Download Chapter 20 Review of Stars` Lifetime Review

Review of Stars’
Lifetime
• Main sequence – all stars
Chapter 20
• Converting hydrogen to helium
• Low mass (<0.4 MSun)
• Ends as inert ball of helium
• Gradually releases all its energy
Stellar Evolution:
The Death of Stars
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Review, cont.
Medium Mass Stars
• Medium mass (0.4 to 4 MSun)
• Helium fusion begins in the core
• Helium flash if < 2MSun
• Consume only the hydrogen in
their cores
• Then leave the main sequence
• Hydrogen shell burning
• Luminosity decreases a bit
• Star expands and cools
• As rate of burning decreases, the
star’s outer layers contract
• This causes an increase in
temperature
• Horizontal (red giant) branch
• New energy causes star to enlarge
• Becomes a red giant
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Asymptotic Giant
Branch (AGB)
HR Diagram
• The helium in the core is
converted into carbon and
oxygen
• Once the helium is exhausted,
the core contracts until stopped
by degenerate electron
pressure
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AGB, cont
Helium Shell Burning
• A thin shell of helium around the
core will begin to undergo
fusion
• The star enters a second red
giant stage
• The expansion of the star’s
outer layers eventually cause
the hydrogen shell burning to
stop
• Carbon ash is
accumulating in
the core
• Helium is “burning”
in a shell
surrounding the
carbon
• Hydrogen is still
burning in a shell
surrounding the
helium
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Stages Shown by a
Cluster
AGB Structure
• The central part (right) is about the
size of the Earth
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Dredge-Ups
Dredge-Ups, cont
• During the final stages of a
star’s lifetime, convection cells
can reach deeper into the star
• First red giant stage
• Helium flash
• A second dredge-up brings more carbon,
nitrogen and oxygen
• Third (if > 2 MSun) during AGB
• The material produced by the NCO
cycle can be brought to the
surface
• Increases the amount of carbon,
nitrogen and oxygen
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• Large amounts of carbon
• Star’s spectrum is heavy in carbon
elements
• Called a carbon star
• Ejects large amounts of material
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Carbon Star, Example
Planetary Nebula
• Strong solar
winds
• Relatively low
surface
temperatures
• A solar mass star does not have
enough mass to start fusing other
materials
• Intense burning of the hydrogen and
helium layers cause pressure that
causes its outer layers to escape
• During the AGB stage, the star
divests itself of its outer layers
• About 3000 K
• Ejected material
often resembles
soot
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Examples of Planetary
Nebula
Planetary Nebula, cont
• The star ends up with two distinct
parts
• The cool envelop of dust and gas that
has escaped
• The hot core
• Still very luminous
• The core can illuminate the cloud of
gas and dust
• This forms a planetary nebula
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Matter
White Dwarf
• The planetary nebula will
continuing expanding into
space carrying many different
molecules with it
• This is the origin of the
“heavier” elements
• As the envelop of the red giant
disperses, the core becomes
visible
• It is small, but luminous
• It is a white dwarf
• “Naked” white dwarfs have lost
their envelops (or at least they are
so dispersed they are invisible)
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Characteristics of White
Dwarfs
Mass-Luminosity
Relationship
• Cool, but do not shrink
• Electron degeneracy creates
pressure not dependent on
temperature
• Mass-radius relationship
• The more massive the white
dwarf, the smaller it is
• The more degenerate matter you
have, the smaller it becomes
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Sample Evolutionary
Tracks
Chandrasekhar Limit
• There is a limit to the amount of
pressure the degenerate electrons
can produce
• Therefore, there is an upper limit to
the mass that a white dwarf can
have
• This limit is the Chandrasekhar limit
• 1.4 Msun
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White Dwarf Example –
Sirius B
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More White Dwarfs
• Characteristics:
• Mass 1.1 MSun
• Radius 0.008 rSun
• Luminosity 0.04
LSun
• Surface Temp
24,000 K
• Average Density
3 x 10 kg/m3
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Another White Dwarf
Binary White Dwarfs
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Black Dwarf
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Sun’s Evolution
• The white dwarf continues to cool
• No nuclear reactions, only stored heat
• The heat dissipates
• Eventually it becomes a cold, burnt
out ember
• A black dwarf
• Not much compression, just cooling
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Other Types of White
Dwarfs
Another Type of White
Dwarf
• White dwarfs produced by stars of ~
1 solar mass contain mainly carbon
and oxygen
• Helium White Dwarfs
• Neon-oxygen white dwarf
• Very low mass stars may never reach
helium fusion
• The star’s envelop can still be ejected,
leaving behind a white dwarf composed
mainly of helium
• None have been detected that have formed
this way
• Some have been found in binary systems
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• A more massive star could reach
temperatures high enough for the
oxygen to combine with the helium and
form neon
• When fusion stops, the neon-oxygen
white dwarf would be left
• The star would need to be near the 8
solar mass maximum that form carbon
cores
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Evolution of High Mass
Stars (> 4 MSun)
Core Contraction
• As high mass stars leave the
main sequence, they tend to
move horizontally along the
giants branch instead of
vertically
• They “burn” successively
heavier elements
• More massive than
Chandrasekhar limit
• Electron degeneracy cannot
stop the contraction of the core
• Enters a new round of fusion
• Exactly which elements will
depend on their mass
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HR Diagram Tracks High
Mass Star Comparison
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HR Diagram – Massive
Stars
• The 4 Solar
Mass star will
burn carbon
then stop
• Higher masses
will burn more
elements
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Fusion Sequence
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Red Supergiant
• As the star moves back and
forth in the red giant area of the
HR diagram,
• Its luminosity decreases
• Its radius increases
• It reaches the red supergiant
stage
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Mass Ejections from
Supergiants
Shell Burning
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V838 Close Up
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Betelgeuse
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Shell Burning in Massive
Stars
Red Supergiant Group
• There is no
noticeable
different in the
appearance of the
star when each
new element starts
to burn
• At the supergiant
stage
• All nuclear fusion
stops at iron
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Evolution and Mass –
Summary
Evolution and Mass –
Summary, cont
• The Russell-Vogt Theorem
• 0.4 M < M* < 1.2 M – Life-cycle
like the Sun
• The most important determinant for the
life-cycle of a star is its mass
• Hydrogen burning by p-p chain
• Helium burn to carbon
• M* < 0.01 M - Planet
• 0.01 M < M* < 0.085 M - Brown
Dwarf
• 0.085 M < M* < 0.4 M – long-lived,
low main-sequence
• M* > 1.2
• Hydrogen burning by CNO cycle
• End as helium white dwarfs
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Evolution and Mass –
Summary, Final
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Core Collapse
• M* > 8 M
• Will have a larger number of
nuclear burning cycles
• Core Mass of M ~ 1.4 solar masses
• Electron degeneracy
• Largest mass for white dwarf
• As the iron builds up, fusion ceases
• Gravity overcomes the thermal
radiation pressure
• Core collapses
• Some estimates are collapse in less
than 1 sec
• Photodisintegration occurs
• The photons have enough energy to
divide the iron atoms
• Continues until protons and neutrons are
separated
• End their lives with a supernova.
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Core Collapse
Supernova
Core Collapse, final
• Core consists of protons, neutrons,
electrons and photons
• High densities cause the electrons
and protons to form neutrons
• Releases neutrinos
• Escape carrying energy with them
• Collapse stops when the neutrons
cannot be forced closer together
• Rebounds and releases energy in a
core-collapse supernova
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Supernovae
Type II Supernova
• Types of Supernovae
• Caused by core collapse
• Type I
• Details were discussed previously
• Type Ia – include strong absorption lines of
ionized silicon
• Type Ib – lack silicon lines, but have strong
helium absorption lines
• Type Ic – have neither silicon or helium lines
• Spectra shows a lot of hydrogen
and helium
• Out shells were blown off during
the supernova
• Type II
• Core collapse of a massive star
• Have strong hydrogen absorption lines
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Type II Example – SN
1987 A
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SN 1987 A
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SN 1987 A
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SN 1987 A Today
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Another Example – SN
1993J
SN 1987 A
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Type Ia Supernova
Type Ia Supernova, cont
• A white dwarf’s mass tends to
increase with each nova cycle
• If the 1.4 solar mass limit is
exceeded, the star starts to collapse
• The temperature increases to the
point where carbon can fuse
• Not all the accumulated mass is
ejected
• This is occur in all parts of the star
~simultaneously
• If a mass > 1.4 solar masses is
accumulated, then another
process occurs
• A carbon-detonation supernova
results
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Type I Supernova, final
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Type Ia – Summary
• Some theories indicate other
paths to the same result
• Collisions between white dwarfs,
for example
• Spectra shows virtually no
hydrogen
• The white dwarf was from a low
mass star and so the system
already lost all of its hydrogen
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Type Ib and Ic
Supernovae
Supernovae Comparison
• Also core collapse
• Progenitor stars have already
lost outer layers
• Accounts for lack of hydrogen
spectral lines
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Supernovae Comparison
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Comparison, cont
• Type Ia and Type II supernovae are
unrelated to each other
• All high mass stars will undergo a
Type II supernova
• Some low mass stars will become
white dwarfs that will experience
Type Ia supernovae
• The two types occur at
approximately the same rate
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Crab Nebula (top)
Vela
Supernova
Remnants
(bottom)
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More SN Remnants
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Even More SN
After the SN?
• Examples of
• Neutron stars
• Black holes
• G292.0+1.8
• Crab
• SN 1987 A
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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 stars
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