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Death of Stars
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Chapter 19
Death of Stars
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Low Mass Stars on the lower Main
Sequence of the H_R Diagram have
extremely long lifetimes. Their entire
original mass of Hydrogen is available as
fuel. When all the hydrogen of the star is
used (fused to Helium) it collapses quietly
to a Helium White Dwarf
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Death of Stars
Stars with masses of about ½ M to
about 8 M follow roughly the evolution
of a 1 M star.
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Death of Stars
Mass Loss Among Red Giants
Stars just larger than 1.4 M lose their
extra mass through accelerated stellar
wind
Stars with masses up to 8 or 9 M
often have their outer layers go unstable
and explode. The result is a Nova
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A Nova is seen from Earth as a
sudden brightening of an
existing star. The explosion is
very bright for a few days to
weeks as the gas expands.
Then it fades as the gas
expands and cools. This
process can be repeated every
3 or 4 hundred years until the
star reaches the mass limit
then it can go White Dwarf.
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More than half the stars in the sky are
double stars and are close enough to share
matter when one goes Giant. The larger
star goes giant 1st and dumps its extra
mass to the smaller Main Sequence star
until it is under the mass limit then it
quietly goes to White Dwarf. When the now
bloated 2nd star goes giant it feeds back to
the White Dwarf where it is ejected by
explosion and we see it as a Nova.
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A large star has a great
number of shell fusion
furnaces. The ‘ashes’
from one furnace
serves as fuel for the
next. The inner most
ash layer is Iron.
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A high-mass star can
continue to fuse elements in
its core right up to iron (after
which the fusion reaction is
energetically un-favored).
As heavier elements are
fused, the reactions go faster
and the stage is shorter as
more shells are added.
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On the left, nuclei gain mass through fusion;
on the right they loose it through fission.
Iron is the
crossing point;
when the core
has fused to
iron, no more
fusion can take
place.
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Many of the elements are formed during normal stellar
fusion. Left, 3 helium nuclei fuse to form carbon; right a
couple of more complex fusion
Reactions. Some are made
during the supernova explosion.
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A Supernova is a one-time event – once it
happens, there is little or nothing left of the
progenitor star.
There are two different types of supernovae,
both equally common:
Supernova I, which is a supernova explosion
around a core which implodes
Supernova II, which is an explosion of the core
resulting in the complete destruction of the star
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A super nova has not
occurred in our part of
the Milky Way since the
invention of the
telescope so we have
not had the opportunity
to study one up close.
We have seen many in
other galaxies as well as
remnants in our galaxy.
A supernova in a distant
galaxy is often brighter than
the entire galaxy it is in.
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Supernovae I arise in two ways.
The first kind, a single star SNI
comes from an explosion in the
Silicon layer around the Iron core
of a large star. The second kind,
comes from interaction of large
binary stars.
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The iron is very reluctant to fuse. Sometimes the Oxygen
and Silicon layers around the core become unstable and
explode, imploding the Iron to a Neutron Star
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Normally a large star would die as a
Supernova. In a binary situation, however,
it dumps its excess mass over to its smaller
companion and becomes a White Dwarf.
The now very large companion finishes its
life and goes giant dumping its excess
matter on the white dwarf.
The now multi-layered star around the
white dwarf is very unstable and explodes
in a Supernova imploding the White Dwarf
to a Neutron Star.
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The Crab Nebula is the
result of a Supernova in
1054. It was observed
and location recorded
by the Chinese. We see
the expanding debris of
the explosion today at
that location.
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The classic results of a Supernova I are the expanding
debris of the explosion, a neutron star and a Pulsar
Often as a large star ages much of the fuel is used up and
deposited as ‘ash’ in the iron core. The inward pressure
on the iron core is enormous, due to the high mass of
the star. As the core continues to become more and
more dense, the protons react with one another to
become neutrons + a flood of neutrinos + much energy.
These local hot spots initiate fusion of the iron which
triggers formation of all of the elements more massive
than iron + more neutrinos and much more energy. The
energy builds up in a cascade effect producing a gigantic
explosion and the complete destruction of the star,
known as a Supernova II
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The classic results of a
Supernova II are:
Collapse of the iron core
Flood of neutrinos
Super explosion debris cloud
Complete disassembly of the
star
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While doing a theoretical
study of Supernovae Zwysic
and Baade in the 1930’s
predicted the existence of
Neutron Stars but they had
never been seen even with
the 200 inch Hale telescope
on Mount Palomar. The first
one found, much later, was
associated with a Pulsar in
the Crab nebula.
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Neutron stars, although they
have 1–3 solar masses, are so
dense that they are very small.
This image shows a 1-solarmass neutron star, about 10 km
in diameter, compared to
Manhattan.
As the parent star collapses, the neutron core spins
very rapidly, conserving angular momentum. Typical
periods are fractions of a second. Again as a result of
the collapse, the neutron star’s magnetic field
becomes enormously strong
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In 1967 Jocelyn Bell led a group of graduate students
at the University of Cambridge in England in a search
for Radio Sources in the sky. They discovered a
source that emitted extraordinarily regular
pulses. After some initial confusion, it was
realized that this was a neutron star,
spinning very rapidly.
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Why do neutron Stars pulse?
Strong jets of matter and
beams of light are emitted at
the magnetic poles, as that is
where they can escape. If the
rotation axis is not the same
as the magnetic axis, the two
beams will sweep out circular
paths. If the Earth lies in one
of those paths, we will see
the star blinking on and off.
A Neutron Star with its
sweeping pulsar beam
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The velocities of the
material in the Crab nebula
can be extrapolated back,
using Doppler shifts, to the
original explosion point
This is the pulsar at the
center of the Crab
Nebula
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Death of Very Large Stars
Summary of the Death of Stars
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End of Chapter 19
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