Transcript Goal: To understand the deaths of stars and how it depends

Goal: To understand the deaths
of stars and how it depends on
mass.
Objectives:
1) To learn about the lives of Red dwarfs
2) To understand what Stars like the sun will do at
the end of their lifetimes
3) To understand how Stars somewhat bigger
than the sun will have different ends
4) To understand how and why Stars quite a bit
bigger than the sun will end their lives
5) To examine the deaths of Stars a whole lot
bigger than the sun
Red Dwarfs
Live so long that we won’t talk about them
much
None have died
Become White Dwarfs
Stars like our sun.
• What will our sun become when it dies?
• A) nothing, will blow itself to bits in a fiery
supernova that destroys everything
• B) a white dwarf
• C) a neutron star
• D) a black hole
Stars like our sun.
• What will our sun become when it dies?
• B) a white dwarf
So, how does the sun get to there?
• Lets back up a bit.
• The sun is currently 4.5 billion years old.
• In about 5 billion more years the sun is
going to start to run out of fuel in its core.
• This leads to trouble.
The beginning of the end!
• With its supply of energy from fusion
dwindling, the core of the sun starts to
contract (gravity is winning).
• This heats up the core.
Hotter more radiative core
• Meanwhile the outer parts of the sun
expand.
• In fact they expand by a factor of 100!
• The sun balloons up to the size of the orbit
of the earth!
Also
• Also, because of the sun’s expansion, the
temperature drops by 50%.
• This makes the white sun red.
• Thus it is a red giant.
• As you might expect, 100X bigger in size
means it will radiate more energy.
• It gets 1000 X brighter!
• Also, the rate it throws off materials off its
surface also increases by a factor of 1000.
In the core
• The core temperature goes up from tens
of millions of degrees to hundreds.
• However we just have Helium and bigger.
• The protons are all gone.
• The solution might look simple, get the
temperature high enough and 2 Helium
atoms will collide.
• What happens if we do that?
Atomic hug
• The 2 Helium atoms combine for a brief
moment and then split apart.
• What they make just is not stable.
• Beryllium 8 is formed and almost instantly
decays back to 2 Heliums
• So, it looks grim.
• We are made of Carbon and Oxygen –
how do we get that if 2 simple Helium
atoms can’t combine?
Triple alpha process
• An “alpha” particle is just the nucleus of a
Helium atom (2 protons and 2 neutrons).
• Imagine that during our atomic hug a 3rd
Helium atom came in.
• We could then create Carbon!
• So, 3 Helium atoms crashing into each
other at almost the same time creates
Carbon.
As you can imagine though
•
•
•
•
Trying to collide 3 atoms is not easy.
So, this process is VERY difficult to do!
Why?
Well first 3 atoms means that the reaction rate
depends on the density to the THIRD power.
• Also, this is very temperature dependant!
• The reaction rate depends on the temperature to
the 41st power!
• A 10% increase in temperature means 50X more
reactions per second.
Start of a new era
• When you get your first reactions you will
heat the core slightly.
• This slight heating creates a chain reaction
as the reaction rate goes up exponentially
• This creates more heating which makes
the rate go up exponentially again.
• We have the makings of an Astronomically
large bomb.
Helium Flash
• When helium starts to fuse in the core it is
a very explosive event!
• The fusion heats the core.
• This causes more the reactions to happen
a lot faster! (10% increase in temp = 50X
faster)
• The sun undergoes a very rapid change
here.
However
• The “flash” is short lived
• This energy will increase the pressure of
the core
• Pressure is a measure of how strongly gas
pushes
• If the interior pressure is suddenly higher
the core pushes outward and expands
Helium flash core consequences
• The Helium flash heats the core.
• The causes the core to expand.
• This keeps the core from increasing in
temperature to quickly
Helium outside core consequences
• The outer edge of the core gets hot enough to
fuse Hydrogen in a new layer called a shell.
• The expansion of the core causes the outer part
of the star to rapidly contract (by a factor of
about 25).
• The contraction makes the outer part of the star
hotter (by about a factor of 2)
• So, the star as viewed from far away shrinks,
gets hotter, and gets dimmer (about 40 times
dimmer)
Time table
• This all occurs in a time frame of about a
week.
• We have never been able to be lucky
enough to watch a star go through this
rapid transition.
Post helium flash
• With time the core will shrink again and the outer
layers will expand (cooling the star but making it
brighter).
• After this the sun will expand back to its previous
size and temperature as what is called an
Asymptotic Red Giant.
• Eventually the Helium will run out (well fairly
quickly – it is radiating energy 1000 times faster
now after all).
• So, what happens when the Helium starts to run
out?
Well…
• Once you get a good Carbon and Helium and
the core gets a bit hotter you can get some
carbon to fuse with Helium to get Oxygen and
maybe some Oxygen with Helium to get Neon.
• However, it won’t get past that. You need 600
million degrees to fuse carbon with carbon
reliably.
• So, what will happen to the sun at this point?
Core continues to collapse
• The core continues to collapse.
• This makes the outer layers expand.
• However, the sun can no longer hold onto
these layers, so they get ejected.
• The sun will loose half of its mass during
this period.
• Will anything stop its collapse?
Electrons to the rescue!
• It is humbling that to save this large star it takes
something as small as an electron to save it.
• At some point the density of the core gets to a
MILLION times the density of water!
• At this point the electrons are crammed so
closely that they repel each other.
• While seeming innocent, this gives enough
outward pressure to repel gravity.
• And the sun is saved!
• This is called electron degeneracy pressure.
What is left?
• What is left is the core (the rest is ejected
into space).
• The remains is half the mass of our
current sun with a diameter of our earth
(which is 1% of the diameter of the current
sun).
• This object is called a white dwarf.
White Dwarfs
• The sun will become a white dwarf at the end of
its lifetime.
• It takes 10% of the main sequence time to get
from main sequence to white dwarf (which for
our sun would be 1 billion years after it leaves
the main sequence).
• White dwarfs are small and very hot.
• With time they cool down.
• There is no fusion, so they slowly loose energy
and get cooler.
What happens to white dwarfs?
• Eventually the cool down and become
black dwarfs.
• And this is the ultimate fate of our sun and
all stars more massive than a red dwarf
but less than 4 times the mass of our sun.
• Now for some pretty pictures (have you
forgotten about all the ejected gas
already?)!
Planetary Nebula
Butterfly Nebula (still planetary)
Ant Nebula
Ring Nebula (4k light years away)
Ghost of Jupiter
Eskimo Nebula
Spirograph Nebula
• 2k lyr away and 0.3 lyr across
Cat’s Eye Nebula
• Binary system?
Stars between 4-8 time the mass of
the sun
• These stars have a different evolution.
• However their evolution is not completely
understood.
• When they reach the Helium Flash they have a
chance of detonating their entire core due to the
core being held together by electrons.
• This would completely destroy the star.
• However, it is not completely understood what
happens in these cases.
Stars 8-25 times the mass of the
sun.
• The start is the same as the sun.
• However, once Helium gets fused into carbon the core is
able to ready 600 million degrees.
• At 600 million degrees Carbon fuses with Carbon to form
an array of heavier elements.
• At a billion degrees Oxygen can fuse with Oxygen.
• 2.7 billion degrees to fuse Silicon.
• In a short period of time (a thousand years) you go from
finishing the Helium burning to creating heavier and
heavier elements.
• Where will it end?
Iron
• The end is Iron.
Once the core reaches Iron
• Well actually it doesn’t reach Iron, the book has
mislead you.
• For the pressures in the core the “Iron” is
actually Nickel.
• Anyhow, once you reach that point you can go
no further.
• Since it takes energy to go higher, you are stuck.
• Stars are like businesses – if they don’t produce
energy (money) – they don’t survive!
So…
• The core collapses.
• This time electrons won’t be able to save it.
They don’t produce enough pressure to win out
over gravity.
• So, the atoms themselves collapse together.
• The core basically becomes one giant atom (and
the electrons fuse with the protons).
• The energy to do this (remember it takes energy
to break down atoms if they are smaller than
iron) comes from the gravitational collapse.
Also,
• Neutrinos are formed which fly outward.
• Since they have little mass and no charge they are not
affected much by matter.
• Once the core reaches the density of matter (400 trillion
times the density of water) the collapse slows.
• The density is now so high that neutrons try to take up
the same space as other neutrons, which is not allowed
to happen.
• This causes a neutron degeneracy pressure (the
neutrons hold up the star).
• The core has become a neutron star!
Meanwhile
• Just outside the core, this causes a rebound to occur
(sort of like a pile up of cars on the freeway when
someone slams on their brakes).
• This causes a reversal and some material now flies
outward.
• The rest of the star is collapsing inward at 15% of the
speed of light (but the star is so big that its radius is
several light minutes).
• The now out flowing matter hits the inward falling layers
and both now move outward.
• A shockwave is produced which moves outward taking
all of the star with it.
SUPERNOVA!
• Once this reaches the surface there is nothing to
stop it, and all of the star except for the neutron
star at the core flies into space at a fraction of
the speed of light.
• This is a SUPERNOVA!!!
• This process only takes a few seconds.
• The materials from the star now shine very
brightly (they are extremely hot and effectively
over a large area) – up to a million times brighter
than the star it leaves.
Supernova
• So, the star can actually outshine the
galaxy for a few days!
• They are bright enough to be seen in the
DAY if it occurs in our galaxy.
• At first you are seeing
the hot gas radiate.
• Eventually a decay of
Nickel to Iron takes over.
With
time
• The gasses
from the star
expand into a
supernova
remnant.
• This allows the
materials from
the star to be
dispersed
throughout the
local area.
Crab Nebula
Why are supernovas important?
• We are made of a lot of Carbon, Nitrogen,
and Oxygen.
• All of the C, N, and O in the universe was
made in the cores of stars.
• Stars making white dwarves hang onto
their metals.
• So, supernovas have given us the C, N,
and O we have today.
But that isn’t all!
• There is more.
• Fusion still occurs during the time of the
supernova.
• The core collapse produces a lot of
neutrons.
• How difficult is it to fuse a neutron with any
other atom (hint think about why it was
difficult to fuse 2 protons together)?
Fusion, neutron style
• With no charge, there is no repulsion
barrier to leap over!
• So, the fusion is easy.
• However you only get 6 min to do it
otherwise the neutrons convert back to
protons!
• So, you add neutrons.
2 ways to do this
• There are 2 ways to do this – slow and fast.
• Slow: you add 1 neutron at a time and wait. If
you add too many neutrons to an atom 1
neutron will turn into a proton and you suddenly
have a different atom.
• Fast: you add them all at once. Then some
neutrons convert to protons.
• These methods create different atoms and/or
isotopes.
• However, this type of supernova does not
produce all of the heaviest elements in the
abundances we have on the earth (so there is
something more – to be discovered later in the
course).
> 25 solar mass stars
• In this case the mass of the core exceeds
the limit at which even neutrons can hold
themselves up (which is about 1.4 times
the mass of the sun).
• In this case the core
does not hold up.
• It collapses even further!
• What stops it this time?
NOTHING!
• Nothing stops the collapse.
• The entire core collapses into a single
point.
• This creates a BLACK HOLE!
• The rest of the star – similar to before – is
blasted outward in a supernova event.
Conclusion
• The fate of a star is locked to its mass.
• Stars like the sun become white dwarves (and
do not supernova) while ejecting a planetary
nebula.
• Stars > 8 solar masses all supernova and
become either neutron stars (8-25 solar masses)
or black holes (> 25 solar masses).
• White dwarves are held up by electrons while
neutron stars are held up by neutrons.
• This death takes 10% of the time the star spent
on the main sequence.