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.

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

Red Dwarfs• For the stars that we look at today, Red Dwarfs are the

smallest (Brown Dwarfs are the only stars smaller, but they don’t really die so we leave them out).

• What does this tell you about the lifetime of the largest Red Dwarves (~0.4 times the mass of the sun)?

• B) They are about 100 billion years• Smaller stars live longer • (by at least a factor of • their mass squared).• Also, since the universe

is only 13 billion years old, that means the Red Dwarfs won’t be dieing anytime soon.

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.