Download To understand the deaths of stars and how it

Goal: To understand the births
and deaths of stars and how they
depend on mass.
Objectives:
1) To lean about the births of stars
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
In the beginning
• All you have is a large cloud of dust and
gas.
• This cloud is very large and very cold.
• They are called Giant Molecular Clouds.
• Somehow (more on
this later on in the
lecture) the cloud
collapses.
Then…
• A small part of the gas cloud collapses to
form the starting solar nebula.
The initial cloud
• Is made of mostly
Hydrogen (~90% by weight).
• Most of the rest is Helium (9%)
• The cloud has some spin. What will that
do?
Spin city
• The small amount of spin
acts like a merry-go-round.
• Much like on a merry-go-round, this
spinning motion pushes things outward.
• However, nothing stops the collapse in the
vertical direction, so the cloud collapses to
a disk.
• The gas in the disk is literally in orbit
around the center of the gas cloud.
And then, something special happens
• In the core of this bit of cloud, there are a
lot of particles falling into the center
(everything not lucky enough to start
orbiting).
• This creates heat from the infalling
materials.
• At some point the central object starts to
radiate this newly acquired heat and
becomes a protostar.
Multiple star systems
• Are very common. At least 50% of all star
systems are multiple star systems (2 or
more stars).
• So, our sun is actually the exception to the
rule.
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.
Hydrostatic Equilibrium
• Currently the core of our sun is in balance
• Gravity tries to collapse it
• Gas pressure and radiation pressure push
outwards
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.
• 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!
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?
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.
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!
• The sun undergoes a very rapid change
here.
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.
Butterfly Nebula (planetary)
Ant Nebula
Ring Nebula (4k light years away)
Eskimo Nebula
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 8-20 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
• 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.
Crab Nebula
> 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.