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Core-Collapse Supernovae

The  most powerful astronomical bombs are neither chemical nor nuclear, but gravitational.  Gravity can convert a substantial fraction of an object's rest mass into thermal energy in a fraction of a second.  In practice, substantial means about 15% of an object's rest mass energy.  If the core of a massive star provides this rest mass energy, it can generate several hundred times the energy of a thermonuclear supernova.  The core collapse of a massive star is associated in current theory will all but one type of supernova.

The reason that a massive star can explode is that its core is larger than the Chandrasekhar mass limit of 1.4 solar masses, making the core unstable to gravitational collapse.  The degeneracy pressure exerted by electrons within the core cannot counteract the core's self-gravity;  only the heat generated through thermonuclear fusion keeps the stellar core from collapsing.  Once thermonuclear fusion runs to completion within the star's core, and it is composed of iron, it rapidly collapses until either the degeneracy pressure of neutrons and protons halts the collapse or the core becomes a black hole.  The first outcome generates a supernova; what the second outcome generates is unclear.

Falling doesn't kill you: stopping does.  This is as true of a star as with you and me.  The collapse of a star's core does not itself generate much heat, because most of the gravitational potential energy is being converted into the kinetic motion of the collapse.  Not until the core “hits bottom” is this kinetic energy converted into the thermal energy that powers the explosion.  The sudden appearance of nucleon degeneracy pressure, which marks the formation of a neutron star, provides this bottom.  In this creation of a neutron star, about 15% of the core's rest mass is converted into heat.  This is dramatically larger than the energy—about 0.05% of the white dwarf's rest mass energy—liberated in a thermonuclear supernova.

The amount of energy required to blow a massive star apart in a core-collapse supernova is larger than the energy necessary to blow a white dwarf apart in a thermonuclear supernova.  The gravitational potential of a stellar core before collapse is similar to that of a white dwarf vulnerable to thermonuclear detonation, because both are at the Chandrasekhar mass limit and the radius of the core during the fusion of carbon, oxygen, and other intermediate-mass elements is not dramatically smaller than the radius of a stable white dwarf.  Blowing a solar mass of material out of the gravitational potential of a stellar core and unbinding a white dwarf in a thermonuclear detonation therefore require about the same amount of energy.  But the amount of mass surrounding a collapsing core is several solar masses, so several times as much energy is required to disrupt the progenitor of a core-collapse supernovae than is required to disrupt a white dwarf.  One implication is that a massive star cannot be blown apart by the thermonuclear detonation of its core, because the amount of energy liberated in such a detonation is only sufficient to blow apart a white dwarf; for this reason, the thermonuclear detonation of a stellar core cannot create a supernova in a massive star.

The energy necessary to disrupt a massive star is easily generated in the gravitational collapse of a core.  Because the radius of a white dwarf is several hundred times the radius of a neutron star, the gravitational potential energy released in creating a neutron star is several hundred times that released in creating a white dwarf.  If only several tenths of one percent of the energy generated by a collapsed core were absorbed by the surrounding star, it would be disrupted.

How does the heat released within a newly-formed neutron star escape to the surrounding mass?  It is transported out of the neutron star by neutrinos.  These fundamental particles interact weakly with matter, so they move easily through most bodies.  For example, both the Sun and the Earth are transparent to them.  They are responsible for cooling the interiors of white dwarfs, because they can escape freely from the interior of a white dwarf once they are created.  When a stellar core collapses, its high density spurs the creation of thermal neutrinos through a variety of processes.  The core itself is not transparent to neutrinos, so they interact and come into thermal equilibrium with the core before escaping.  The mass surrounding the core is almost transparent to these neutrinos, so very few neutrinos give up energy to the surrounding mass.  The tiny amount of energy lost by the neutrinos to the surrounding mass, however, is more than sufficient to blow this mass away from the core.  The most energetic explosions in the universe therefore hide most of their energy from our sight; we see the brilliant light and the high speed of the supernova debris, but we almost never see the neutrinos that carry away almost all of the energy generated in the birth of the neutron star.

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