Transcript Chapter 17

Chapter 17: Evolution of
High-Mass Stars
Massive stars have more
hydrogen to start with but they
burn it at a prodigious rate
The overall reaction is still
4 H  He  2e  2

There are 3 gamma ray
photons instead of two
and it consumes
hydrogen much faster but
requires higher
temperatures.
High-mass
stars have
convection
in their
cores
Because of convection
in their core, high
mass stars never
develop a degenerate
helium core so they
never have a helium
flash.
As high mass stars evolve off the
main sequence they move back
and forth on the H-R diagram
For high-mass stars the
core temperature will get
hot enough to fuse carbon
Carbon will start fusing when the
core temperature reaches
800,000,000 Kelvin. Unlike
hydrogen fusion which only
produces helium or helium fusion
which only produces carbon,
carbon fusion can produce lots of
different elements.
As the
temperature
gets higher,
heavier
elements start
to fuse
The products of one fusion will
form the elements for the next
level of fusion making the core
start to look like an onion with
multiple shells of fusion
As they evolve, some stars
move into the “instability strip”
In the instability strip
stars can begin to
pulsate. The first
pulsating star to be
discovered was Delta
Cephei so we call this
type of star a Cepheid
variable
For Cepheid variables, the
brightness varies in a regular way
Another type of pulsating
variable is RR Lyrae
The
pulsations
are due to
a layer of
helium that
becomes
ionized
The period of their variation
is related to their luminosity
Once an iron core starts to
form, the end comes quickly
Each stage of fusion requires
higher temperatures, releases
less energy and lasts less time
Higher
fusion also
releases
most of its
energy as
neutrinos
Since neutrinos don’t
interact with normal
matter much, they
quickly escape the
star and don’t help in
balancing the inward crush of gravity.
Because of the nuclear force,
fusing iron requires energy
rather than releasing it
At this point temperature in
the core is nearly 5 billion K
The peak of the
blackbody curve
that is being
produced is in the
high energy
gamma ray range
Due to degeneracy, extreme
temperature and pressure, the
core starts to collapse
Once the iron becomes degenerate, the
core starts to collapse as more mass is
added. As it collapses, it gets hotter. As
it gets hotter, the gamma rays get more
energetic until they have enough energy
break up the iron. The process is called
photodisintegration
g + 56Fe→134He + 4n
After photodisintegration,
reverse beta decay relieves the
electron degeneracy pressure
Things get so crowded the electrons are squeezed into
the nucleus where they combine with protons to make
neutrons in a process called Reverse Beta Decay
Reverse Beta Decay
causes neutrinos to pour
out of the core
The conversion of electrons
and protons into neutrons
causes the core to rapidly
shrink in size which
produces a blast of heat
which forms a shock wave.
In addition, huge amounts of
energy is being carried off
by the neutrinos
The
shock
wave
pushes
the rest
of the
star
outward
The result is a Type II
Supernova
Supernovae come is
several types
Type Ib through Type II are from the core collapse of a
massive star. Type Ia is the thermonuclear detonation
of a white dwarf star.
The last naked-eye
supernova was SN 1984A
The first supernova to be observed in 1987, it was
visible to the naked eye for several weeks if you were in
the southern hemisphere. It occurred in the Large
Magellanic Cloud, a satellite galaxy 180,000 ly away.
Type II Supernovae are important
for the elements they produce
The Solar System formed
from the debris of several
supernovae
After the supernova, a neutron
star is left behind
Because of the
peculiarities of
their emissions,
they are called
Pulsars
Pulsars are spinning Neutron Stars
The Lighthouse Model
The Crab
pulses in all
wavelengths
Pulsars emit by
synchrotron radiation
Charged particles spiraling in a
magnetic field produce
synchrotron radiation. The
stronger the magnetic field, the
shorter the wavelength emitted
If a pulsar is radiating, where
is the energy coming from?
Pulsars in a binary system will
emit x-rays and gamma rays
How do we know all this stuff
about stellar evolution?
Since the stars in a cluster are all born at
about the same time, we can study
clusters to learn how stars evolve
The H-R Diagram of a cluster
shows which stars are starting
to evolve off the main sequence
The actual
H-R
diagram of
clusters
match the
theory
very well
By
comparing
clusters of
different
ages we can
understand
how stars
evolve