Massive Things: Galaxies, Stars and Black Holes Topic 2: Stars Big Bang, Black Holes, No Math 1 Big...

Massive Things: Galaxies, Stars and Black Holes

Topic 2: Stars

Big Bang, Black Holes, No Math

Big Bang, Black Holes, No MathASTR/PHYS 109

Dr. David TobackLecture 22 & 23

Topic 2: Stars

Was due Today – L23• Reading:

– (Unit 5)• Pre-Lecture Reading Questions:

– Unit 5 Revision (if desired): Stage 1• End-of-Chapter Quizzes:

– Chapter 15• Papers:

– Paper 3 Revision (if desired): Stage 1 due Wednesday

• Honors Papers:– (Stage 2)– Final paper due on the last day of class

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Unit 5: Big Objects

1. Galaxies2. Star Birth and Death3. More on Black HolesIt turns out that the way

Galaxies and Stars form is very similar… start there

The way stars “die” depends on the star itself… sometimes they die to form a Black Hole

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Important Buzz Words

Wanted to make sure we explained some of the important buzz words in astronomy:–Red Giant–Supernova–White Dwarf–Neutron Star–Black Hole

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Today’s Lecture

•A star is born•Nuclear reactions and gravity

keep stars alive and make them shine

•The life of stars: shining and converting hydrogen into heavier elements

•Life and death of stars like our Sun

•Life and death of massive stars5

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Where are we now in the history?

A few hundred million years after the bang, stars start forming

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Quick Summary of Galaxy Formation

•Half a million years after the Big Bang we have lots of neutral stuff floating around in space•Half a billion years after the bang the stuff has clumped into galaxies

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Stellar Clumps• The galaxies start as giant

spinning areas of neutral, massive stuff kept together by gravity

• Eventually, local areas that orbit around the center of the galaxy are close enough to each other that they also start clumping due to gravity

• Eventually, the hydrogen and helium atoms can start to interact with each other

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Star Formation

•This is some text..

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Step 2

•More…

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Step 3

•This is some text…

Beginnings of

planets!

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Will be the star

Stars have some things in common with Spiral

GalaxiesWill be the

planets

Gee… kinda looks like Saturn also…

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Describing StarsWe tend to use metaphors to

describe Stars1. Can think of them like

people: They have a birth, a life and a death

2. Can think of them like a car: They run on “fuel,” and “die” when they run out of fuel

3. Can think of them like a balloon: A giant collection of atoms that are “forced” to stay inside the balloon walls

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Proto-stars

•Gravity brings together the stuff in a galaxy that has mass •When the atoms (mostly hydrogen and a little helium) get close enough they start moving quickly into the center•However, need to get really close before they will interact

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Low energy (temperature) hydrogen reactions

Proton

Electro-magnitism Reaction

Same charges repelProton=Hydrogen

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A Star is Born•When there are enough atoms, the large amount of mass makes the pull to the center so strong that the hydrogen starts moving quickly into the center become higher temperature

•When atoms reach a high enough temperature (10 Million Kelvin) they start to have nuclear interactionsCreates the light we see (makes stars shine)

Atoms inside the Sun don’t move in the same way the Earth orbits outside it

Call this a star

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Proton + Proton Deuterium + Electron + Neutrino

Hydrogen Reactions at high Temperatures

Proton

DeuteriumNuclear Reaction

Anti-Electron

Neutrino

Proton=Hydrogen

This is what we call FusionOut-going particles get LOTS of energy

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The Life and Death of Stars

A star’s life is effectively a battle between:

1.Gravity trying to crush everything into a tiny point

2.The nuclear interactions “opposing” the gravity

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Only in the center of Stars

•The center of a star is called the core •This is where the particles have the highest energy and density This is where all the fusion occurs 19

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Where does the energy come from?

•Mass of Deuterium is smaller than the mass of two protons

•So what? E=mc2, so the “mass energy” gets converted into kinetic energy in the collision

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Proton + Proton Deuterium + Electron + Neutrino

Hydrogen Reactions

Proton

DeuteriumNuclear Reaction

Anti-Electron

Neutrino

Proton=Hydrogen

Fusion produces particles with LOTS of energy

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Proton + Deuterium 3He + PhotonAgain, lots of energy to the particles

Hydrogen and DeuteriumDeuteriu

Proton

Nuclear Reaction

Photon

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3He + 3He 4He + 2 HydrogensMore energy is released

Creating Stable Helium

Nuclear Reaction

Hydrogen

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Nuclear “Burning”•In each nuclear reaction additional energy is released

Some of the energy “turns into” photons (and neutrinos)–This is what makes stars shine

Some energy goes into the increased speed of the atoms–This is what keeps the star from crushing itself

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Thinking of a Star as a Balloon

•The hydrogen and helium are the gas inside the balloon, the fusion speeds them up so they “try” to leave

•The gravity is what keeps it all together, like the walls of the balloon

Hydrogen and Helium gas

Gravity holds it togethe

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Small speed objects can’t leave the Earth

Look at the different things that can happen at various speeds on various size “heavenly bodies”

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Another example with the same speed bullet but on the Moon

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If we move to an asteroid, then it CAN leave

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Can build things that shoot fast enough to leave the

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Why doesn’t the Balloon deflate or pop?

If an atom has a high speed or there is only small gravity (star is small mass) then the atom will leave the star

SmallGravity

Hydrogen and Helium

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HydrogenAnd Helium

Why doesn’t the Balloon deflate or pop?

• If gravity is strong (large mass star), or the speed is small then gravity drags the atoms back to the center–Acts “like” the walls

of a balloon–The more massive the

star the stronger the walls

–Atoms don’t leave

LargeGravity

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A Stable Star• The nuclear reactions

speed up the atoms– “Try to make the

balloon pop”–Create light

• Gravity pulls it back together

• The size of the balloon depends on this balance– Can stay stable for

billions of years

NuclearReactions

Gravity

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The Fuel of a Star

The Hydrogen and Helium provide the “fuel” that both:

1. Creates the light we see 2. Keeps the star stableConverts light atoms into

heavy atoms

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What happens when it runs out of fuel? 34

The Life of a Star

The star starts as a “ball” of mostly hydrogen, with the fusion in the core

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What happens when the Hydrogen Runs out?

•Without hydrogen fuel to make things “expand,” gravity crushes atoms closer and closer together– It takes a temperature of 100 million Kelvin to fuse Helium, this may not happen for many stars

•From there what happens next depends on the “mass” of the star

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Start with Stars like our Sun

• Mid-sized stars (between 8% and 8 times the mass of our Sun) all typically have a similar life

• Smaller than 8% of the Sun won’t get hot enough to fuse hydrogen– Isn’t really a star at all–Call this a brown dwarf

• More than 8 times the mass of our Sun complicated things can happen

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The Life and Death of our Sun

• Sun is using its hydrogen to create light• Helium produced falls to the

center (the core)• Today: Core only about 15

million Kelvin not hot enough to convert helium into Carbon

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Future: White Dwarf

When most of the hydrogen is used up, the core gets crushed

Eventually the atoms get REALLY close to each other and the strength of the repulsion between the electrons from quantum mechanics is so big that it balances out the gravity

Outer part becomes a Red Giant and then diffuses into space– Inner part stabilizes

•Call this a “White Dwarf”

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Lots of White Dwarfs in the Universe

•Hot core creates heavy elements

•Can shine for quite awhile–This is why we call it “white”

•Really dense: An object with the mass of the Sun shrunk to about the size of the Earth–This is why we call it a “dwarf”

•How dense? A pair of standard dice would weigh about 5 tons

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White Dwarf Black Dwarf

•Eventually a white dwarf star runs out of the rest of its fuel (nothing left but iron) and it stops emitting light

•Call this a “Black Dwarf” •However, predictions are that it takes so long for this to happen that none should exist… none observed

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Next move to heavier stars…

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The Life of a Heavy Star• For heavy stars the first part of their life

is the same as for lighter stars: after the hydrogen runs out, gravity crushes the core

• What is different is that for heavy stars there is so much mass that gravity can continue to crush the core, and the temperature can rise significantly

• For stars with more than 8 times the mass of the Sun, the core can reach a temperature of 100 million Kelvin, and helium fusion can start

• Helium can fuse to make heavier elements

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Stages in a Massive Star’s Life

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4He4 + 4He4 8Beryllium

Converting Helium to Beryllium

8Beryllium

Nuclear Reaction

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8Beryllium + 4He 12Carbon + Photon

Converting Beryllium to Carbon

8Beryllium

12CarbonNuclear Reaction

Photon

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The Most Massive Stars

•For the heaviest stars, after the helium is used up can start fusing Carbon•Then Neon•Keep going…Get “shells” of the various types of atoms•This is where the atoms WE/Earth get created

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How Long do Stars Live?

Weird: stars with huge mass die faster than lighter stars. Why?

•The more massive the star, the more it crushes atoms in the center and raises their temperature

•The hotter/denser the star, the faster the nuclear reactions occur The sooner it burns out–Some stars can last as little as a hundred million years

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Death of Very Massive Stars

• If the star is much more massive than our Sun it runs out of fuel quickly– 1x Sun ~10 billion years– 10x Sun ~30 million years– 100x Sun ~100,000 years

•Different things happen as it runs out of fuel gravity is so strong it can REALLY crush the star

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More Crushing Neutron Star

• After the fuel runs out, if the mass of the star is large enough, gravity crushes the atoms into each other

• The electrons are pushed so close to the protons that they start to interact–Turn into Neutrons (more on the

physics of this interaction in Chapter 19)

• The star turns into a giant ball of neutrons about the size of Manhattan

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Proton + Electron Neutron + Neutrino

Creating Neutron Stars

Proton Neutron

Electron

Neutrino

(leaves the

Up Quark

Down Quark

Up Quark Down Quark

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Neutron Stars• Neutrons don’t like to be “too

close” to each other because of Quantum Mechanics star can stabilize

• In this process the star has collapsed from the size of the Earth into a ball of neutrons just a few kilometers across

• Incredibly dense!–Billions of tons per cubic inch –A marble made from neutron star

material would weigh the same as the Earth

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The crushing…

•The inner part of the star is quickly crushed into a neutron star

•What happens to the outer part? Falls towards the center!

•Hits the dense neutron core and bounces back into space as a giant explosion

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Huge Explosion = Supernova

• The explosion can be so big that it can shine as bright as 10 billion suns for a couple of weeks

• The temperatures in the explosion are so high and the atoms are so densely packed that really heavy atoms can be created and then blown into space–This is how we get stuff like

uranium on the Earth• We call the “remnants” of

supernovas “nebulae”

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Moving Towards Black Holes

•If the remaining neutron star has a “critical mass” (about 3MSun) it can continue to collapse

•Nothing left to oppose the crush of gravity! Continues to collapse until it becomes a Black Hole

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For Next Time – L22• Reading:

– Chapter 16 if we finished Chapter 16 (else just Chapter 15)

• Papers:– Paper 3 Revision (if desired): Stage 1 due

Wednesday• Honors Papers:

– (Stage 2)– Final paper due on the last day of class

Topic 2: Stars

For Next Time – L23• Reading:

– Chapter 16 if we finished Chapter 16 (else just Chapter 15)

• Papers:– Paper 3 Revision (if desired): Stage 1 due

Wednesday• Honors Papers:

– (Stage 2)– Final paper due on the last day of class

Topic 2: Stars

Full set of Readings So Far

•Required:–BBBHNM: Chaps. 1-17

•Recommended:–TFTM: Chaps. 1-5–BHOT: Chaps. 1-7, 8 (68-85), 9 and 11 (117-122)–SHU: Chaps. 1-3, 4(77-93), 5(95-114), 6, 7 (up-to-page 159) –TOE: Chaps. 1-3

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Clicker Question

Why do stars shine? a)When atoms fall back to their

lowest energy level, they give off light

b)Gravity is pushing atoms to fuse

c)Nuclear reactionsd)Heavy nuclei

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Clicker QuestionWhat is the main reason the atoms in a

star don't just all fall to the center of the star?

a)Because of the surface of the star keeps them from falling to the center the same way the surface of the Earth keeps things from falling to the center

b)Because all the other atoms are in the way

c)Because of the pull of gravityd)None of the above

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Clicker Question

What are the two main interactions in stars and how do they relate to each other?

1.Gravity and nuclear fusion2.Gravity and degeneracy pressure

3.Nuclear fusion and degeneracy pressure

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The Life of a Star Continued…

As the hydrogen gets used up the core “ball” becomes a smaller ball of mostly helium

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The “Burning” Process

•At the beginning of a stars life, most of the atoms are hydrogen they get converted to helium first

•Then the helium starts getting converted into heavier elements like carbon, oxygen and iron Produces many of the heavy elements that will go into the universe after it dies

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Death of Stars

When the hydrogen “fuel” runs out different things happen for very

massive stars and small mass stars

separately

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Next look at the nuclear reactions that keep the star

from getting “crushed” by

gravity

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Prep for Next time – L23•Reading:–Chapter 18

•Pre-Lecture Reading Questions:–Chapter 18

•End-of-Chapter Quizzes:– If we finished Chapter 16 then end-of-chapter quiz 16 (else just through Chapter 15)

•Papers:–Paper 2 Revision: Was due before class today. Let us know if you were mis-graded

–Paper 3: Calibrations, reviews and Self-assessment due Wednesday before class

Topic 2: Stars

Was due Today – L24•Reading:

– Chapter 18•Pre-Lecture Reading Questions:

– Chapter 18•End-of-Chapter Quizzes:

– Chapter 15 was due, but I’ll give extensions since Chapter 14 was busted.

– You need to go back and check Chapter 14. Many of you need to go back and fix the wrong answers you were accidentally getting credit for

•Papers:– Paper 3: Was due today. One of the

calibrations was busted. I’ll fix that soon. After I do that, I’ll open up a chance for Revisions

Topic 2: Stars

Prep for Next time – L24•Reading:

– Chapter 18•Pre-Lecture Reading Questions:

– Chapter 18•End-of-Chapter Quizzes:

– If we finished Chapter 16 then end-of-chapter quiz 16 (else just through Chapter 15)

– You need to go back and check Chapter 14. Many of you need to go back and fix the wrong answers you were accidentally getting credit for

•Papers:– Paper 3: Was due today. One of the

calibrations was busted. I’ll fix that soon. After I do that, I’ll open up a chance for Revisions

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Prep For Next Time – L10•Note: May change depending on how far we

get in lecture•Reading:–BBBHNM: All reading through Chapter 18

•Reading Questions:–All reading questions through 18

•eLearning Quizzes:–If we finished Chapter 16 then end-of-chapter quiz 16 (else just through Chapter 15)

•Papers:–Paper 3 revisions text (if desired) due before class on Monday–Paper 2 revisions (if any) calibration, reviews and self-assessment due before class on Monday

Topic 2: Stars

Was due today– L11•Reading:–BBBHNM: All reading through Chapter 18

•eLearning Quizzes:–Through Chapter 16

•Papers:–Text of Paper 3 revisions (if desired) were due before class–Paper 2 revisions (if any): calibration, reviews and self-assessment due before class

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Prep for Today (Is now due) – L22

• Reading:– BBBHNM 17

• Reading questions:– Chapter 17

• End-of-Chapter Quizzes:– Chapter 15

• Paper Stuff– Paper 2 Revision: Calibrations and

Review were due before class– Paper 3: Calibrations and Review

were due before class

Topic 2: Stars

Prep For Next Time – L11•Note: May change depending on how far we

get in lecture•Reading:–BBBHNM: All reading through Chapter 19

•eLearning Quizzes:–If we finished Chapter 16 then end-of-chapter quiz 16 (else just through Chapter 15)

•Papers:–Rest of Paper 3 assignment (if you resubmitted) due before class on Tuesday

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Prep for Next Time – P21

• Reading– BBBHNM 17

• Reading questions– Chapter 17

• End-of-Chapter Quizzes– If we finished Chapter 16 then end-of-

chapter quiz 16 (else just 15) • Papers

– Paper 2 Revision: Calibrations and Review were due before class

– Paper 3: Calibrations and Review were due before class

– Paper 4: Will assign paper 4 next time

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Prep for Next Time – P22

• Reading– BBBHNM 17

• Reading questions– Chapter 17

• End-of-Chapter Quizzes– If we finished Chapter 16 then end-of-

chapter quiz 16 (else just 15) • Papers

– Paper 2 Revision: Calibrations and Review were due before class

– Paper 3: Calibrations and Review were due before class

– Paper 4: Will assign paper 4 next time

Topic 2: Stars

Full set of Readings So Far

•Required:–BBBHNM: Chaps. 1-17

•Recommended:–TFTM: Chaps. 1-5–BHOT: Chaps. 1-7, 8 (68-85), 9 and 11 (117-122)–SHU: Chaps. 1-3, 4(77-93), 5(95-114), 6, 7 (up-to-page 159) –TOE: Chaps. 1-3

Topic 2: Stars

Future: The Red Giant Phase

When most of the hydrogen is used up, the core gets crushed

Helium falling towards the center speeds up and interacts

Heats up the hydrogen in the outer shell

Inflates to giant red shell that emits red lightBigger than the size of the Earth

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White Dwarf

Eventually the atoms get REALLY close to each other and the strength of the repulsion between the electrons from quantum mechanics is so big that it balances out the gravity–Outer part is blown away•Planetary Nebula

– Inner part stabilizes •Call this a “White Dwarf”

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Shouldn’t a Star just Collapse?

•Why don’t the atoms just fall to the center of the star?•Do they orbit?•If the star is giving off light aren’t the atoms losing energy, so they should fall in

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Today’s Lecture

•Galaxies Form•Stars Form…•Stars Die… and die in different ways

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The life and death of our Sun

Our Sun will go through a Red Giant

phase in about 5 billion years

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Our Sun Today

•Sun is using its hydrogen to create light•Helium produced falls to the center (the core)–Core is not hot enough to convert helium into Carbon

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For Next TimeTopics• Galaxy Formation Done• Stars Done• Black Holes NextReading for Next time:

BBBHNM: Chap 17Full reading for Unit 5:• BBBHNM: Chaps. 1-17• TFTM: Chaps. 1-5• BHOT: Chaps. 1-7, 8 (68-85), 9 and 11 (117-

122)• SHU: Chaps. 1-3, 4(77-93), 5(95-114), 6, 7 (up-

to-page 159) • TOE: Chaps. 1-3Lecture prep: Turn in on eLearning

Two questions from Chapter 17 you want to know the answer to

Topic 2: Stars

Shouldn’t a Star just Collapse?

•Atoms in stars give off lots of light as energy•If the atoms are losing energy, why don’t they just slow down and have everything fall to the center of the star?

Topic 2: Stars

A giant lump of atoms?

•Why doesn’t everything just fall to the center of the Earth?•The Earth is solid, but stars aren’t–Earth is made of iron and heavy atoms

Topic 2: Stars

The Reading So Far and For Unit 5

Reading for Unit 5:• BBBHNM: Chaps. 15-17• SHU: 4(87-93)• TOE: 3• BHOT: 8(76-85)Full reading through Unit 5:• BBBHNM: Chaps. 1-17• TFTM: Chaps. 1-5• BHOT: Chaps. 1-7, 8 (68-85), 9 and 11

(117-122)• SHU: Chaps. 1-3, 4(77-93), 5(95-114), 6,

7 (up-to-page 159) • TOE: Chaps. 1-3

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Other Stuff•Q-Drop date is tomorrow–No final–Only papers 1-5 (due last day of class)

•Elearning:–Unit 5 in progress

•Make sure you pick up the latest version of the online textbook!!!–Been updating it all semester

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Papers 1 and 2

•Paper 1 is done•Paper 2 grades are on eLearning –Working on getting the revised grades fixed–Paper 2 Revision is done

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Papers 3,4 and 5• Paper 3:

– Let us know if you think you have been mis-graded

– Revisions now open• Paper 4:

– Assigned after we start Chap 17– Probably today– Due 1 week after we finish Chap 17

• Paper 5 (last paper, no final)– Assigned after we start Chap 20

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Other Stuff

•Elearning:–Unit 5 in progress

•Make sure you pick up the latest version of the online textbook!!!–Been updating it all semester

Topic 2: Stars

Examples

•Our sun: Has another ~5 billion years before it goes through a Red Giant Phase •Others can last as little as a hundred million years

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The Life and Death of Stars

Black Hole

Big Star(Supergiant)

Small Star(Red Giant)

Brown Dwarf

White Dwarf Neutron Star

Slightly more complicated than this…

Explodes (Superno

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Writing Assignments

Short Assignments 1 & 2Re-do’s are still possible. Want to revise again? Talk to me

eLearning:Unit 4 now dueNeed to be working on Unit 5

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Papers AFTER Paper 2

Two options1.Two short papers like the first one

• One on Black Holes (due one week after we finish Black Holes)

• One on Dark Matter (due last day of class)

2.Research PaperIf you want this option Stage 1

must be approved ASAP• Was due last week

• Final paper due the last day of class, Tuesday Dec 8th

Topic 2: Stars

For Next Time

• More on the properties of Black Holes…

Full reading for Unit 5:• BBBHNM: Chaps. 1-17• TFTM: Chaps. 1-5• BHOT: Chaps. 1-7, 8 (68-85), 9

and 11 (117-122)• SHU: Chaps. 1-3, 4(77-93),

5(95-114), 6, 7 (up-to-page 159)

• TOE: Chaps. 1-3

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Why do we call it a black hole?Look at objects

this big

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Escape Velocity

•The escape velocity for the Earth is about 11.2 km/sec

•The escape velocity for the sun is about 620 km/sec

•The escape velocity of a neutron star is about half the speed of light!

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Why do we call it a Black Hole?

A Black Hole is so dense that it’s escape velocity is GREATER than the speed of light light can no longer escape!More on this soon…

Topic 2: Stars

Wait a Billion YearsAfter less than ~1 billion years, galaxies and the first stars start to form

Formation of the first stars

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Remaining Lectures

•Two lectures left: Today and Thursday–Stars and Black Holes–Won’t get back to Inflation and Dark Energy (Lectures posted)

•Recitation tomorrow as usual•Extra recitation at the Dixie

Chicken at 2:00 PM tomorrow (Wednesday)

Topic 2: Stars

Remaining Assignments

•Writing Assignments Due Thursday–However, you may turn it in, via email, on Monday

– If you want to revise your paper you’ll need to pick it up from me directly

•Don’t forget to finish your WebCT stuff

Topic 2: Stars

• Textbook?• BHOT 8 starting on page 76.• SHU 4, 5• SHU 8 for MACHOs (need this?), 9 Dark Matter (Cold or

hot?)• SHU 10 is Quasars…• SHU 11: In search of black holes• TOE 3, 4• TCP 17Reading:

– TOE: 3– BHOT: 8– SHU: 4, 7– TCP 16 and 17– Others???

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Summary of the Buzz Words in the Newspapers

•White Dwarf•Brown Dwarf•Neutron star•Supernova•Black Hole

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TCP 17 cont…

•Once the critical mass is reached nothing can hold back gravity and it collapses on itself and becomes a black hole.

•This stretches space time•Cool pictures on page 529

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TCP 17 cont…• How black holes are formed• A black hole originates in the collapse of the

iron core that forms just prior to the supernova of a very high mass star

• Any star born with more than about 8MSun dies in a supernova (previous lecture), but most of the star’s mass is blown into space by the explosion.

• As a result most of the collapsed cores left behind by most supernovae become neutron stars

• However, if the supernovae doesn’t blow away too much of the mass the remaining neutron star can have enough mass so that the neutron pressure cannot overcome gravity

Topic 2: Stars

•If the mass of the neutron star is so large that the gravity can overcome the neutron degeneracy that the core will

Topic 2: Stars

•This mass is about 3MSun

•Stars with 1½ times the mass of the sun eventually can’t support itself against its own gravity and will collapse into a black-hole

•Chandrasekhar limit

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•Its as if we have a giant nucleus of neutrons only it’s held together by gravity not the strong force

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•In some cases they may explode or manage to throw off enough matter to reduce their mass below the limit

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TCP 17 cont…

•(again p514)

Topic 2: Stars

Why is Iron Special? Need This

• For the elements lighter than iron: every time you create a new, heavier nucleus, you release energy

• For the elements heavier than iron: it takes energy to create them (don’t create light and doesn’t add to the heat of the star that keeps it from collapsing)

• Thus, after all the atoms have turned into iron you can’t any more heat from crushing atoms together to form heavier atoms

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TCP 17 cont…

• Aside on neutron stars:• As neutron stars collapse they

start spinning faster and faster (kinda like an ice skater pulling her arms in)

• Picture from 218, chapter 10?11?• It emits light along its axis (picture

on p527)• We see this beam of light as it

sweeps past us. Kinda like a light house

• Figure on p527• Give these the creative name of

pulsars

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TCP 17 cont…

•Actually we have found neutron stars that have planets orbiting them… kinda cool

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•Weird stuff: more massive than the sun but just about 10 Km across, essentially a giant nuclei but different–Almost entirely neutrons (no protons)–Gravity, not the strong force, holds it all together

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•If the mass of the star is one or two times the mass of the sun, the crush of gravity would be repelled by the exclusion principle between the neutrons and protons, rather than between the electrons

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TCP 17 cont…• In the iron core (or if there is electron degeneracy) not

really sure which, and the gravitational pressure continues then the electrons have enough energy such that that when they collide with protons they convert into a neutron and a neutrino

• P511, Fig 16.16 (something better)?) • The iron core with a mass comparable to our Sun (is this

the 1.4?) and once the electrons are gone, they don’t have the degeneracy pressure so gravity can start squeezing again making the electrons and protons more energetic so they convert into more neutrons until everything is just a just a bunch of neutrons

• The neutrons also have a degeneracy that opposes the collapsing of gravity and our giant ball of iron that used to be the size of the earth collapses into a ball of neutrons just a few kilometers across

• Its as if we have a giant nucleus

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•Picture 17.2 (page 521)

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•Pictures on p507 and 8 of TCP. Also 16.16

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The Birth of Stars

•Gravity condenses hydrogen and helium into small clumps

•Atoms get close enough that they start interacting with each other

•As the atoms collide with one another we start to get nuclear reactions–“A Star is Born”

Topic 2: Stars

•Sometimes when a very massive star collapses, the outer regions may get blown off in a tremendous explosion called a supernovae

•Results in a great cloud of hot shining dust blown out into space, spreading all the elements far and wide

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•Pressure•TCP: Page 440, Fig S4.10

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–Supernovae–White Dwarf–Neutron Star–Black hole…–Pulsars–Worm holes?–MACHOS?–Red Giants?

Topic 2: Stars

•Another issue: Things speed up as they fall towards the center of the star: Just like a rock falling towards the earth speeds up brings energy into the star!

Topic 2: Stars

•This is a test…

Topic 2: Stars

•This is some text

Topic 2: Stars

Neutron Stars

Topic 2: Stars

•Nova video

Topic 2: Stars

•This is some text

Topic 2: Stars

•Don’t want to say too much here about the exclusion principle, eventually gravity can overcome the quantum mechanical effect of particles needing to not be in the same position/velocity state

Topic 2: Stars

Heat and pressure• As the atoms are drawn towards the

center of the star they get faster (like a rock falling towards the surface of the earth)

• As atoms combine in nuclear reactions they release energy and the atoms get more energetic

• These higher energy atoms can try to escape the gravity of the star

• Can think of this like a gas: If the atoms are getting more energetic we say they are being heated

• As they atoms get more energetic (higher velocity) they can get further from the center of the star, again the same analogy as throwing a rock from the surface of the earth

Topic 2: Stars

• When the “gas” of energetic atoms gets heated its as if there is a “pressure” pushing the atoms away from the center like a balloon. If you heat up a balloon (or put more air in it) the atoms hit the walls of the balloon harder and we say it is under more pressure. What keeps the balloon together is the force that the plastic walls of the balloon exerts; what keeps the star together is the force of gravity pushing the atoms back towards the center of the star

• We say that gravity exerts a “pressure” on atoms in the star

• In a balloon, the inward pressure of the balloon walls balances the outward pressure of the gas. In a star gravity the pressure of gravity balances the outward pressure of the gas

Topic 2: Stars

•Probably want to finish the story about neutron stars and white dwarfs and such… That could also go into the next section…

•Next some SHU 4 cont stuff..

Topic 2: Stars

Stellar Cooking

•The hydrogen effectively gets converted into helium and releases lots of energy in the form of photons, electrons and neutrinos

•The heat released in this reaction, which is like a controlled hydrogen bomb explosion, is what makes a star shine

•Raises the temperature of the star

Topic 2: Stars

•Probably want that astronomy book that did the different types of stars…

Topic 2: Stars

•After this fuel is spent the center can collapse into a very dense state such as a black hole… Are we getting into the next section?

Topic 2: Stars

Big Picture

•The big picture about galaxy formation, stellar formation and a star’s life is the balance between the stuff that tries to compress it together and the stuff that keeps it from crushing itself into a tiny little thing.

Topic 2: Stars

Gravity

•Let’s use the earth as an example. Let’s say I have a rock near the surface of the earth. That rock will fall to the surface of the earth.

• Its been attracted•This is true for all objects near

the earth. They fall towards the center of the earth

Topic 2: Stars

Other?

• Ok… So all the things on the earth fall to the center of the earth… why doesn’t our rock fall to the center? Because it hits the ground.

• So what?… what about the ground keeps it from falling to the center? I.e., what force OPPOSES gravity and keeps it from falling further? Why doesn’t the stuff already at the surface of the earth fall more? Why is the SURFACE of the earth not falling towards the center?

Topic 2: Stars

Opposition• The reason it doesn’t fall towards the

center of the earth is that the “stuff” of the ground “pushes back”

• How does it push back? • The atoms of the stuff that make up the

earths surface are bound together (this is chemistry in some real sense)

• For the rock to “pass through” the surface would mean having to break the bonds that hold the atoms together

• These bonds can be VERY strong• The bonds in the atoms of paper are

much weaker than the bonds of steel

Topic 2: Stars

Summary• So… The surface of the earth is where it

is because gravity is pushing it towards the center, but the stuff below the surface is “pushing back” since the push of gravity isn’t strong enough to break the bonds of the stuff below it

• If the bonds weren’t strong (but did exist) like in water, if the rock hit water it would fall through the water until it hit something it couldn’t fall through.

• Bottom line: What’s important is that gravity causes things to compress and some other force keeps things from compressing

Topic 2: Stars

Galaxy Formation (BHOT 8 continued)

•Galaxy formation:•After the big bang we just had

lots of stuff floating around in space

•Can think of this as just being a giant tank of gas.

•After a few million years the electromagnetic attractions (electrons to nuclei) would start forming atoms (already said this)

Topic 2: Stars

• The gravitational attraction would eventually stop expansion in some regions and start them to collapse

• As they were collapsing, the gravitational pull of matter outside these regions might start them rotation slightly

• As the collapsed region got smaller, it would spin faster– just like ice skaters spin faster on ice as they draw in their arms.

• Eventually, when the region got small enough it would be spinning fast enough to balance the attraction of gravity to balance the attraction of gravity and in this way, we believe, disk-like rotating galaxies were born Others would form other times of galaxies like elliptical galaxies.

• In this manner the clouds coalesce into starts like our sun, burning hydrogen into helium and radiating the resulting energy as heat and light

Topic 2: Stars

Star Formation (TOE 3… BHOT 8 cont…)

• As time went on the hydrogen and helium gas in the galaxy would condense under their own gravity

• This would create proto-stars. Read: effectively a ball of gas in space, just concentrated in one area because of the gravitational attraction

• As these contracted (again by gravity), the atoms would collide with one another

• As things fell to the “center” their speed would increase (like a ball falling to the surface of the earth picks up speed) the temperature would increase until eventually it became hot enough (particles are energetic enough) to start nuclear fusion reactions (i.e., when they collide)

• Picture of a fusion reaction that converts hydrogen into helium and releases a lot of light (photons)

Topic 2: Stars

•These would convert the hydrogen to make more helium. The heat released in this reaction, which is like a controlled hydrogen bomb explosion, is what makes a star shine.

•Pictures on p507 and 8 of TCP. Also 16.16

Topic 2: Stars

Star Formation (BHOT 8 cont… TOE 3)

• This additional heat also increases the pressure of the gas until it is sufficient to balance the gravitational attraction and the gas stops contracting

• In this manner the clouds coalesce into starts like our sun, burning hydrogen into helium and radiating the resulting energy as heat and light

• This can stay stable for billions of years

Topic 2: Stars

BHOT 8 cont…

•Our star: Can last for another 5 billion years or so

•Others can last as little as a hundred million years

Topic 2: Stars

BHOT 8 cont…• What happens when the star starts to run out

of fuel?• Since there isn’t enough hydrogen around to

keep making helium at a fast enough rate, the heat can’t keep the gravity off any more and the star starts to contract (i.e., get smaller)

• This causes the atoms closer together and the star gets hotter again and we burn even the remaining hydrogen

• What happens when the hydrogen is all gone and all we have left is the helium produced from fusion?

Topic 2: Stars

• Instead of converting hydrogen to helium (because there isn’t much hydrogen left), the helium starts interacting (again chemistry) and gets converted into heavier elements like carbon and oxygen (this is where the carbon and oxygen in the universe comes from). Need a picture here

• After this fuel is spent the center can collapse into a very dense state (white dwarf, neutron star or a black hole)

Topic 2: Stars

TCP 17

•Do we have enough on stellar formation?

•The afterworld of stars: White dwarfs, neutron stars and black holes

Topic 2: Stars

• Paradoxically: the more fuel a star starts off with the sooner it runs out. This is because the more massive the star is the hotter it needs to be to balance its gravitational attraction. And the hotter the star the faster the nuclear reactions occur and the sooner it will use its fuel

• Important point: Because the strength of the force of gravity depends on the mass of the star (and so in effect on its size and weight) the star’s fate depends on its size

Topic 2: Stars

• Then the star life cycle starts and the hydrogen burns and turns into helium

• As the hydrogen gets used up gravitational pressure increases and the helium starts to fuse

• One by one the heavier elements are produced, each in turn fueling reactions to produce the heaviest element as gravity compresses the star into a denser and denser mass

Topic 2: Stars

TCP 17 cont…

•The battle between the “crush” of gravity and the outward pressure of the nuclear burning that governs a star’s life

•Here we talk about what happens as that fuel begins to run out

Topic 2: Stars

TCP 17 cont…

•Start with small stars (kinda did this already), like our sun

•After all the nuclear burning all we are left with is the products of the burning

•Typically this is carbon•Slightly bigger ones will be able to crate iron

Topic 2: Stars

•Since it happens in roughly the same way, they all look the same

•Since we measure the amount of light we can tell how far away it occurred

•This is pretty damn cool!

Topic 2: Stars

TCP 17 cont…

• Theoretical calculations show that a white dwarf cannot be larger, in mass, 1.4 Sun

• This is the Chandrasekar limit• At this mass the compression is so

big that the electrons get so fast that they approach the speed of light

• Unfortunately, they can’t get any fasters than light so the game breaks down

Topic 2: Stars

TCP 17 cont…

•After 1.4 MSun gravity is stronger than the electron degeneracy pressure and the star collapses further, heating the star until the temperature is big enough for carbon fusion

•The star explodes in a White Dwarf Supernova

•Not sure how this ends…

Topic 2: Stars

• Mention the difference between massive star supernovae and the “carbon bomb” white dwarf supernovae

• White dwarf supernovae are especially cool because they always happen when the WD reaches the critical mass of 1.4MSun when you get the carbon bomb stuff and the thing explodes (we call this a type 1a supernova)

Topic 2: Stars

Death of White Dwarf’s

•Depending on its size, once iron is formed in the star, it either dies slowly and sheds its elements into space–Why does it sped its elements into space?

Topic 2: Stars

Stuff on Electron Degeneracy

• Do we want an aside about degeneracy?

• Since there is no burning to counteract the gravity it shrinks until the electrons get so close together that, for quantum mechanical reasons, they can’t get any closer (electron degeneracy)

• A white dwarf is supported by the exclusion principle repulsion between electrons in the matter

Topic 2: Stars

TCP 16 and 17 Continued

•This incredible compression releases a huge amount of energy and is what we call a supernova

•(It can shine so brightly that it looks like about 10 billion suns for about a week)

•Page 114

Massive Things: Galaxies, Stars and Black Holes Topic 2: Stars Big Bang, Black Holes, No Math 1 Big...

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