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Only consider the
first ascent for this
quiz
Quiz #7
• On the H-R diagram, a lower mass star that is
evolving off the main sequence will become redder in
color and have a tremendous increase in luminosity.
Write out the equation for luminosity in terms of
surface temperature and radius. Then discuss which
parameter is primarily controlling the luminosity as
the star evolves.
The Death of High Mass Stars
• When a high mass star runs out of hydrogen in
its core, the core begins to shrink. The outside
of the star expands and the star moves right on
the H-R diagram.
• The temperature is cooling and the radius is
growing, but the luminosity is virtually constant.
• Since L = σT4(4πR2); T4 must be changing at
the same rate as R2
• The star becomes a supergiant
(luminosity class I star)
The Death of High Mass Stars
• As the star tracks to the right for the first
time the inert helium core is contracting
and hydrogen shell burning is occurring.
• At the farthest right, helium core burning
begins, converting helium into carbon.
And still hydrogen shell burning.
• The star begins to move to the left on the
H-R diagram.
The Death of High Mass Stars
• When the helium runs out in the core, the
core begins to contract again, there is
helium shell burning into carbon, and
hydrogen shell burning into helium.
• The star moves right again, toward cooler
temperatures and larger radii.
The Death of High Mass Stars
• Finally the carbon core is hot enough to
fuse carbon into oxygen and nitrogen.
• The star moves back to the left on the H-R
diagram. There is a core changing carbon
into oxygen and nitrogen, a shell changing
helium into carbon, and a shell changing
hydrogen into helium.
A rule of thumb.
• Every time a high mass star moves to the
right (cooler temp) on the H-R diagram,
the core is inert, but contracting.
• Every time a high mass star moves to the
left, the core is fusing one element into
another.
• Throughout all of this there is shell burning
going on.
Final stage.
• The core of the high mass star fuses:
• hydrogen into helium
•
helium into carbon
•
carbon into oxygen and nitrogen
• oxygen and nitrogen into sulfur and silicon
•
And finally silicon into IRON.
• At last the core is iron. This is where everything
stops with a bang!
The final core and shells of a
high mass star
Fusing Iron does not release energy.
Think of it like a stairway
• A ball at the top of a stairway has potential
energy and can release it to make kinetic
energy. This can continue all the way
down to the floor.
• Once on the floor the ball can be made to
go back up the stairs but energy is not
released. It has to be provided by
someone in order to move back up again.
Nuclear Fusion
• So, Iron (Fe) can fused into other elements. But
just like the ball at the bottom of the stairs, this
process doesn’t release energy. It actually takes
energy from the system to make other elements.
• When the star’s core is completely Fe, it can be
fused into other elements, but in so doing,
energy is not released. Instead the reaction
robs the surrounding core of kinetic energy.
Making the core cool down. This drops the
pressure in the core and gravity wins the fight.
• In chemistry this type of reaction would be called
endothermic. The reaction doesn’t provide
energy it uses energy from its surroundings
causing the surroundings to cool.
• With no pressure to fight against gravity the core
collapses. NOT contracts, but collapses. Like a
house of cards.
• The gravity is too great for the electrons to hold
up the core like what happens in white dwarfs.
• If the core mass is greater than 1.4 solar
masses, the electrons are forced into the
protons to make neutrons.
• The neutrons become wave-like and hold up
against gravity. If the core is less than 3-4 solar
masses.
Remember all the way back to the start of
the course.
• The atom is mostly empty space. The
nucleus is tiny compared to the size of the
electron shells.
• If all the space inside your atoms were
removed, so that the nuclei were
“touching” each other, you would be the
size of a pin head.
If the Nucleus is a
grape on the 50 yard
line in Commonwealth
Stadium, then the
electrons are tiny
gnats flying in the
cheap seats
• A second atom would be the size of
another stadium, right next to the
Commomwealth Stadium. It also has a
grape for a nucleus. When the electrons
can no longer hold the nuclei apart, all the
space between the nuclei disappears, until
the nuclei are in “contact”. In other words,
the neutrons stop the contraction.
• The core becomes an enormously big
nucleus
• The result is a core with radius ~ 6 miles
and a mass > 1.4 solar masses.
• The density of a neutron star is so high
that one table spoon full of neutron star
material would weigh as much as an entire
mountain.
• Remember, a similar amount of white
dwarf material would weigh as much as a
car.
Neutron star
• The core is now a neutron star, if the core
mass is less than ~ 3 solar masses. If
more a black hole forms.
• Gravity is so strong that the collapse
occurs at nearly the speed of light, and the
material above the core follows the
collapse down at similar speeds.
Layers above core follow it in at speeds close to
the speed of light. At the center they run into the
densest object in the universe.
What will happen when the layer hits the
neutron core?
1. It will stop and become a
layer on the surface of
the neutron star
2. It will penetrate inside
the neutron star and
allow fusion in the core
to start again
3. It will rebound off the
core and head outward
in the star
Layers bounce off the neutron star and head back
out at nearly the speed of light. There they run
into other layers on their way toward the center.
The collision forms a shock wave that moves out
through the star
The shock wave blows the star apart in
about 2 hours.
• The result is a supernova explosion.
Supernova 1987a as it looked in 1994
Shock
wave
It will take hundreds of years for the shock wave to
reach the molecular clouds around it, but when it
does it will set off star formation.
Also, in the explosion, the elements heavier
than iron are created.
• Neutrons are from the core collide with
nuclei and build up the heavy elements.
Supernova in a distant galaxy
• Many radioactive isotopes are created in
the explosion. They are unstable and
decay down to stable nuclei.
• In supernova explosions, emission lines in
spectra can show these isotopes. Many of
which only live for a few hours or a few
days. This shows that the isotopes are
created in the explosion.
• It is also interesting that the half-life of
isotopes can be determined from these
observations.
The decay rate of radioactive isotopes are
used to measure the age of many things.
• When a radioactive isotope decays in a
rock, it produces a new element (called a
daughter isotope.)
• The decay rate tells us how fast a given
sample of an isotope will decay
• If we know the decay rate and the amount
of the isotope that has decayed in the
rock.
• It is possible to tell how old the rock is.
• We can figure out the decay rate of any
given isotope by taking a sample of the
isotope and measuring how fast it decays.
• One assumption: The decay rate for a
given isotope has always been the same,
over the entire lifetime of the rock.
Why does the decay rates of isotopes in
supernova allow astronomers to show that these
rates remain constant?
60
30
1. If it is the rate is the
same in during the
violence of a supernova
it must be the same
everywhere
2. Supernova that are one
billion light years away
exploded 1 billion years
ago.
3. It doesn’t, it only shows
what the decay rates are
for3 a supernova.
1
2
4
5
6
7
8
9
10
0
21
22
23
24
25
26
27
28
29
30
33%
11
12
13
1
33%
14
15
16
2
33%
17
18
19
3
20
NGC 4526, distance ~ 200 million light years.
• Elements like Gold, Silver, Platinum, are
very rare, because they only form in the
hour or so during the supernova explosion.
• When the shock wave of material collides
with the molecular clouds, it sets off star
formation AND also seeds the cloud with
new elements.
• The result is that when new stars form,
they have more heavy elements than the
previous generation of stars.
The neutron star spins very
rapidly.
• Stars rotate and before the core collapse
the core of the star was rotating as well.
• Why might we expect the neutron star to
be rotating extremely fast?
• Conservation of angular momentum tells
us that as the radius shrinks the velocity
increases. L = mvr
• Where L is angular momentum
• If the radius of the core was to shrink from
1 x 105 km down to 10 km then the radius
would be 10,000 times smaller.
• The new velocity would have to be 10,000
times faster.
Pulsars– spinning neutron stars
• Neutron stars have very strong magnetic
fields. They can redirect material near the
surface of the neutron star, out along the
magnetic poles. (bi-polar outflow again)
• When the material hits other atoms it
produces radio signals that beam out
along the magnetic poles.
• As the neutron star spins, the beam of
light hits the earth and we see a pulse.
The light-house effect
• The pulsar is similar to a light house. As
the beam of light passes by us we get a
very large signal. When the beam moves
away the signal dies out.
• Some pulsars give a burst of light every
second. This means the neutron star is
spinning once every second.
• The fastest neutrons stars spin 1000 times
every second.
Crab Nebula – exploded in 1054 AD.
Pulse signal from Crab pulsar.
Spins once every 0.0331 seconds or about
30 times every second
As the neutron star spins the beams of light
are sometimes directed at the Earth.