The Death of Stars

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The Death of Stars
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Parts of a MS star

What holds a star up while it is on the MS?
On the Main Sequence
How does energy get out?
Radiation Convection
May take a million years to
reach the surface.
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The smallest stars
Brown Dwarfs Stars with core mass lt .08 Msun
(failed stars). Brown Dwarfs do not get hot
enough to fuse H, but they do fuse Deuterium for
a very short time. Deuterium is an isotope of H,
with a neutron. About 1,000 Brown Dwarfs have
been found. They radiate in the infrared
wavelength.
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Red Dwarfs Stars with a core mass of .08 to
0.4 solar mass
Coolest and dimmest of all
MS stars. They remain on MS hundreds of billions
of years. When all the H is converted to He
fusion ceases, they cool down, moving down and to
the right in the H-R diagram.
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Red Dwarfs are very low mass stars with no more
than 40 of the mass of the Sun and represent the
majority of the stars.
They have relatively low temperatures in their
cores red dwarfs transport energy from the core
to the surface by convection.
.
A low-mass main-sequence star of spectral classes
M and L. Red dwarf stars range from about 0.6
solar mass at class M0 down to 0.08 solar mass in
cool M
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Death of Low Mass Star
Final State
Core Mass
0.5 - 1.4
White Dwarf
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Thermodynamics
When Fusion stops, core shrinks temperature of
core rises.
When the Envelope expands its temperature cools
down.
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Evolution of Low-Mass Stars
0.5 - 1.4 Msun
Main Sequence Phase Energy Source H fusion in
the core Using P-P cycle H fuses to He Slowly
builds up an inert He core
He Fusing
Envelope
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When the H in the core is almost completely
converted into He, H fusion stops in core.
The left over H is pushed out into a
shell ring around the He core.

The core collapses heats up.


The increasing temp will cause the H shell to
fuse forming He that will join the core
Outer layer expands and cool forming a Red Giant
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• (On the H_R Diagram)
•                                                   
                                        

• The star gets brighter and redder, climbs up the
  Giant Branch. (Takes 1 Byr)

At the top of the Giant Branch, the stars
envelope is about the size of Venus orbit
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• The core will contract until it gets hot enough
  to fuse the He in the core into Carbon Oxygen.

When the fusion begins, the burning occurs
rapidly because of the H shell burning and the He
burning in the core. This is called the Helium
Flash. Some of the outer layers are blown
outward causing the star to loose mass.
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The star gets hotter, and moves onto the
Horizontal Branch
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Once the He fuses and forms C O, the core
contracts.

• C-O core collapses and heats up
• He burning shell outside the C-O core
• H burning shell outside the He burning shell
•  The core never gets hot enough to fuse the
  Carbon
• Oxygen                                    

Outside Envelope swells cools because of H
He burning Climbs the Asymptotic Giant Branch
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Climbs the Giant Branch again, slightly to the
left , and higher, becoming a super red giant. .
                                                 
                                       
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Core and Envelope separate, takes 100,000 yr C-O
core continues to contract
With weight of envelope taken off, core never
reaches Carbon fusion temp of 600 Million K
Outer envelope gets slowly ejected . This is a
non-violent ejection a series of puffs or
burps. Expanding envelope forms a ring nebula
around the contracting C-O core.
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A Planetary Nebula forms                   
                                                  
               
Hot C-O core is exposed, moves to the left
Becomes a White Dwarf
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Planetary Nebulae
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Fig. 13.16c
Butterfly Nebula
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Planetary Nebula
Ejection not explosive
Nebula shell expands
Outer shells of red supergiant puffed off
The nebula is ionized, and heated by the.
Ultra-violet radiation from the hot star

• After 50,000 years, the nebula spreads so far
  that the nebulosity simply fades from view.

Hot dwarf left behind Cools down to form a WD
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Contraction of the core is stopped by electron
degeneracy. The electrons repel each other as
they are pressed closer together and a White
Dwarf forms.
White Dwarfs have a mass that is less than 1.4
Mo They will shine for a long time but no fusion
is taking place.

• One teaspoon weighs about 5 tons.

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Electron energy levels

• Only two electrons (one up, one down) can go
  into each energy level.
• In a degenerate gas, all low energy levels are
  filled.

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White Dwarfs are planetary in size, but have a
stellar mass
Radius (a little smaller than Earth!)
Temp. anywhere from 100,000 to 2500 K.
White dwarfs shine by leftover heat, no fusion.
WD will cool off and fade away slowly, becoming a
"Black Dwarf.
White Dwarfs mass lt than the Chandrasekhar mass
(1.4 Solar Mass)
Takes 10 Tyr to cool off , so none exists yet.
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Sirius B
White Dwarfs are so small, that they can only be
seen if close-by, or in a binary systems.
The most famous W.D. is Sirius companion .
Sirius B Temp. 25,000 K Size 92 Earth's
diameter Mass 1.2 solar masses
Sirius B
The mass of a star, in the size of a planet.
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A lone white dwarf is a cooling corpse but a
white dwarf in a binary system can be revived
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There is more !! A White Dwarf in a binary
system
White Dwarf
I
Evolving (dying) star
Roche Lobes
II
Evolving (dying) star
White Dwarf
Accretion Disk
III
Evolving (dying) star
Roche Lobe filled
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W.D. can take on material but, if the W.D.
exceeds 1.4 solar masses (Chandrasekar limit)
powerful explosions take place and they could
happen more than once. The star will get down
below 1.4 solar mass.
Type 1a super NOVA!!
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Since the Type 1a supernova is always a white
dwarf they can be used to judge very great
distances (using the inverse square law).
Type Ia No hydrogen lines in the spectrum Type
II Hydrogen lines in the spectrum
There is a further subdivision of I into Ia, Ib,
Ic
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Low Mass Stars
Sun
Becomes Red Giant when H is almost gone
Envelope separates from core and forms a
planetary nebula
Orbit out to almost Venus
Red Giant
If the White Dwarf is a binary star, a Supernova
type 1a can form, if its mass becomes greater
than 1 ¼ solar masses
Red Super Giant
Becomes a Red Super Giant
Core forms a White Dwarf
White Dwarf becomes a Black Dwarf (dead star)
Only H , He in shells, C O in core left C
O do not fuse
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Wanted
Of course you know the relationship is just going
to end in a Type 1a supernovae...but I suppose
its better to have transferred mass and exploded
than to have never transferred mass at all...
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Stellar Graveyard High Mass Stars
Final Core Mass
Final State
1.4 lt M lt 3.0
Neutron Star
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Evolution of Massive Stars
Massive stars have the same internal changes as
we saw in low mass stars ,
except
massive stars evolve more rapidly due
to rapid nuclear burning, and massive stars
produce heavier elements
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Evolution of High-Mass Stars

• High-Mass Stars
• O B Stars core mass gt1.4 and lt3 Msun
• Burn Hot
• Live Fast
• Die Young

• Main Sequence Phase
• Burn H to He in core using the CNO cycle
• Build up a He core, like low-mass stars
• But this lasts for only 10 Myr

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• Red Supergiant Phase
• After H core exhaustion
• Inert He core contracts heats up the
• H burning in a shell .
• Envelope expands due to the burning H shell and
  cools
• Envelope size of
• orbit of Jupiter
•                                            

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Moves horizontally across the H-R diagram,
becoming a Red Supergiant star
                                                  
                                 
Takes about 1 Myr to cross the H-R diagram.
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Core Temperature reaches 170 Million K Helium
Flash Helium ignites This Helium flash is not
as explosive as the one for low mass stars.
Helium Fusion produces C O in core Star heats
up and becomes a Yellow Supergiant.
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Star becomes a Yellow Supergiant.
Yellow
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• When He exhausted in core
• Inert C-O core collapses heats up the
• H He burning in shells. Star expands
• and becomes a Red Supergiant again
•                                                 
                                     

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• C-O Core collapses until Tcoregt 600 MillionK
• Carbon in the Core ignites.

C fuses to form Ne , and O

• Core at the end of
• Carbon Burning
• Phase

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Nuclear burning continues past Helium
Things happen fast!
1. Hydrogen burning 10 Myr 2. Helium burning 1
Myr 3. Carbon burning 1000 years 4. Neon
burning 10 years 5. Oxygen burning 1 year 6.
Silicon burning 1 day Finally builds up an
inert Iron core
End of the line!!
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Massive star at the end of Silicon Burning
Onion Skin of nested nuclear burning shells
                                                  
                                                 
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Protons electrons form neutrons neutrinos.
Collapse is final.

• At the start of Iron Core collapse
• Radius 6000 km (Rearth)
• Density 108 g/cc

• A second later!! , the properties are
• Radius 50 km
• Density 1014 g/cc
• Collapse Speed 0.25 c !

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Supernova explosion
Neutron degeneracy pressure halts the collapse
Material falling inwards rebounds. Outer layers
of the atmosphere, including shells, are blown
off in a violent explosion called a supernova.
The star will outshine all the other stars in the
galaxy combined.
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Elements heavier than Lead are produced in the
explosion and ejected into space. Stars do
recycle.
The ejected material often attain speeds of
100,000 km/sec.
Close to 150 supernova remnants have been
detected in the Milky Way.
There are smaller numbers of massive stars and so
smaller amount of explosions.
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The Famous Supernova
SN 1987A
type II Supernova
At maximum
Before
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Supernova remnants
Cygnus Loop (HST) greenH, redS, blueO
Cas A in x-rays (Chandra)
Vela
Remnant of SN386, with central pulsar (Chandra)
SN1998bu
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The rings of SN 1987A are from previous mass loss
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1a is binary with a White Dwarf
Type II Hydrogen lines in the spectrum
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Supernova explosion
1, The iron core collapses
2. Neutrons stop the collapse
3. The rebound of the core sends shock waves
causing an explosion that blows the outer
atmosphere into space as a super nova
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• The Crab Nebula.
• A supernova that, according to the Chinese,
  exploded in 1054.
• Despite a distance of 7,000 light-years, the
  supernova was brighter than Venus for weeks
  before fading from view after nearly two years.

• Even today, the nebula
• is still expanding at
• more than 3 million
• miles per hour.

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• Structure of a Neutron Star
• Diameter 12 km in diameter
• Mass -about 1.4 times that of our Sun.
• One teaspoonful of material would weigh a billion
  tons! Rotation Rate 1 to 100 rotations/sec

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• Lighthouse Model

Spinning magnetic
field generates a
a strong electric field.

The magnetic axis is miss-aligned with the
rotation axis of the neutron star . The star's
rotation sweeps the beams outward as it
rotates. If we are in the sight path, will see
regular, sharp pulses of light (optical, radio,
X-ray.)
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Pulsars emitted sharp, 1 millisecond-long pulses
every second at an extremely repeatable rate.
A typical pulsar signal, received with a radio
telescope
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The connection between pulsars and neutron stars
was the discovery of a pulsar in the crab nebula.
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Iron
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Proto-stars (born in
cool gas GMC)
Main Sequence Stars
(H Fusion)
Core Mass (CM) gt 1.4 MO
Core Mass (CM) 0.5- 1.4 MO
Red Giant
Red Dwarf 0.08 - 0 .5 MO
Red Super Giant
Brown Dwarf
Red Super Giant
Yellow Super Giant
CMlt0,08
Planetary Nebula
Red Super Giant
CM 0.5 1.4 MO
White Dwarf
White Dwarf
Supernova (Type II)
Binary can produce Type ia supernova
CM gt 3
CM gt 1.4 lt 3
Neutron Star
Black Hole
Black Dwarf
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Outer layers of the atmosphere, including shells,
are blown off in a violent explosion called a
supernova
Massive star
Neutron Star
Red Supergiant
High Mass
Becomes a Red Supergiant when H exhausted
Yellow Supergiant
Supernova
Becomes Yellow Supergiant when He exhausted Orbit
size of Jupiter
Very High Mass
Red Supergiant
Black Hole
Becomes Red Supergiant
Massive Stars
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Black Holes
We know of no mechanism to halt the collapse of a
compact object with mass gt 3 Msun.
It will collapse into a single point a
singularity
gt Becoming a Black Hole!
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Massive stars form the followingH, He, C, Ne, O,
Si, Fe . Iron will not fuse. Low mass stars form
only H, He, C, O
Honeycutt H Has He Caused
C No Ne Oxford
O Student Si Injury
(Iron) Fe
To memorize this sequence, use this
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Thanks to the following for allowing me to use
information from their web site Nick
Stobel Bill Keel Richard Pogge NASA