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Chapter 21: Stellar Explosions
© 2017 Pearson Education, Inc.
Stellar Explosions
Chapter 21: Stellar Explosions
• The Final Fate of Stars
• Low Mass Stars
– Forming White Dwarfs
– The Fate of a White Dwarf
• Novae
• High Mass Stars
– Fusion of Heavy Elements
– Collapse of the Iron Core
– Photodisintegration
• Supernovae
• Making elements heavier than Iron
• The Cycle of Stellar Evolution© 2017 Pearson Education, Inc.
Final Fate of Stars
• A low mass star (up to ¼ solar masses) cannot achieve
the temperatures required to fuse He. After the main
sequence
– 4 11H → 4
2He + 2 e+ + 𝜈𝑒It forms a Helium white dwarf.
© 2017 Pearson Education, Inc.
• A star of mass greater than ¼ solar masses but less
than 8 solar masses will achieve the temperatures
required to fuse He to C and O
– 3 42He → 12
6C
– 42He + 12
6C → 168O
and forms a Carbon-Oxygen white dwarf
© 2017 Pearson Education, Inc.
Final Fate of Stars
• Even larger stars, of mass greater than 8 solar
masses but less than 12 solar masses can go on to
fuse O with He to form Ne
– 42He + 16
8O → 2010Ne
ending up as Neon-Oxygen white dwarfs.
• Stars of mass greater that 12 solar masses will have
a very different history. Electron degeneracy
pressure cannot sustain the core
• This leads to a catastrophic explosion that leaves
behind a neutron star.
• The explosion is called a supernova.
© 2017 Pearson Education, Inc.
Final Fate of Stars
© 2017 Pearson Education, Inc.
White Dwarfs, Binaries and Novae
• As the white dwarf cools, its size
does not change much; it simply
gets dimmer and dimmer, and
should finally cease to glow (black
dwarf). White dwarfs are sustained
by electron degeneracy.
• However, we sometimes see white
dwarfs suddenly brighten, i.e., turn
nova.
• Nova Persei, a WD that suddenly
brightened by a factor of 40,000!
• In general, a nova is a star that
flares up very suddenly and then
returns slowly to its former
luminosity.
© 2017 Pearson Education, Inc.
White Dwarfs, Binaries and Novae
• We know today that a nova is a white dwarf that is
undergoing an explosion on its surface
• The explosion results in a rapid and temporary increase
of its luminosity
• What causes a white dwarf to undergo surface
explosions?
© 2017 Pearson Education, Inc.
White Dwarfs, Binaries and Novae
• A white dwarf that is part of a semidetached binary
system can undergo repeated novas.
© 2017 Pearson Education, Inc.
White Dwarfs, Binaries and Novae
• Material falls onto the white dwarf from its main-
sequence companion.
• This forms a swirling disk of matter around the white
dwarf. The disk gets hotter and hotter (by friction or
viscosity) even as it falls in and becomes more and
more luminous.
• The gas becomes denser as it builds up on the
surface because of the great mass and small radius
of the white dwarf.
• When enough material has accreted, fusion can reignite very suddenly, burning off the new material.
© 2017 Pearson Education, Inc.
White Dwarfs, Binaries and Novae
• The sudden start of fusion can generate shock waves that blow the surface layers into space.
• Material keeps being transferred to the white dwarf, and the process repeats.
• A nova represents one way in which a dead star in a binary system can extend its life.
© 2017 Pearson Education, Inc.
Supernovae
• Supernovae are far more catastrophic and
luminous explosions than novae.
• Supernovae are not “super” versions of novae.
These explosions have a very different origin.
• They can achieve luminosities that are over millions
of times that of the sun.
• They are one-time events. Once they occur, there is
little or nothing left of the progenitor star.
• Supernovae are classified by duration (light-curves)
and by their absorption spectra. The most important
consideration is the presence or absence of
Hydrogen.
© 2017 Pearson Education, Inc.
Supernovae
• There are two types of supernovae, both equally
common, which exhibit very different light-curves:
– Type I, is a carbon-detonation supernova
– Type II, is a core-collapse supernova that occurs at
the death of a high-mass star.
© 2017 Pearson Education, Inc.
Type I: Carbon Detonation Supernova
Carbon-detonation supernova:
• Each time a white dwarf in a binary goes nova, it ejects
some of the matter it has collected from its companion.
• Still, not all of it is eliminated and the white dwarf grows
in size.
• In time this white dwarf will accumulate too much mass
from its binary companion.
• If the white dwarf’s mass exceeds 1.4 solar masses
(Chandrashekar limit), electron degeneracy can no
longer keep the core from collapsing.
• Carbon fusion begins throughout the star almost
simultaneously, resulting in a carbon explosion.
© 2017 Pearson Education, Inc.
Type I: Carbon Detonation Supernova
• This graphic illustrates this mechanism.
• Nothing is left of the white dwarf after the Carbon
Detonation. Type I supernovae are Hydrogen poor.
• In this way, the heavier elements are transferred
back to the Interstellar medium
© 2017 Pearson Education, Inc.
Type I: Carbon Detonation Supernova
• On the right is an image of
the supernova remnant G
299, which resulted from a
Type I supernova explosion.
• (Type I supernovae are
used to measure the
expansion of the universe)
• The supernova remnant SN
1572. It resulted from a Type
I supernova, visible to the
naked eye and observed in
1572, in particular by Tycho
Brahe. © 2017 Pearson Education, Inc.
Larger Mass Stars
• Larger mass stars are able to fuse heavier elements.
The sequence goes like this
– 42He + 20
10Ne → 2412Mg
– 42He + 24
12Mg → 2814Si
• This is the He capture sequence. It requires higher
temperatures at every step and each step occurs faster.
The last two stages occur appreciably only in stars of
mass > 12 solar masses. Advanced nuclear fusion of the
heavier elements then starts to occur:
– 126C + 16
8O → 2814Si
– 42He + 28
14Si → 3216S
– 2814Si + 28
14Si → 5628Fe
© 2017 Pearson Education, Inc.
Larger Mass Stars
• Core temperatures of stars of mass greater than 12
solar masses allow the fusion of elements up to Fe.
• Other pathways also occur, e.g., an intermediate mass
element like Nitrogen is formed during the following
reaction:
– 11H + 12
6C → 137N → 13
6C + n + 𝑒+ + 𝜈𝑒
– 136C + 1
1H → 147N
• All these reactions release energy.
• Iron is the heaviest element that is energetically
favorable to fuse.
• Fusion to even heavier elements obviously does occur,
but now it removes energy from the star.
© 2017 Pearson Education, Inc.
Larger Mass Stars
• But He capture continues, releasing energy, as well:
– 42He + 28
14Si → 3216S
– 42He + 32
16S → 3618Ar
– 42He + 36
18Ar → 4020Ca
– 42He + 40
20Ca → 4422Ti
– 42He + 44
22Ti → 4824Cr
– 42He + 48
24Cr → 5226Fe
• and… at the cost of energy:
– 42He + 52
26Fe → 5628Ni
– 42He + 56
28Ni → 6030Zn
– Etc.
© 2017 Pearson Education, Inc.
Large Mass Stars
© 2017 Pearson Education, Inc.
Nuclear Binding Energy
© 2017 Pearson Education, Inc.
Fate of the Iron Core
• Iron lies at the lowest point of the curve, so with the
appearance of substantial amounts of iron, fusion
ceases.
• The iron core has no way to generate the energy
required to sustain itself and it begins to collapse.
• With the collapse, the core temperature rises to
about 10 billion degrees K.
• Gravitational energy is released as heat in the form
of photons.
• According to Wien’s law these photons have a very
small wavelength
© 2017 Pearson Education, Inc.
Fate of the Iron Core
© 2017 Pearson Education, Inc.
Photodisintegration
• Wien’s Law:
𝜆 =2.898 × 10−3
𝑇= 2.898 × 10−13 m
• They are smaller than the nucleus and extremely
energetic. They act as high energy bullets and begin
a process of photodisintegration of the heavy
elements in the core.
• In less than one second the process undoes all the
effects of fusion, splitting the Iron nuclei into smaller
and smaller pieces.
• This process cools the core, reduces the pressure
and accelerates the core’s collapse.
© 2017 Pearson Education, Inc.
Photodisintegration
• Soon the core ends up where it began, made of
protons, electrons and neutrons, but at much
higher densities.
• The gravitational force (weight of the core) is far too
much for electron degeneracy to counterbalance it.
• The electrons then begin to combine with the
protons to form neutrons. This process is called
neutronization
𝑝 + 𝑒− → 𝑛 + 𝜈𝑒• Since there are equal numbers of protons and
electrons the central region of the core is now made
of neutrons.
© 2017 Pearson Education, Inc.
Type II: Core Collapse Supernova
• When the neutronization process begins, the core
density is about 1012 kg/m3
• A very large number of neutrinos are produced in this
process. These neutrinos carry out about 1046 Joules
of energy (in about 10 seconds) because they interact
weakly with matter.
• As they get closer together, the neutrons begin to
exert an outward degeneracy pressure on the core.
• The core continues to collapse, compressing the
matter to a density of 1017 kg/m3, at which point the
neutron degeneracy pressure causes the collapse to
halt. The outer layers rebound off the inner core.
© 2017 Pearson Education, Inc.
Type II: Core Collapse Supernova
• A massive shock wave sweeps through the star,
ejecting all the outer layers (including the heavy
elements just formed) into space. This is a Core-
Collapse Supernova.
• A neutron core survives this type of a supernova
explosion. Type II supernovae are Hydrogen rich.
© 2017 Pearson Education, Inc.
Supernova Remnants
• Supernovae leave
remnants—the
expanding clouds of
material from the
explosion.
• The core is bounded by an
expanding shock wave,
which is expanding into
space and sweeping up
material from the ISM
Electrons ejected from the
core are in the middle,
emitting radio synchrotron
radiation.© 2017 Pearson Education, Inc.
The Crab nebula
resulted from a Type II
Supernova (2 kpc).
Supernova Remnants
There are in general three kinds
of remnants:
• Crab-like remnants (Crab)
• Shell-like remnants like
Cassiopeia on the right (3.4
kpc, Type II). A shock wave
plows through space.
Sometimes the shells contain a
central neutron core like Vela,
on the right (294 pc, Type II).
• Composites: appear shell-like
in radio and crab-like in X-rays,
or crab-like in both frequencies
but also have shells.
© 2017 Pearson Education, Inc.
Formation of Elements
• He capture (also known as the alpha process) can
only lead to the formation of elements with atomic
mass divisible by four.
• This is the most common process, so these
elements will be more abundant than others.
• But it is not the only way elements can form.
© 2017 Pearson Education, Inc.
Formation of Elements
• Elements can also form by direct capture of free protons
and neutrons by heavy nuclei to form even heavier
nuclei.
• By the time 2814Si appears the temperature in the core is
about 1 billion K. At this point, the photons are so
energetic that photodisintegration begins and there is a
competition between forming heavier nuclei and
breaking them up.
• The result is that some heavy nuclei (like 2814Si) are
destroyed creating new 42He and promoting He capture
to form even heavier elements.
• The presence of a variety of elements, He, protons and
neutrons then produces other, less common, elements.
© 2017 Pearson Education, Inc.
Formation of Elements Heavier than Iron
• He capture cannot proceed after Iron is formed.
Making elements heavier than iron occurs via
neutron capture.
• Neutrons have no charge, so they are easier for
heavy elements to capture.
• Neutron capture is a very slow process and is often
referred to as the s-process.
• Being slow, the heavier nuclei have time to decay
into lighter nuclei, so it ends up being a competition
between capture and decay.
• This is why there are only trace quantities of the
heaviest elements.
© 2017 Pearson Education, Inc.
Formation of the Heaviest Elements
• Neutron capture, or the s-process, can function to
produce only relatively stable nuclei.
• Any element with an atomic mass greater than 209
(Bismuth) cannot be produced by neutron capture
because the decay rate is greater than the capture rate.
• However, in the last 15 minutes of a supernova
explosion there are so many neutrons that the capture
rate exceeds the decay rate as the neutrons are jammed
into the nuclei.
• This is known as the rapid process (r-process)
because it proceeds very rapidly.
• So the heaviest elements are formed after the star has
died!
© 2017 Pearson Education, Inc.
The Cycle of Stellar Evolution
• Star formation is
cyclical: Stars form,
evolve, and die.
• In dying, they send
heavy elements into the
interstellar medium.
• These elements then
become parts of new,
next generation stars.
• And so it goes.
© 2017 Pearson Education, Inc.
Summary
• Once hydrogen is gone in the core, a star burns
hydrogen in the surrounding shell. The core
contracts and heats; the outer atmosphere expands
and cools.
• Helium begins to fuse in the core as a helium flash.
The star expands into a red giant as the core
continues to collapse. The envelope blows off,
leaving a white dwarf to gradually cool.
• A nova results from material accreting onto a white
dwarf from a companion star.
• The same white dwarf may undergo many novae.
© 2017 Pearson Education, Inc.
Summary, cont.
• A Type I supernova is a carbon explosion, occurring
when too much mass falls onto a white dwarf.
• Very massive stars become hot enough to fuse
carbon, then heavier elements, all the way to iron. At
the end, the core collapses and rebounds as a Type
II supernova.
• All heavy elements are formed in stellar cores or in
supernovae.
• The heaviest elements are formed after the star is
dead.
• Stellar evolution can be understood by observing
star clusters.
© 2017 Pearson Education, Inc.
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