Announcements• Reading for next class: Chapter 19• Star Assignment 9,

due Monday April 12Angel Quiz

• Cosmos Assignment 1,Due Monday April 12Angel Quiz

Death of Stars

1) White Dwarf

2) Neutron Star

3) Black Hole

4) Nothing

WHITE DWARFS -corpse of small mass stars

• Core contracts until electrons are squeezed so much, their velocity increases according to the Uncertainty Principle

• Produces extra Pressure, stops contraction, at Size about Earth

• White Dwarf slowly cools & becomes redder

A white dwarf is about the same size as Earth

More Massive White Dwarfs are Smaller

• More Mass More gravityNeed larger PressureSqueeze electrons

more to increase their speed and pressure

Smaller White Dwarf

Maximum Mass for White Dwarfs

• Pressure of “degenerate” electrons can only support so much mass before electron speed would = speed of light. Electrons get squeezed onto protons.

• Maximum Mass of White Dwarfs = 1.4 Msun

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S. Chandrasekhar

What happens

to a White Dwarf

that gains more mass?

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Question:

What happens to a white dwarf when it accretes enough matter to reach the 1.4 MSun limit?

A. It explodesB. It collapses into a neutron starC. It gradually begins fusing carbon in its core

Question:

What happens to a white dwarf when it accretes enough matter to reach the 1.4 MSun limit?

A. It explodes (White Dwarf SUPERNOVA)B. It collapses into a neutron starC. It gradually begins fusing carbon in its

Fate of Large Mass Stars

Core contracts & gets hotter

Onion like layered structure

Fuse heavier nuclei up to Iron

Iron core shrinks, but can’t fuse to heavier nuclei & release energy

Fate of Large Mass Stars

Iron core shrinkse- + p -> n + No Pressure Iron core collapsesSupernova

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What is the source of Energy for a Supernova Explosion?

a) Chemical Energy?

b) Nuclear Energy?

c) Gravitational Potential Energy?

d) Dark Energy?

e) Thermal Kinetic Energy?

What is the source of Energy for a Supernova Explosion?

a) Chemical Energy?

b) Nuclear Energy?

c) Gravitational Potential Energy

d) Dark Energy?

e) Thermal Kinetic Energy?

Test Supernova Theory

• Supernova 1987A close by in Magellanic Cloud• Burst of neutrinos observed

Core collapsed and became very hotEnergy ~ 108 Lgalaxy ~ 1019 Lsun, Core mass 1.4 Msun

• Burst lasted several secondsNeutrinos diffused out

• Progenitor star (unexpected)Blue not Red supergiantSmaller, shock reached surface faster (2 hrs between & )

Supernova are the source of all heavy elements

• Explosion returns to space the elements produced nuclear fusion during a stars life: C, N, O, Ne, Mg, Si, S, Ca, Fe

• Elements heavier than iron are only made during supernova explosions

What is left after a Supernova Explosion?

1. Neutron Star

2. Black Hole

What is a Neutron Star?

• Ball of neutrons

• Remnant core of a massive star supernova

• Supported by Pressure of degenerate neutrons (v ~ h/mn x)Because mn >> me, must be squeezed much more to get large velocity & pressure

Neutron Star ~ Size of Lansing

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eutronStar

Discovery

• Theorized by J. Robert Oppenheimer and Volkoff in 1930s

• Discovered by Jocelyn Bell Burnell

• Part of her PhD thesis• Found regular pulses of

radio waves

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Crab Pulsar,f=30 /s.

P = 1/30 s

How do we see Neutron Stars?

• Gravity near NS very strong (mass of Sun in Size of Lansing)

• Gas falling into NS (from companion binary star) speeds up to almost speed of light, becomes very hot

• Emits x-rays in beam along rotation axis,~ lighthouse beacon

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X-rays Visible light

Test of Neutron Star Model

• Observe Crab Pulsar is slowing down

• Is slowing down because losing rotational KE. Calculate rate of energy loss from rate slowing down based on assumption is NS

• Compare rate of energy loss to observed rate of energy emission from entire Crab nebula

• They agree!!!Must be NS

Maximum Mass of Neutron Stars• Neutron stars are supported against gravity

by the pressure of “degenerate” neutrons• More Mass More Pressure neutrons move Faster neutrons more Squeezed together,

v ~ h/mn xMaximum possible velocity = speed of

lightMaximum mass neutron star ~ 3 Msun

If supernove remnant mass > 3 Msun

Gravity overcomes PressureRemnant collapsesGravity increases

Fgravity = G M1 M2 / D2

Black Hole

Student Questions:• What is a black hole• Do they exist• How do they form• Explain curved space-time• Is a BH a hole in the universe• How can we know anything about them• How can we find them• Can one live forever inside them• What is on the other side• Why does time run slower• How can more heat make gravity stronger• Where does stuff go that falls into them

What is a Black Hole?

• An object whose GRAVITY is so strong, not even Light can escape it (that is you would have to go faster than the speed of light to escape)

Question:

What happens to the escape velocity from an object if you shrink it?

A. IncreasesB. DecreasesC. Stays the same

Question:

What happens to the escape velocity from an object if you shrink it?

A. IncreasesB. DecreasesC. Stays the same

Formation of Black Holes

If the collapsing core of a massive star which produces the supernova explosion has more mass than the pressure of degenerate neutrons can support (> 3 Msun)

Nothing can stop its collapseThe escape velocity reaches the speed of lightNothing can go faster than the speed of lightBlack Hole

Surface of a Black Hole

• Surface where escape velocity = speed of light is surface of a Black Hole, called Event Horizon

• Outside Event Horizon can escape,inside can not

Question:

• What happens to the SIZE of a BH if it gains more mass?

a) Increases

b) Decreases

c) Stays the Same

Question:

• What happens to the SIZE of a BH if it gains more mass?

a) Increases (Gravity stronger, so escape velocity = speed of light farther away)

b) Decreases

c) Stays the Same

If nothing can escape from a BH, How do we know its there?

If gas falls into a BHBH gravity makes it speed upConservation of Angular Momentum

makes it form an Accretion Disk, orbiting at nearly the speed of light

Friction makes it very hotEmits X-Rays

Black Hole Accretion DiskQuickTime™ and a

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How do we know it’s a Black Hole?

• Only Neutron Stars and Black Holes have strong enough gravity to make infalling gas hot enough to emit x-rays.

• If can determine mass of suspect (in a binary system) & Mass > 3 Msun

Must be Black Hole

Do we see any Black Holes?

Black Holes are NOT holes in the Universe

What would

you see as you

approach a Black

Hole

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What happens as you fall into a BH?

• Tides: gravity is stronger on your feet than your head, because they are closer

• Gravity is towards center of BH, squeezes you from sides

What do your classmates see?

To answer this need to know a little of Einstein’s theory of Motion and Gravity:

• Gravity is Motion in Warped Space - Time

• You can’t tell the difference between acceleration by gravity and any other constant acceleration

• E = mc2, energy and mass are same thing measured in different units

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Mass warps Space - TimeWarped Space - Time tells

Mass how to Move

Forget time, think just about warped space

Orbits in Warped Space - Timec = circular, e = elliptical, u = unbounded

Elevator & Rocket

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Gravity = AccelerationLight Beam in an Elevator or Gravity

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Gravity Attracts LightLight generates Gravity

Reasonable since E = mc2

• Black Holes Gravity attracts light

• Light loses energy escaping from environs of a Black Hole

• Escaping Light is redshifted to longer wavelengths and periods

Your classmates would see you slow down as you approached

the BH event horizon

• Can use period of light as a clock

• Redshifted light oscillates with a longer period

• Time appears to run slower near event horizon

• You would appear to stop and hover (& fade out) as you approached the Event Horizozn

What would you notice as you passed the Event Horizon

Nothing special

• For you time does not slow down in a BH.

• You quickly crash into the previous matter inside the BH(But you couldn’t tell us about it)

What can we know about Black Holes?• Nothing can escape from inside an Event

Horizon• Long range forces can exert influence

outside Event Horizon1. Gravity2. Electric Force

• Can determine: 1. Mass2. Charge3. Spin

Mini Black Holes can Evaporate

Mini BH produce strong tides (stellar BH don’t have strong enough tides)

Lose energy by work of tidal gravity on material outside the event horizon

Since energy = mass, they lose mass and get smaller

Evaporate