Transcript Ch12&13 Life and Death of Stars

The Life and Death of Stars
We are “star stuff” because the elements
necessary for life were made in stars
• Stars are born in molecular clouds consisting mostly
of hydrogen molecules
• Stars form in places where gravity can overcome
thermal pressure in a cloud
Orion Nebula is
one of the
closest starforming clouds
Infrared light from Orion
Solar-system formation is a
good example of star birth
As gravity forces a cloud to
become smaller, it begins to
spin faster and faster
Conservation of angular
momentum
Protostar to Main Sequence
• Protostar contracts and heats until core
temperature is sufficient for hydrogen fusion.
• Contraction ends when energy released by
hydrogen fusion balances energy radiated from
surface.
• Takes 50 million years for star like Sun (less time
for more massive stars)
Luminosity
Stars more
massive than
100 MSun
blow apart
How massive are
newborn stars?
Temperature
Stars less
massive
than
0.08 MSun
can’t
sustain
fusion
Degeneracy Pressure:
Laws of quantum mechanics prohibit two electrons from
occupying same state in same place
Pressure
Gravity
If M > 0.08 MSun, then gravitational
contraction heats core until fusion begins
If M < 0.08 MSun, degeneracy pressure stops
gravitational contraction before fusion can
begin
Life as a Low-Mass Star
• What are the life stages of a low-mass star?
• How does a low-mass star die?
High-Mass Stars
> 8 MSun
IntermediateMass Stars
Low-Mass Stars
< 2 MSun
Brown Dwarfs
Most of life is relatively boring
• Things get interesting after about 9 billion years….
• A star remains on the main sequence as long as it
can fuse hydrogen into helium in its core
Thought Question
What happens when a star can no longer fuse
hydrogen to helium in its core?
A.
B.
C.
D.
Core cools off
Core shrinks and heats up
Core expands and heats up
Helium fusion immediately begins
Thought Question
What happens when a star can no longer fuse
hydrogen to helium in its core?
A.
B.
C.
D.
Core cools off
Core shrinks and heats up
Core expands and heats up
Helium fusion immediately begins
So the core is contracting and heating up…
• The core is inert He, but outside the core, in the
radiation-zone, there is still plenty of H.
• As the core contracts it, of course, brings some of
the rest of the star with it. A layer of H in the
radiation-zone gets sufficiently hot to start
fusing!
• We call this hydrogen-shell burning when you
have a shell of H (around the He core) fusing
into He.
But don’t forget -- the core is still shrinking, even though there
is some fusion in the shell going on.
So the thermostat is broken. Shell burning doesn’t do anything
for the core. But it does fight back against the gravity of the
rest of the star.
That means that the star poofs out and expands into a
Red SubGiant. Radius and Luminosity are bigger.
Stage 1: H-shell burning SubGiant
But what about the core? Eventually the contraction of the
core heats it up high enough for Helium Fusion to start
Helium fusion requires higher temperatures than hydrogen
fusion because larger charge in bigger atoms leads to greater
repulsion.
Fusion of two helium nuclei doesn’t work (the beryllium
barrier), so helium fusion must combine three He nuclei to
make carbon.
BUT! Note that Electron Degeneracy
Pressure is supporting the core when
He-burning begins.
• That means that He-burning won’t immediately
cause the core to expand back outward.
• Instead there is a Helium Flash where a huge amount of
fusion occurs quickly and a lot of energy is released. The
Degeneracy Pressure keeps the core’s temperature hot so
there’s no lessening of the fusion rate.
Fortunately, this lasts only a short time.
• Thermal Pressure
eventually does become
larger than Degeneracy
Pressure, and so lets the
core expand.
• Core expansion means the
H-burning shell expands
too and cools off a bit.
• The thermostat is fixed, and the star goes back into
equilibrium for a while, burning He in the core and some H
in the shell.
• The star shrinks back out of Red Giant phase, but not all
the way back to the main sequence.
Helium burning stars neither shrink nor grow
because thermostat is temporarily fixed.
…except a solar-mass star will never get hot enough to fuse
Carbon into something else. No more fusion can happen!
You’ll have two shells burning around the core -- a H shell
and a He shell, but no more fusion in the core.
Again, the shells do nothing for the core, but they poof out
the star even larger than before.
It is at this point that probably our Sun will engulf
Earth!
Stage 3: He and H shell burning Red Giant
A star like our
sun dies by
puffing off its
outer layers,
creating a
planetary
nebula.
Only a white
dwarf is left
behind
Stage 5: White
Dwarf
Stage 4: Outer
layers lost to
planetary nebula
A white dwarf is about the same size as Earth
White dwarfs shrink when you add mass to them because
their gravity gets stronger
Shrinkage of White Dwarfs
• Quantum mechanics says that electrons in the
same place cannot be in the same state
• 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?
The White Dwarf Limit
• Einstein’s theory of relativity says that
nothing can move faster than light
• When electron speeds in white dwarf
approach speed of light, electron
degeneracy pressure can no longer
support it
• Chandrasekhar found (at age 20!) that
this happens when a white dwarf’s mass
S. Chandrasekhar
reaches 1.4 MSun
• He actually puzzled this out on the boat
from India to England before he started
his grad studies in physics. (Once at
Cambridge his advisor told him he was
crazy and to drop this work…..it won him
the Nobel Prize)
Hydrogen that accretes
onto a while dwarf builds
up in a shell on the surface
When base of shell gets
hot enough, hydrogen
fusion suddenly begins
leading to a nova
Nova explosion generates a burst of light lasting a few weeks
and expels much of the accreted gas into space
Thought Question
What happens to a white dwarf when it accretes enough
matter to reach the 1.4 MSun limit?
A. It explodes
B. It collapses into a neutron star
C. It gradually begins fusing carbon in its core
Thought Question
What happens to a white dwarf when it accretes enough
matter to reach the 1.4 MSun limit?
A. It explodes
B. It collapses into a neutron star
C. It gradually begins fusing carbon in its core
Two Types of Supernova
Massive star supernova:
Iron core of massive star reaches
white dwarf limit and collapses into a
neutron star, causing explosion
White dwarf supernova:
Carbon fusion suddenly begins as white
dwarf in close binary system reaches
white dwarf limit, causing total explosion
These two types have different patterns of
luminosity, so we can tell them apart….
Nova or Supernova?
• Supernovae are MUCH MUCH more luminous!!!
(about 10 million times)
• Nova: H to He fusion of a layer, white dwarf left
intact
• Supernova: complete explosion of white dwarf,
nothing left behind
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
Not to scale!
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
Not to scale!
Life as a High-Mass Star
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. But the extra mass soon produces the
temperatures and pressures necessary to start He
fusion.
•
Helium Core Burning: He fuses to C in core (no
flash)
High-mass
stars become
supergiants
after core H
runs out
Luminosity
doesn’t change
much but
radius gets far
larger
The fusion in the high-mass star is a sequence of
similar events that repeat themselves:
• X is fusing in the core, making Y, but the
core eventually runs out of X.
• Core contracts, allowing layers around the
core to heat up, initiating an X-burning shell
around the core.
• The shell burning does nothing for the core,
but does change the star’s overall radius.
• Core continues to contract, eventually getting
hot enough to let Y start fusing into Z.
How do high mass stars make the
elements necessary
for life?
I.e.: how do these stars actually
make those heavier elements?
Big Bang made 75% H, 25% He
stars make everything else
Helium fusion can make carbon in low-mass stars
CNO (explained in the book) cycle can change C
into N and O
Helium-capture reactions add two protons at
a time and make more common elements
Helium capture builds C into O, Ne, Mg, …
Advanced nuclear fusion reactions require extremely high
temperatures
Only high-mass stars can attain high enough core temperatures
before degeneracy pressure stops contraction
Advanced reactions make heavier elements
Advanced nuclear burning occurs in multiple shells -Like an onion!
Iron is dead end
for fusion
because fusion
reactions
involving iron
do not release
energy
(Iron has the
lowest mass per
nuclear particle)
Evidence for
helium
capture:
Higher
abundances
of elements
with even
numbers of
protons
How does a high mass star die?
Core degeneracy
pressure goes away.
Gravity is so strong
that it forces the
electrons to combine
with protons, making
neutrons and
neutrinos
Neutrons collapse to
the center, forming a
neutron star.
The collapse of the core also
produces a HUGE explosion, a
Supernova
Energy and neutrons released in supernova explosion enables elements
heavier than iron to form AND releases them into space.
Elements made
during supernova
explosion
Crab Nebula: Remnant of supernova observed in 1054 A.D.
before
after
Supernova 1987A is the nearest supernova observed in the last 400
years
The next
nearby
supernova?
Life Stages of High-Mass Star
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
Not to scale!
5. Supernova leaves neutron star
behind
What is a neutron star?
A neutron star
is the ball of
neutrons left
behind by a
massive-star
supernova
Degeneracy
pressure of
neutrons
supports a
neutron star
against gravity
A neutron star is about the same size as a small city
• The first neutron stars were discovered using radio
telescopes that found very regular pulses of radio
emission coming from a single part of the sky.
• These are Pulsars
Pulsar at center
of Crab Nebula
pulses 30 times
per second
A pulsar’s rotation is not aligned with magnetic poles
Pulsars are rotating
neutron stars that act
like lighthouses
Beams of radiation
coming from poles
look like pulses as
they sweep by Earth
We can even see the radiation coming out of pulsar’s poles
X-rays
Visible light
Thought Question
Could there be neutron stars that appear as pulsars to
other civilizations but not to us?
A. Yes
B. No
Thought Question
Could there be neutron stars that appear as pulsars to
other civilizations but not to us?
A. Yes
B. No
What happens to
a neutron star in
a close binary
system?
Matter falling toward a neutron star forms an
accretion disk, just as in a white-dwarf binary
Accreting matter
adds angular
momentum to a
neutron star,
increasing its
spin
Episodes of
fusion on the
surface lead to
X-ray bursts
Thought Question
According to conservation of angular momentum,
what would happen if a star orbiting in a
direction opposite the neutron’s star rotation fell
onto a neutron star?
A. The neutron star’s rotation would speed up.
B. The neutron star’s rotation would slow down.
C. Nothing, the directions would cancel each other
out.
Thought Question
According to conservation of angular momentum,
what would happen if a star orbiting in a
direction opposite the neutron’s star rotation fell
onto a neutron star?
A. The neutron star’s rotation would speed up.
B. The neutron star’s rotation would slow down.
C. Nothing, the directions would cancel each other
out.
What is a black hole?
A black hole is an object whose gravity is so
powerful that not even light can escape it.
Thought Question
What happens to the escape velocity from an object if
you shrink it?
A. It increases
B. It decreases
C. It stays the same
Thought Question
What happens to the escape velocity from an object if
you shrink it?
A. It increases
B. It decreases
C. It stays the same
Hint:
The “surface” of a black hole is the radius at
which the escape velocity equals the speed of
light.
This spherical surface is known as the event
horizon.
The radius of the event horizon is known as the
Schwarzschild radius.
A black hole’s mass
strongly warps
space and time in
vicinity of event
horizon
No Escape
Nothing can escape from within the event
horizon because nothing can go faster than
light.
No escape means there is no more contact with
something that falls in. It increases the hole
mass, changes the spin or charge, but otherwise
loses its identity.
BLACK HOLES DON’T SUCK
• As one gets farther away
from the object, the
strength of gravity
decreases
(remember…inverse
square law).
• Away from the black
hole, it doesn’t matter if it
is a black hole or a star of
the same mass.
• You will orbit a black
hole exactly the same as
you would a star of the
same mass.
Neutron Star Limit
• Quantum mechanics says that neutrons in the
same place cannot be in the same state
• Neutron degeneracy pressure can no longer
support a neutron star against gravity if its mass
exceeds about 3 Msun
• Some massive star supernovae can make black
hole if enough mass falls onto core
Beyond the neutron star limit, no known
force can resist the crush of gravity.
As far as we know, gravity crushes all the
matter into a single point known as a
singularity.
Neutron star
3 MSun
Black
Hole
The event horizon of a 3 MSun black hole is also about as big as a
small city
Thought Question
How does the radius of the event horizon change
when you add mass to a black hole?
A. Increases
B. Decreases
C. Stays the same
Thought Question
How does the radius of the event horizon change
when you add mass to a black hole?
A. Increases
B. Decreases
C. Stays the same
What would it be like
to visit a black hole?
If the Sun shrank
into a black hole, its
gravity would be
different only near
the event horizon
Black holes don’t suck!
Light waves take extra time to climb out of a deep hole in
spacetime leading to a gravitational redshift
Black Hole Verification
•
Do Black Holes really exist? After all you can’t
see them…..
• Need to measure mass
 Use orbital properties of companion
 Measure velocity and distance of orbiting gas
•
It’s a black hole if it’s not a star and its mass
exceeds the neutron star limit (~3 MSun)
The jet emitted by the galaxy M87 in this image is
thought to be caused by a supermassive black
hole at the galaxy's centre
At the center of the
Milky Way stars
appear to be
orbiting something
massive but
invisible … a
black hole?
Orbits of stars
indicate a mass of
about 4 million
MSun