernova
• type II supernova
• supernova remnant
• standard candle

Key theories

Iron-core collapse and the neutrino mechanism

When an iron core exceeds the effective Chandrasekhar mass it collapses until nuclear densities halt it, launching a shock that stalls; energy deposited by the intense neutrino flux is thought to revive the shock and drive the explosion, leaving a neutron star or black hole.

Thermonuclear type Ia supernovae

A carbon-oxygen white dwarf pushed toward the Chandrasekhar limit by accretion or merger ignites a runaway thermonuclear burning that completely disrupts the star; because these explosions are highly uniform, they serve as standardizable cosmological distance indicators.

Mechanisms

In a massive star the inert iron core grows until it surpasses the mass degeneracy pressure can support, then collapses in under a second; the inner core stiffens at nuclear density and rebounds, but the shock stalls until neutrinos pouring out of the proto-neutron star deposit enough energy to relaunch it, blowing off the envelope. In a white dwarf instead, runaway carbon fusion releases enough energy to unbind the entire star.

Clinical relevance

Supernovae disperse the heavy elements that enrich galaxies and seed new stars and planets, drive interstellar shocks and cosmic-ray acceleration, leave behind neutron stars and black holes, and provide, in type Ia events, the standard candles that revealed the accelerating expansion of the universe.

History

Baade and Zwicky coined the term supernova in 1934 and linked it to neutron-star formation, Colgate, Arnett, Bethe, and others developed the neutrino-driven mechanism, and the 1987A supernova in the Large Magellanic Cloud provided the first detection of neutrinos from a collapsing core.

Debates

Robustness of the neutrino-driven explosion mechanism

Reproducing successful core-collapse explosions in simulations has long been difficult; whether neutrino heating aided by multidimensional instabilities suffices, or whether rotation and magnetic fields are essential in some cases, remains under active investigation.

Key figures

• Fritz Zwicky
• Walter Baade
• Hans Bethe
• Stanford Woosley

Seminal works

• woosley2002
• baade1934

Frequently asked questions

Why does a collapsing core explode rather than just implode?

The inner core suddenly stiffens at nuclear density and rebounds, launching a shock wave; although this shock stalls, the enormous flux of neutrinos escaping the newborn neutron star can deposit enough energy behind it to relaunch the shock and blow off the star's outer layers.

Are all supernovae the same?

No, there are two main kinds: core-collapse supernovae mark the death of massive stars and leave a neutron star or black hole, while type Ia supernovae arise from the thermonuclear disruption of a white dwarf and leave no compact remnant; they differ in their spectra and light curves.