Transcript Chapter 17 Lecture Presentation

http://www.highpoint.edu/~afuller/PHY-1050
• Read:
– Death From The Skies Chapter 3: “The Stellar Fury of
Supernovae”
– Death From The Skies Chapter 7: “The Death of the Sun”
• Pre-Lecture Quiz:
– MasteringAstronomy Ch18 pre-lecture quiz due March 31
– MasteringAstronomy Ch19 pre-lecture quiz due April 14
• Homework:
–
–
–
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MasteringAstronomy Ch16 assignment due March 29
MasteringAstronomy Ch17 assignment due April 12
MasteringAstronomy Ch18 assignment due April 19
MasteringAstronomy Ch19 assignment due April 24
How does a star’s mass affect nuclear fusion?
• The mass of a mainsequence star determines
its core pressure and
temperature.
• Stars of higher mass have
higher core temperature
and more rapid fusion,
making those stars both
more luminous and
shorter-lived.
• Stars of lower mass have
cooler cores and slower
fusion rates, giving them
smaller luminosities and
longer lifetimes.
High-Mass Stars
> 8MSun
IntermediateMass Stars
Low-Mass Stars
< 2MSun
Brown Dwarfs
Star Clusters and Stellar Lives
• Our knowledge of the
life stories of stars
comes from comparing
mathematical models of
stars with observations.
• Star clusters are
particularly useful
because they contain
stars of different mass
that were born about
the same time.
Life Cycle of a Low-Mass Star
Main Sequence Lifetimes and Stellar Masses
http://www.highpoint.edu/~afuller/PHY1050/textbook/17_MSLifetimeAndMass.htm
What happens when a star can no longer fuse
hydrogen to helium in its core?
A.
B.
C.
D.
The core cools off.
The core shrinks and heats up.
The core expands and heats up.
Helium fusion immediately begins.
What happens when a star can no longer fuse
hydrogen to helium in its core?
A.
B.
C.
D.
The core cools off.
The core shrinks and heats up.
The core expands and heats up.
Helium fusion immediately begins.
Life Track After Main Sequence
• Observations of star
clusters show that a star
becomes larger, redder,
and more luminous
after its time on the
main sequence is over.
Red Giants: Broken Thermostat
• As the core contracts, H
begins fusing to He in a
shell around the core.
• Luminosity increases
because the core
thermostat is broken—
the increasing fusion
rate in the shell does
not stop the core from
contracting.
Helium fusion does not begin right away because it
requires higher temperatures than hydrogen fusion—
larger charge leads to greater repulsion.
Fusion of two helium nuclei doesn’t work, so helium
fusion must combine three helium nuclei to make carbon.
What happens in a low-mass star when core
temperature rises enough for helium fusion to
begin? (Hint: Degeneracy pressure is the main
form of pressure in the inert helium core.)
A. Helium fusion slowly starts.
B. Hydrogen fusion stops.
C. Helium fusion rises very sharply.
What happens in a low-mass star when core
temperature rises enough for helium fusion to
begin? (Hint: Degeneracy pressure is the main
form of pressure in the inert helium core.)
A. Helium fusion slowly starts.
B. Hydrogen fusion stops.
C. Helium fusion rises very sharply.
Helium Flash, I
• The thermostat of a low-mass red giant is
broken because degeneracy pressure supports
the core against gravity instead of the energy
released from nuclear fusion.
• Hydrogen continues to burn in a shell
surrounding the helium core.
Helium Flash, II
• Because hydrogen fusion
continues in outer shell,
helium “ash” continues to
get dumped onto helium
core.
• As helium continues to
pile up in the core,
eventually helium fusion
is triggered, causing the
core temperature to
rapidly rise.
• Helium fusion rate
skyrockets until thermal
pressure takes over and
expands the core again.
A 5 Msun star with a helium core and a
hydrogen-burning shell shortly after shell
ignition.
Helium-burning stars neither shrink nor grow
because core thermostat is temporarily fixed.
Life Track After Helium Flash
• Models show that a red
giant should shrink and
become less luminous
after helium fusion
begins in the core.
• The exact path of the
life track depends on
the star’s mass.
Life Track After Helium Flash
• Observations of star
clusters agree with those
models.
• Helium-burning stars are
found on a horizontal
branch on the H-R
diagram.
• Combining models of
stars of similar age but
different mass helps us to
age-date star clusters.
Life Track After Helium Flash
1 Msun Star
5 Msun Star
What happens when the star’s core runs out of
helium?
A.
B.
C.
D.
The star explodes.
Carbon fusion begins.
The core cools off.
Helium fuses in a shell around the core.
What happens when the star’s core runs out of
helium?
A.
B.
C.
D.
The star explodes.
Carbon fusion begins.
The core cools off.
Helium fuses in a shell around the core.
Double Shell Burning
• After core helium fusion
stops, helium fuses into
carbon in a shell around the
carbon core, and hydrogen
fuses to helium in a shell
around the helium layer.
• This double shell–burning
stage never reaches
equilibrium—fusion rate
periodically spikes upward
in a series of thermal
pulses.
• With each spike, convection
dredges carbon up from
core and transports it to
A 5 Msun star well after all the helium in the
core has been exhausted, with just a
carbon-oxygen core.
Planetary Nebulae
• Double shell burning ends with a pulse that
ejects the H and He into space as a planetary
nebula.
• The core left behind becomes a white dwarf.
• Despite the name, this phenomenon has
nothing immediate to do with planets
End of Fusion
• Fusion progresses no further in a low-mass star
because the core temperature never grows hot
enough for fusion of heavier elements (some
helium fuses to carbon to make oxygen).
• All that remains is the exposed core of the star,
called a white dwarf.
• Degeneracy pressure supports the white dwarf
against gravity and it slowly cools.
• After several trillion years, the white dwarf will
eventually reach temperatures around 10 K and
become known as a black dwarf.
Understanding the Individual Stages of a Low-Mass
Star’s Death Sequence
http://www.highpoint.edu/~afuller/PHY1050/textbook/17_DeathSeqStar.htm
Life Track of a Sun-like Star
Life Cycle of a High-Mass Star
Life Cycle of a High-Mass Star
• Late life stages of high-mass
stars are similar to those of
low-mass stars:
– Hydrogen core fusion (main
sequence)
– Hydrogen shell burning
(supergiant)
– Helium core fusion (supergiant)
• High-mass stars, however,
have the ability to continue
fusing elements well past
helium.
• As such, the paths of highmass stars on the H-R diagram
are different from those of
low-mass stars.
CNO Cycle
• High-mass mainsequence stars fuse H
to He at a higher rate
using carbon, nitrogen,
and oxygen as catalysts.
• Greater core
temperature enables
hydrogen nuclei to
overcome greater
repulsion.
High-mass stars make the elements
necessary for life
High core temperatures allow helium to fuse
with heavier elements.
Big Bang made 75% H, 25% He. Stars make
everything else.
Insert image, PeriodicTable2.jpg.
Helium fusion can make carbon in low-mass
stars. (It can also make beryllium and oxygen
in what is known as the triple-alpha cycle.)
CNO cycle can change carbon into nitrogen
and oxygen.
Helium capture builds carbon into oxygen,
neon, magnesium, and other elements.
Advanced Nuclear Burning
Core temperatures in stars with >8 MSun allow
fusion of elements as heavy as iron.
Insert image, PeriodicTable5.jpg
Advanced reactions in stars make elements
like Si, S, Ca, Fe.
Evidence for helium capture
Higher abundances of elements with even
numbers of protons.
Multiple Shell Burning
Advanced nuclear burning proceeds in a series of
nested shells.
Time Frames
Because C, O, and Si burning produce nuclei
with masses progressively closer to Fe, less and
less energy is generated per gram of fuel. As a
result, the time scale for each succeeding
reaction becomes shorter.
The Death Sequence of a High-Mass Star
http://www.highpoint.edu/~afuller/PHY1050/textbook/IF_17_12_HighMassDeathSeq.htm
The Party Ends With Iron
Iron is a dead end for fusion because nuclear reactions
involving iron do not release energy. (This is because iron has
lowest mass per nuclear particle.)
The Party Ends With Iron
• Iron builds up in core until degeneracy
pressure can no longer resist gravity.
• To make matters worse, fusion of iron will
require—not release—energy.
• The core then suddenly collapses, creating a
supernova explosion.
Supernova Step 1: Photodisintegration
• At the billion-degree temperatures now present
in the core, the photons possess enough energy
to destroy heavy nuclei, a process known as
photodisintegration.
• This destroys heavy elements created in each
stage of fusion.
• This process requires energy, so thermal energy is
removed from the gas that would otherwise have
resulted in the pressure necessary to support the
star’s core.
Supernova Step 2: Creation of Neutrinos
• Free electrons that had
assisted in supporting the star
through degeneracy pressure
now collide with the protons
produced through
photodisintegration.
• The result of this is that a
massive amount of the star’s
mass is converted into
neutrons and neutrinos.
• Neutrons collapse to the
center, forming a neutron
star.
• Neutrinos escape to space
mostly uninhibited.
Supernova Step 3: Core Collapse
• Through the photodisintegration of iron,
combined with the creation of neutrons
and neutrinos, most of the core’s
support in the form of electron
degeneracy pressure is suddenly gone
and the core begins to collapse
extremely rapidly.
• The inner core collapses so fast, it
decouples from the outer core,
completely separating from it, causing
the outer core to go into free-fall.
• During the collapse, speeds can reach
almost 70,000 km/s (0.25 c), and within
about one second a volume the size of
Earth has been compressed to a
diameter of 100 km.
• This process takes roughly a quarter of a
second.
Supernova Step 3: Suspended Shells
• Since “word” that the core has collapsed
propagates through the star at a much smaller
speed, there is not enough time for the outer
layers to immediately learn about what has
happened.
• The outer layers, including the O, C, and He
shells, as well as the outer envelope, are left in a
precarious position of being almost suspended
above the catastrophically collapsing core.
Supernova Step 4: Neutron Degeneracy
• The inner core continues to
collapse until it reaches a point
where neutron degeneracy is
strong enough to resist gravity
and support the core.
• The result is that the inner
core rebounds somewhat,
sending pressure waves
outward into the in-falling
material from the outer core.
• This “core bounce” takes only
20 milliseconds to occur and is
known as a “prompt
hydrodynamic explosion.”
Supernova Step 5:
Shock Wave Propagation
• The pressure waves speed up and
become full shock waves that work their
way toward the surface.
• If the remainder of the iron core is less
than roughly 1.2 Msun, the shock waves
“snowplow” the H-rich envelope and the
remainder of the nuclear-processed
matter in front of it.
• If the remainder of the core is more
massive than 1.2 Msun, then the shock
wave stalls. Neutrinos are blocked by
this stalled shock wave. Eventually the
build-up of neutrinos pushes the shock
wave back into motion, releasing roughly
1047 J. (The sun produces 1045 J of energy
over its entire lifetime on the main
sequence.)
Supernova Step 6: The Remnant
• Energy released by the collapse of
the core drives the star’s outer
layers into space.
• If the initial mass of the star on the
main sequence is not too large (<
25 Msun), the remnant inner core
will stabilize and become a neutron
star, supported by degenerate
neutron pressure.
• However, if the initial stellar mass is
much larger, even the pressure of
neutron degeneracy cannot support
the remnant against the pull of
gravity. The final collapse will
produce a black hole.
• The Crab Nebula is the remnant of
the supernova seen in A.D. 1054.
Supernova Time Frame
Insert figure, PeriodicTable6.jpg
Energy and neutrons released in supernova explosion
enable elements heavier than iron to form, including
gold and uranium.
A Star’s Mass Determines Its Destiny
The Role of Mass
• A star’s mass determines its entire life story
because it determines its core temperature.
• High-mass stars with >8 Msun have short lives,
eventually becoming hot enough to make iron,
and end in supernova explosions.
• Low-mass stars with <2 Msun have long lives,
never become hot enough to fuse carbon nuclei,
and end as white dwarfs.
• Intermediate-mass stars can make elements
heavier than carbon but end as white dwarfs.
Low-Mass Star Summary
1. Main sequence: H fuses to He in core.
2. Red giant: H fuses to He in shell around He core.
3. Helium core burning: He fuses to C in core while H
fuses to He in shell.
4. Double shell burning: H and He both fuse in shells.
5. Planetary nebula leaves white dwarf behind.
Reasons for Life Stages
• Core shrinks and heats until it’s hot enough for fusion.
• Nuclei with larger charge require higher temperature
for fusion.
• Core thermostat is broken while core is not hot enough
for fusion (shell burning).
• Core fusion can’t happen if degeneracy pressure keeps
core from shrinking.
High-Mass Star Summary
1. Main sequence: H fuses to He in core.
2. Red supergiant: H fuses to He in shell around He core.
3. Helium core burning: He fuses to C in core while H
fuses to He in shell.
4. Multiple shell burning: Many elements fuse in shells.
5. Supernova leaves neutron star behind.