Nuclear Interactions in
Supernovae
By: Alec Fisher
11/14/2012
How Do They Die?
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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 carbonoxygen 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
The Eagle Nebula
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!
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. (2006-01-12). "The Physics of Core-Collapse
Supernovae". Nature Physics 1 (3): 147–154.arXiv:astroph/0601261. Bibcode 2005NatPh...1..147W. doi:10.1038/nphys172.
• Hinshaw, Gary (2006-08-23). "The Life and Death of
Stars". NASA Wilkinson Microwave Anisotropy Probe(WMAP) Mission.
Retrieved 2006-09-01.
• Staff (2006-09-07). "Introduction to Supernova Remnants". NASA
Goddard/SAO. Retrieved 2007-05-01.
• http://astronomynow.com/080604Secondsupernovaepointtoquarkstars.ht
ml
• http://astro.unl.edu/naap/distance/supernovae.html
• http://www.astronomy.ohiostate.edu/~pogge/Ast162/Unit3/extreme.html
• http://www.dailymail.co.uk/sciencetech/article-1336832/What-blackhole-really-look-like-Eerie-Nasa-image-shows-universes-mysteriousphenomenon-close-up.html