Download Astronomy 110 Announcements: Life and Death of a Low Mass Star

Life and Death of a Low Mass Star
Astronomy 110 Announcements:
Stages:
• No reading quiz today
• Homework #3 due at start of class
• Pick up Homework #4 in front of class (due
next Wednesday, June 22nd)
• Reading for Monday: pp. 330 – 345
(schedule has been updated for rest of year
on website)
1
1. H-burning main
sequence
2
2. H-shell burning
Red Giant
3
4
6
3. He burning star
4. Double shell
burning star
5. Planetary Nebula
5
6. White Dwarf
Stage #2: Red Giant
Stage #2: Red Giant
(H-burning shell, inert He core)
(H-burning shell, inert He core)
• After 10 billion years, sun’s
core H is used up
• Inert He core (not hot
enough to burn yet)
• He core shrinks with
surrounding layers
• Surrounding shell of H gets
hot enough for fusion
• H shell burning goes at rapid
pace! more energy than
during main sequence
• Star expands and cools:
Red Giant Phase
Hydrogen core burning
Hydrogen shell burning
Stage #2: Red Giant
(H-burning shell, inert He core)
The “Broken Thermostat”
• During MS, temp & size
regulated by solar thermostat
• During H-shell burning, no
thermostat
• H-shell burning makes more
He, dumped onto core
• Core contracts further due to
increased weight
• H-shell burning accelerates
• Sun grows to 100 times its
size
Helium Flash
Helium fusion requires higher temperatures than hydrogen
fusion because larger charge leads to greater repulsion
Fusion of two helium nuclei doesn’t work, so helium fusion
must combine three He nuclei to make carbon ! “Triple
Alpha Process”
Stage #3: Helium burning star
• Thermostat is broken in low-mass red giant because
degeneracy pressure supports core
• Core temperature rises rapidly when helium fusion
begins
• Helium fusion rate skyrockets until thermal pressure
takes over and expands core again
Helium burning stars neither shrink nor grow
because thermostat is temporarily fixed.
Question
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
Stage #4: Double-shell burning star
• Similar to the red giant phase:
– Inert Carbon core
– Surrounding shells: burning H and He
• Star becomes very cool and luminous, expanding in
size
• Gravity is very weak at stellar surface leading to
mass loss through a strong stellar wind (stage #5)
• Continuing contraction of core leads to greater &
greater luminosity, however, it never gets hot enough
to fuse carbon
• Star collapses to a dense, small, hot object: white
dwarf
Question
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
Stage #5: Planetary Nebula
A low mass star dies by shedding its outer layers
Stage #6: White Dwarf
The central remaining core is very hot, leading to
ionization of the surrounding gas shell.
Star clusters
help us test
models of
stellar
evolution
because they
contain stars
of same age
but at different
life stages
Life stages
of a lowmass star
like the Sun
White dwarfs
cool off and
grow dimmer
with time
A white dwarf is about the same size as Earth
White dwarfs shrink when you add mass to them because
their gravity gets stronger
The White Dwarf Limit
Shrinkage of White Dwarfs
Einstein’s theory of relativity says
that nothing can move faster than
light
• Quantum mechanics says that electrons in the
same place cannot be in the same state
When electron speeds in white
dwarf approach speed of light,
electron degeneracy pressure can
no longer support it
• Adding mass to a white dwarf increases its
gravity, forcing electrons into a smaller space
• In order to avoid being in the same state some of
the electrons need to move faster
• Is there a limit to how much you can shrink a
white dwarf?
S. Chandrasekhar
Chandrasekhar found (at age 20!)
that this happens when a white
dwarf’s mass reaches
1.4 MSun
12.3 Life as a High-Mass Star
• Our Goals for Learning
• What are the life stages of a high mass star?
• How do high-mass stars make the elements
necessary for life?
What are the life stages of a high
mass star?
• How does a high-mass star die?
High-Mass Stars
> 8 MSun
IntermediateMass Stars
Low-Mass Stars
< 2 MSun
Brown Dwarfs
High-Mass Star’s Life
Early stages are similar to those of low-mass star:
•
Main Sequence: H fuses to He in core
•
Red Supergiant: H fuses to He in shell around
inert He core
•
Helium Core Burning: He fuses to C in core (no
He flash because core temp is high and sustains
thermal pressure—no degeneracy)
IN
4 protons
OUT
nucleus
2 gamma rays
2 positrons
2 neutrinos
4He
Total mass is
0.7% lower
Review: Proton-proton chain is how hydrogen fuses into helium
in Sun.
High Mass Stars also fuse H ! He, but with CNO cycle
CNO cycle is just
another way to fuse H
into He, using carbon,
nitrogen, and oxygen as
catalysts
CNO cycle is main
mechanism for H fusion
in high mass stars
because core
temperature is higher
High-mass
stars become
supergiants after
core H runs
out
Luminosity
doesn’t
change much
but radius gets
far larger
How do high mass stars make
the elements necessary
for life?
Big Bang made 75% H, 25% He – stars make everything else
Helium fusion can make carbon in low-mass stars
Helium-capture reactions add two protons at a time
CNO cycle can change C into N and O
Advanced nuclear fusion reactions require extremely high
temperatures
Helium capture builds C into O, Ne, Mg, …
Only high-mass stars can attain high enough core
temperatures before degeneracy pressure stops contraction
Advanced nuclear burning occurs in multiple shells
(“layers of an onion”)
Advanced reactions make heavier elements
Iron is dead
end for fusion
because nuclear
reactions
involving iron
do not release
energy
(Fe has lowest
mass per
nuclear particle)
Evidence for
helium
capture:
Higher
abundances of
elements with
even numbers
of protons
Death of High Mass Star
How does a high mass star die?
Core degeneracy
pressure goes away
because electrons
combine with
protons, making
neutrons and
neutrinos
Neutrons collapse to
the center, forming a
neutron star
• Left with inert iron (Fe) core.
• Briefly supported by electron degeneracy pressure.
• Outer shells still burning and dumping tremendous amounts
of mass onto the core.
• Gravity overcomes the electron degeneracy and squishes the
protons and electrons together, yielding neutrons.
Neutron Star
• Original core mass ~ 10 Msun, and size is ~1 REarth.
Collapses to only a few kilometers in size!
Death of High Mass Star
• Neutron star has its own
degeneracy pressure that
can stop the collapse
• If the star is too massive,
nothing can stop collapse
! Black Hole
• Generation of neutrons
gives off extreme amounts
of neutrinos that produce a
shock wave, sending the
outer layers of the star off
at a furious pace.
• Supernova explosion!
• Brighter than 10 billion
Suns for a few days
Energy and neutrons released in supernova explosion enables elements
heavier than iron to form
Iron builds up
in core until
degeneracy
pressure can no
longer resist
gravity
Elements made
during
supernova
explosion
Core then
suddenly
collapses,
creating
supernova
explosion
before
after
Supernova 1987A is the nearest supernova observed in the last 400
years
Crab Nebula: Remnant of supernova observed in 1054 A.D.
12.4 Summary of Stellar Lives
The next
nearby
supernova?
• Our Goals for Learning
• How does a star’s mass determine its life story?
• How are the lives of stars with close companions
different?
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
How does a star’s mass
determine its life story?
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
Not to scale!
Life Stages of High-Mass Star
Reasons for Life Stages
" Core shrinks and heats until it’s
hot enough for fusion
1. Main Sequence: H fuses to He in
core
" Nuclei with larger charge
require higher temperature for
fusion
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
" Core thermostat is broken
while core is not hot enough
for fusion (shell burning)
4. Multiple Shell Burning:
Many elements fuse in shells
" Core fusion can’t happen if
degeneracy pressure keeps core
from shrinking
Not to scale!
5. Planetary Nebula leaves white
dwarf behind
Not to scale!
5. Supernova leaves neutron star
behind
Life of a 20 MSun star
Life of a 1 MSun star