astr/phys 109 dr. david tobacktobackgroup.physics.tamu.edu/toback/109/lectures/last...white dwarf...

Massive Things: Galaxies, Stars and Black HolesTopic 2: Stars

Big Bang, Black Holes, No Math

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Big Bang, Black Holes, No MathASTR/PHYS 109Dr. David TobackLectures 25 & 26



Rest of the Semester-L26

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Date Lecture number (class number)

Due Before class/11:55PM What we do in Class

Assigned after class

Wed Nov 11th 25 EOC 14 (A-D) Chapter 16 EOC 14 (E-G)

Mon Nov 16th 26 EOC 14 (E-G), Rest of Chapter 16 + Chapter 17

EOC 15 & 16, Paper 4, Unit 6 Reading

Wed Nov 18th 27 EOC 15 & 16, Unit 6 Reading Chapter 18 EOC 17 & 18, Unit 6 PLRQ Quiz

Mon Nov 23rd 28 (Last day of Class)

Paper 4 Text, Unit 6 Quiz Chapters 19 and 20

Chapter EOC 19, AMS II and EOC 20

Wed Nov 25th Reading day, no class

Mon Nov 30th Reading day, no class EOC 17 & 18

Tues Dec 1st No Final EOC 19, AMS II and EOC 20, Reviews, Feedback and any Revisions must be done by today

Notes: • Mis-graded on any Assignment? Let us know• Will start calculating grades on Wed December 2nd

Honors Section:• Stage 3 due Wed Nov 18th

• Stage 4, Final Draft, due Monday Nov 23rd



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Unit 5: Big Objects1. 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

Today



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 atoms35



What happens when it runs out of fuel? 36

The Life of a StarThe 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 never happen for many stars

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



Start with Mid-Sized 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 in the first place– 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 (get to that later…)

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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 is only about 15 million Kelvin not hot enough to convert helium into Carbon 39



Future: White Dwarf

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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 marble-size chunk would weigh about 2 tons



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 yet… 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: fuse hydrogen, and 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

• When Helium fuses it makes heavier elements44



Stages in a Massive Star’s Life

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4He4 + 4He4 8BerylliumThis happens a lot in the early universe, but Beryllium quickly decays

Converting Helium to Beryllium4He

4He

8Beryllium

Nuclear Reaction



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8Beryllium + 4He 12Carbon + PhotonEven though Beryllium doesn’t last long, the core is so dense that the Be nucleus quickly finds another helium nucleus and we get Carbon (and more energy)

Converting Beryllium to Carbon8Beryllium

4He

12Carbon

Nuclear Reaction

Photon



The Most Massive Stars

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•For the heaviest stars, after the helium is used up the carbon can start to fuse

•Then Neon•Keep going…up until iron

–Get “shells” of various types of atoms

•This is where the atoms of the Earth get created



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 uses up its fuel

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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 50



More Crushing Neutron Star• After the fuel runs out, if the mass of the star is large enough, things change quickly!

• Gravity crushes the electrons and protons so close to each other that they start to interact– Turn into Neutrons (more on the physics of this interaction in Chapter 19)

• The star crushes itself into a ball of neutrons about the size of Manhattan

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Proton + Electron Neutron + Neutrino

Creating Neutron Stars

Proton Neutron

ElectronNeutrino(leaves

the star)

Up QuarkDown QuarkUp Quark Down Quark



Neutron Stars• Neutrons don’t like to be “too close” to each other because of Quantum Mechanics core can stabilize

• In this process the core 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 amount as the Earth does 53



What about the outer part? • The inner part of the star (the core) is quickly crushed into what becomes the neutron star

• What happens to the outer part? Atoms fall towards the center!

• Hits the dense neutron core and bounces back into space as a giant explosion– This is how we get stuff like Carbon, oxygen, iron etc. on the Earth

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Huge Explosion = Supernova• The explosion can be so violent that it can shine as bright as 10 billion suns for a couple of weeks– As bright as an entire galaxy

• 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 (what was the core of the star) has a “critical mass” (about 3MSun) it can continue to collapse under its own weight

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



Lecture on Chapter 16 now complete

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Big Objects and Black HolesTopic 3: Properties of Black Holes


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Unit 5: Big Objects

1.Galaxies2.Star Birth and

Death3.More on Black

HolesToday



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Where are we now in the history?

Half a billion years after the bangBlack holes start forming



Paper 4: The Assignment• Abbreviated Description: What is the evidence for Stellar Black Holes? Note there is an emphasis on what is a black hole and how it forms. – Explain it to someone who isn’t taking the class (no jargon)

• Make sure you read ALL the instructions• Same format as usual

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Outline

• What makes Black Holes black• What a black hole would look like to a nearby observer

• Evidence for Black Holes• Different types of Black Holes• A few words on why black holes are so important in cosmology and our understanding of the Big Bang 6



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Moving Towards Black Holes

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

• Nothing strong enough to oppose the crush of gravity! Continues to collapse until it becomes a single point in space

• Call this a Black Hole



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Why do we call it a black hole?

Call it a Black Hole because light

can’t “escape”Say more about what this means



What they are… and aren’t• Black holes aren’t demonic, sucking power holes

• A black hole is just another thing a star can turn into when it runs out of fuel

• It is basically a really massive, non-shining, ex-star

• Then again, something with that much mass but a size smaller than a proton does have some unusual properties

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What it IS: A small, massive THING in Space

• Shrink Sun to 10% of its size

• Shrink to 1%• Neutron star is 0.004% of the original size

• What if it were crushed into a black hole? 1.4 Million Kilometers across



Most people have seen the curved Space-Time figure, so let’s

understand those!The size of the Sun… Remember:

Diagram shows the CURVATURE of Space-time

Draw the dent to be deepest is where the force is largest Where space is falling to the center “fastest”

Nothing to “See”11



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How the Curvature Changes as we Compress the SunWhat if we

compressed the Sun into a Neutron Star?

Far outside the Sun you can’t really tell the difference• Force is the same

You can tell the difference if you are very close to the Sun itself• Force is biggerThe sun is now a few kilometers across



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Compress the Sun into a Black Hole

Infinite curvature

Curvature is VERY different really close to where the mass

isNothing “curved” to SEE, deeper

just means where space

falling in fastest

Remember: The black hole is just a point in space



Weird…What if our Sun were to (magically??) turn into a Black Hole?

• The gravitational attraction far outside wouldn’t change, so the planets would continue to orbit

• Wouldn’t suck in any more stuff than the sun does

• Then again, without the light it would get really cold and we’d all die…

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Rest of the Semester-L26

49

Date Lecture number (class number)

Due Before class/11:55PM What we do in Class

Assigned after class

Wed Nov 11th 25 EOC 14 (A-D) Chapter 16 EOC 14 (E-G)

Mon Nov 16th 26 EOC 14 (E-G), Rest of Chapter 16 + Chapter 17

EOC 15 & 16, Paper 4, Unit 6 Reading

Wed Nov 18th 27 EOC 15 & 16, Unit 6 Reading Chapter 18 EOC 17 & 18, Unit 6 PLRQ Quiz

Mon Nov 23rd 28 (Last day of Class)

Paper 4 Text, Unit 6 Quiz Chapters 19 and 20

Chapter EOC 19, AMS II and EOC 20

Wed Nov 25th Reading day, no class

Mon Nov 30th Reading day, no class EOC 17 & 18

Tues Dec 1st No Final EOC 19, AMS II and EOC 20, Reviews, Feedback and any Revisions must be done by today

Notes: • Mis-graded on any Assignment? Let us know• Will start calculating grades on Wed December 2nd

Honors Section:• Stage 3 due Wed Nov 18th

• Stage 4, Final Draft, due Monday Nov 23rd



Full set of Readings So Far•Required:–BBBHNM: Chaps. 1-18

•Recommended:• TFTM: Chaps. 1-5• BHOT: Chaps. 1-7, 8 (68-76), 9 and 11 (117-137), 12

• SHU: Chaps. 1-3, 4(77-86), 5(95-114), 6-8 (up-to-page 164)

• TOE: Chaps. 1-3• Seeds (Cosmology in the 21st Century)

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astr/phys 109 dr. david tobacktobackgroup.physics.tamu.edu/toback/109/lectures/last...white dwarf...

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