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Announcements• There will be a star map on the exam. I will

not tell you in advance what month.

• Grades are not yet posted, sorry. They will be posted by exam time on Wednesday.

• Grades including the 3rd midterm will be posted by Monday 5/3.

• Final is optional. I will announce the room Wednesday at exam time.

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Life History of a Star

Loss of Energy to SpaceGravitational Contraction of CoreContraction is halted temporarily

by nuclear fusionEnergy generation in core

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HOT COOL

BRIGHT

FAINT

HRDiagram

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Star Birth

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Small Mass Stars

become RED

GIANTS

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Large Mass Stars

becomeRED

GIANTS

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Small mass stars can not get hot enough to fuse Carbon

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White Dwarfs & Neutron Stars

• White Dwarfs– Supported by pressure of degenerate electrons– About the size of the Earth– Mass < 1.4 Msun

• Neutron Stars– Supported by pressure of degenerate neutrons– About the size of Lansing– Mass < 3 Msun

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Degenerate Pressure

• Pressure due to motion caused by squeezing particles very close togetherDepends only on density, not on temperature

• Uncertainty Principle location x speed ~ h/mass Means uncertainty in

h is a small constant number

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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

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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

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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

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

Horizon• Long range forces can exert influence

outside Event Horizon:1. Gravity2. Electric Force

• Can know:1. Mass2. Charge3. Spin

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HR Diagram for brightest

northern hemisphere

stars

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Test:Cluster

HR Diagrams

Same DistanceSame Age

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HR Diagram

for clusters

of different

ages

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Gas - Star - Gas Cycle

Interstellar Gas

H, He, C, N, O, Fe, etc.

StarSN orWind

Fusion of He, C, N, O& heavier atoms

Gas cloud contracts

Return ofgas enrichedin heavy atoms

SN fuse atoms > iron

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QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Halo Stars: 0.02-0.2% heavy elements (O, Fe, …) only old stars

Disk Stars: 2% heavy elements stars of all ages

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MilkyWay

Cartoon

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Density Excess?

Higher density proto-galactic clouds form stars more rapidly, use up all their gas before it can form a disk.

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Rotation?

Larger rotation produces more disk-like distribution of matter.

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Galaxies are close together

Mergers may make Ellipticals

Burst ofstarformation

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(hundreds to thousands of galaxies)

1. Denser cloud2. More collisions

Elliptical galaxies are much more common in

huge clusters of galaxies

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Distance & Age

Universe opaque

space

time

light

Here & Now

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Fig 22.18

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Relic Radiation

• Universe was once hot and opaque, see radiation from that time (as from surface of a star)– Comes at us from all directions

(inside fog bank)– Has a thermal spectrum– Cold now, expansion cools

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Spectrum is Thermal, T=2.7 K

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CMB Radiation

Radiation is nearly the same from all directions,

Doppler Shift due to motion of the Milky WayT/T ~ 10-3

After subtracting emission from MW, seePrimoridial fluctuations from when universe became transparent,T/T ~ 10-5

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Relic Elements

TheoryObservations

Universe is 75% H 25% He

Deuterium abundanceconstrains density ofordinary matter

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What do we know about Dark Energy?

• Constitutes 2/3 of energy in universe

• Is smoothly distributed and invisible

Doesn’t clump into galaxies likeMatter, visible or dark

• Has negative pressure

produces Acceleration

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Problems with the Big Bang Model

1. How can two pieces on opposites sides of the universe have the same temperature at the time the universe became transparent?

They are too far apart to have communicated with each other within the age of the universe, since light from them has just now reached us half way between.

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Problems with the Big Bang Model

2. Why is the space-time geometry of the universe so nearly flat, equivalent to the sum of the Ordinary Matter, Dark Matter and Dark Energy = Critical Density?

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Inflation

At very beginning of Big Bang, the Universe underwent a tremendous expansion (inflation).

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Inflation

Before Inflation the two parts of the universe were close enough together to communicate with each other

Fig 23.14

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Inflation

• Expansion smoothes out fluctuations and makes things appear flatter (e.g. blowing up a balloon).