“I Always Wanted To Be Somebody. I Should Have Been More .

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“I always wanted to besomebody. I should havebeen more specific.”Lily Tomlin

HW3 and Quizzes 10 & 11 due Tuesday.Test 2 will be on blackboard Monday &Tuesday, April 18-19.Sample test will be posted next week.

Evolution so far:Protostars: Energy fromgravity (shrinking)Main Sequence: Energyfrom fusion converting Hto He in their coresRed giants: Energy fromgravity (shrinking core).He core shrinks, H shellexpands.Horizontal branch: Fusionof He - CAGB: Energy from gravity8MSun

Stage 2: During the main sequence, stars convert H intoHe in their cores. We have relations for how bright theyare and how long they spend on the main sequence

Gravity is again thesource of heat forpressure againstcollapse.During the Red Giant phase, the core is shrinking (andgetting hotter) and the shell surrounding the core is expandingin reaction to the hotter core.

Giant stars are really giant!Our Sun on this scale is just a dot.

When our Sun becomesa red giant star, it willexpand out to aroundVenus’ orbit.The Earth will still behere, but it will be veryhot.Image credit: sci-news.com

AGB stars are very similar to Red Giants, just bigger(and with a He burning shell).

The planetary nebula.

The shock waves sent out by the spasmodic He burning shellincreases the size of the atmosphere, until it is no longerreally connected to the core.

EnrichmentDuring He burning, spare neutrons react withother elements in the star to build up "heavier"elements (like Sr, Ba, and Pb)During the planetary nebula phase, theseelements (along with the H and He) are put backinto space for future generations of stars to use.

Stars 8MSun end upas white dwarfs.

Once the envelope is expelled back into space, all that is leftis the core: now called a White DwarfWhite dwarfs are stars that are doing nothing but cooling(and shrinking a little bit)It is the end state of 98% ofall stars (including our Sun)

White dwarfs White dwarfs are supported by electrondegeneracy pressure: their electrons are pushed sotightly together, that they can't get any closertogether or they will merge with protons in thenuclei. More massive white dwarfs are smaller.The weight on top pushes the atoms closer together,making the star smaller.

Just like trying to push the same end of 2magnets together, the closer they get, theharder it is to push.

White dwarfs White Dwarfs are about the size of theEarth. But with 60% the mass of our Sun

White dwarfs Density: about one million g/cc!One teaspoon of white dwarf hasas much matter as an entirebaseball team!

Evolution so far:Protostars: energy from gravityMain Sequence: energy fromfusion converting H to He in theircoresRed giants: energy from gravity8MSunHorizontal branch: fusion ofHe to CAGB: energy from gravityPlanetary nebula: energy fromgravity and spasmodic shellHe fusion (and shell H fusion).White dwarf:

Evolution of starswith less than 8 solarmasses.(98% of all stars)GravityFusion H to HeGravityFusion He to CGravityGravityElectrondegeneracypressure.Stars 8MSun end up aswhite dwarfs.

Thecompletepicture onthe HRdiagram.Be sure youcan do this.AND thatyouunderstandwhy starsevolve.

So what?Any star 8 solar masses willbecome a white dwarf. Theymight be near 1.4 solarmasses, but always below it.So why does theChandrasekhar limit have anymeaning?

If you have ablue and yellowstar on themain sequence,you know thatthe blue star ishotter than theyellow star.Many stars arein binary ormultiple star

If you have a blue andyellow star on the mainsequence, you know thatthe blue star is hotterthan the yellow star.That meansthe blue staris moremassive thanthe yellowstar.

If you have a blue andyellow star on the mainsequence, you know thatthe blue star is hotterthan the yellow star.That meansthe blue staris moremassive thanthe yellowstar.More massivestars evolvefaster.

If you have a blue andyellow star on the mainsequence, you know thatthe blue star is hotterthan the yellow star.That means the blue staris more massive than theyellow star.More massive stars evolvefaster.So the blue starevolves and becomes awhite dwarf while theyellow star is still on

If you have a blue andyellow star on the mainsequence, you know thatthe blue star is hotterthan the yellow star.That means the blue staris more massive than theyellow star.More massive stars evolvefaster. So the blue starevolves and becomes awhite dwarf while theyellow star is still on themain sequence.Then the yellow starexpands into a redgiant.

If you have a blue andyellow star on the mainsequence, you know thatthe blue star is hotterthan the yellow star.That means the blue staris more massive than theyellow star.More massive stars evolvefaster. So the blue starevolves and becomes awhite dwarf while theyellow star is still on themain sequence.Then the yellow starexpands into a red giant.Now the white dwarf’sgravity can take materialfrom the companion.

White dwarfs that are in binaries can actuallytake mass from their companions.The white dwarf can then exceed theChandrasekhar limit

So what happens if a white dwarf exceeds theChandrasekhar limit?

It Explodes!

They Explode!When they exceed the Chandrasehkar limit, theycollapse. This causes them to heat up.Then their degenerate carbon cores beginrunaway C fusion.This happens so drastically that they becomesupernovas (exploding stars).

Our SunOur Sun is not in a binary, so it will not explode. Itwill become a normal white dwarf which will cool,roughly forever.As the core cools, it will crystallize.So what?

Our SunOur Sun is not in a binary, so it will not explode. Itwill become a normal white dwarf which will cool,roughly forever.As the core cools, it will crystallize.So what?What is the core of our Sun made of at that point?What is the special name for the crystallized formof that element?

For the rest of space.why are the AGB and planetary nebula phases soimportant for the rest of space?

For the rest of space.why are the AGB and planetary nebula phases soimportant for the rest of space?Chemical EnrichmentOur solar system2% metalsPlanetary nebulareturns 3% metalsH and He are only slightly reduced.

For the rest of space.why are the AGB and planetary nebula phases soimportant for the rest of space?Chemical EnrichmentOur solar system2% metalsPlanetary nebulareturns 3% metalsWhat about before our Sun?

For the rest of space.why are the AGB and planetary nebula phases soimportant for the rest of space?Chemical EnrichmentOur solar system2% metalsPlanetary nebulareturns 3% metalsBefore our Sun?Previous star:1% metalsPlanetary nebulareturns 2% metalsAnd before that?

For the rest of space.why are the AGB and planetary nebula phases soimportant for the rest of space?Chemical EnrichmentOur solar system2% metalsPlanetary nebulareturns 3% metalsBefore our Sun?Previous star:1% metalsPlanetary nebulareturns 2% metalsAnd before that?Previous star:0.5% metalsPlanetary nebulareturns 1% metals

If we just keep going back to previous generationsof stars, what happens to the ‘metals’?Our solar system2% metalsPlanetary nebulareturns 3% metalsBefore our Sun?Previous star:1% metalsPlanetary nebulareturns 2% metalsAnd before that?Previous star:0.5% metalsPlanetary nebulareturns 1% metals

EnrichmentTakeaway: low-mass stars can make elementsup to Pb and this is recycled into the galaxyduring the planetary nebula phase.

Evolution so far:Protostars: energy from gravityMain Sequence: energy fromfusion converting H to He in theircoresRed giants: energy from gravityHorizontal branch: fusion ofHe to CAGB: energy from gravityPlanetary nebula: energy fromgravity and spasmodic shellHe fusion (and shell H fusion).White dwarf: electrondegeneracy pressure

Thecompletepicture onthe HRdiagram for98% of allstars (likeour Sun).

The other sideStage 3b: Supergiants.Stars on the main sequence withmore than 8 solar masses willbecome supergiants.

SupergiantsThe cores of more massive stars are already hotter.As they have more mass, they get more energy fromgravity without having to change their size much.

SupergiantsThe cores of more massive stars are already hotter.As they have more mass, they get more energy fromgravity without having to change their size much.Supergiants are able to begin converting He to C/Overy soon after exhausting H in their core.

SupergiantsThe cores of more massive stars are already hotter.As they have more mass, they get more energy fromgravity without having to change their size much.Supergiants are able to begin converting He to C/Overy soon after exhausting H in their core.But their cores are hotter than on the main sequence,so the envelope expands.

SupergiantsSupergiants are able to begin converting He to C/O verysoon after exhausting H in their core.When that's depleted, they convert C to O, Ne, Na and MgWhen that's depleted, they convert O to Mg, S, P, and SiThen Si to Co, Fe, and NiBetween each nuclear burning stage, the shell expands andthe core contracts, heating up before it can burn the nextfuel.

Late structure of asupergiant:Like an onion.

Fusion TimescalesFor a 25 solar mass star:1) H fusion (main sequence) lifetime: 10 million years2) He fusion can last 1 million years.2) Carbon fusion can last 1000 years3) Neon fusion can last 3 years!4) Oxygen fusion can last 4 months!!5) Silicon fusion can last for 5 days!!!

So what happens when you've builtup an Iron core?What can Iron do to support itself?

NOTHING!

Stage 4b: SupernovaThe Iron core cannotsupport itself and thestarimplodes/rebounds.

The Crab Nebula: A remnant from a supernova in1054.

Observed by Brahein 1572

Kepler's supernovaremnant from 1604.

Iron FusionIt takes energy to fuse Iron. So when Iron gets toohot and compressed, rather than providing energyto support the star, it begins fusion and takesenergy away from the star.The core collapses in less than 1 second!

Iron FusionThe core collapses in less than 1 second!When it becomes too compressed, protons andelectrons combine to become neutrons.However, neutrons do not want to combine, so theycan support the core (at least for a short time) andthe core rebounds- sending the shell exploding outinto space.

In 1987, a supernova went off in one of ourneighboring galaxies. This is the closest supernovasince the invention of the telescope.

Stellar recyclingSupernova send many solar masses of materialback out into space, for future generations ofstars and planets to use.Supernova can create any element as atomsare smashing together at billions of degrees K.

A really interesting (model) binary

But what's left after the supernova ofa massive star?

M 8M 8M 25 M

But what's left after the supernova ofa massive star? A Neutron Star: Main sequence mass up to 25solar masses. A Black Hole: Main sequence mass greater than25 solar masses, there is no stopping the collapse. Itwill become a black hole.

End States of StarsFor main sequence stars with more than 8, but less than25 solar masses: They end up as Neutron Stars. 10 to 30km across. Neutron stars have an average mass of 1.4 solar masses. Neutron stars cannot get larger than about 2.5 solar masses.

The structure ofneutron stars.A sugar lump ofthis matter onEarth wouldweigh 400 billiontons.

How do you detect something 20kmacross?LGM

How do you detect something 20kmacross?Pulsars A special kind of neutron star that "beams" radiowaves in our direction. Spin (on average) once per second. No pulsars spin slower than every 5 seconds Strong magnetic fields cause the "beam"

Why do pulsars spin so fast?

Why don't pulsates 'pulse' longer thanevery 5 seconds?

Neutron stars can exist in pairs.

They havebeen used to(successfully) test thetheory ofgeneralrelativity.

Our solar system 2% metals Planetary nebula returns 3% metals Before our Sun? Previous star: 1% metals Planetary nebula returns 2% metals And before that? Previous star: 0.5% metals . the core contracts, heating up before it can burn the next fuel. Late str

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