Nuclear Interactions in Supernovae By: Alec Fisher 11/14/2012

How Do They Die? What is the process that leads to their deaths? What happens when their life is at an end? What happens to stars like our sun? What happens to stars bigger than our sun?

Nuclear Fusion Stars undergo nuclear fusion within their cores in order to keep themselves alive. A star is in a constant state of thermal equilibrium with itself, perfectly balancing out the crushing gravitational force from its massive core and the continuously exploding outward pressure from its nuclear reactions.

P-P Cycle

When helium is formed from fusing hydrogen, it is gathered closer to the core, and this layering process continues until there is no more fuel source. But what happens when there is no more fusion occurring? Shell structure of stars

The End of a Star’s Life When a low mass star ( a star < 8 solar masses) runs out of fusion material, its core contracts inward while its outer layers of hydrogen and helium expand. The outer layers expand because when the carbon- oxygen core contracts, the increased pressure causes a rise in heat, which excites the molecules in the outer layers of the star. This excitation gives the molecules more room to move and causes them to vibrate. This vibration is expressed as an increase of the volume of the star.

The outer layers of the star eventually cool down and escape the core, leaving a white dwarf behind surrounded by a giant cloud of molecular hydrogen and helium. But this is not the only way a star can die. Fun Fact: 1 teaspoon of a white dwarf would weigh 5 tons! The Eagle Nebula

TYPE 1 SUPERNOVA

Type 1 supernova can only occur in a binary star system. They occur when a white dwarf has reached its mass limit A binary star system

Electron Degeneracy When the core of a sun like star contracts inward during death, some force keeps the white dwarf from collapsing into nothingness. This force is known as the Electron Degeneracy pressure, and is caused by the Pauli Exclusion Principle.

Electron energy levels of carbon

When a dwarf star is in a binary system with a star in the red giant phase, it will strip the outer layers off of the giant to form an accretion disk around itself. Electron Degeneracy pressure holds the white dwarf from collapsing in on itself as it takes in more and more mass. White dwarf accreting matter from its companion

Nova As the mass of the white dwarf increases, the energy levels of the degeneracy pressure increases, causing the electrons to approach the speed of light. This causes the white dwarf to heat up, but since it’s under electron degeneracy it can’t expand, and the accreted mass (hydrogen and helium mostly) is compressed onto the surface of the dwarf. When the mass gets hot enough (around 20 million kelvin) It will cause the hydrogen to start unstably fusing under the CNO cycle on the surface of the dwarf.

Nova continued The hydrogen is burned in a runaway reaction, and an enormous amount of energy is released from all the hydrogen being fused in a short amount of time. This causes an explosion on the surface of the dwarf, which doesn’t affect the star, but increases its brightness by 50,000 to 100,000 times that of the sun.

Chandrasekhar Limit The white dwarf will continue to accrete mass, the pressure and density of the dwarf rises and the temperature increases from this increase in weight bearing down on the dead core. Eventually (at around 1.38 solar masses) the temperature is increased so much, carbon starts fusing within the core, and then oxygen.

The Death of the Dwarf The dwarf can’t regulate itself from degeneracy, and the nuclear reactions are uncontrollable. The carbon and oxygen are fused together almost instantaneously, releasing about 10^44 joules of energy at once. The energy is released through the unbinding of every particle, and a giant explosion is the result.

Type 1 supernova observed in different electromagnetic wavelengths

Type II Supernovae Type II supernova discovered in 2005

Death of stars that are over 8 solar masses Between roughly 8 – 20 solar masses, a neutron star is the result. Over 20 solar masses a black hole will form.

Onion like layer of a massive star

Formation of Neutron Stars and Black Holes During the core collapse, the density of the core becomes so great that the process of electron capture occurs. Neutron degeneracy then takes over and a neutron star is formed. The Average Neutron Star

Formation of Neutron Stars and Black Holes High mass stars have so much material, it rains back down onto the neutron star. Tolman–Oppenheimer–Volkoff limit Artists Impersonation of a black hole

Sources Woosley, S.; Janka, H.-T. ( ). "The Physics of Core-Collapse Supernovae". Nature Physics 1 (3): 147–154.arXiv:astro- ph/ Bibcode 2005NatPh W. doi: /nphys172.arXivastro- ph/ Bibcode2005NatPh Wdoi /nphys172 Hinshaw, Gary ( ). "The Life and Death of Stars". NASA Wilkinson Microwave Anisotropy Probe(WMAP) Mission. Retrieved "The Life and Death of Stars"NASAWilkinson Microwave Anisotropy Probe Staff ( ). "Introduction to Supernova Remnants". NASA Goddard/SAO. Retrieved "Introduction to Supernova Remnants" ml ml state.edu/~pogge/Ast162/Unit3/extreme.html state.edu/~pogge/Ast162/Unit3/extreme.html hole-really-look-like-Eerie-Nasa-image-shows-universes-mysterious- phenomenon-close-up.html