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Announcements• Reading for next class: Chapters 22.6, 23• Cosmos Assignment 4,

Due Wednesday, April 21, Angel Quiz• Monday, April 26

Quiz 3 & Review, chapters 16-23• Wednesday, April 28,

Midterm 3: chapters 16-23

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What are Galaxies?

Galaxies are vast collections of stars (~1011) and sometimes gas and dust as well

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Universe is Expanding

• You and I are NOT expanding

• The solar system is NOT expanding

• The Milky Way Galaxy is NOT expanding

• Our local group of Galaxies is NOT expanding

• Nothing that is bound together by a force is expanding

• SPACE between groups of galaxies IS expanding

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Hubble’s Law

Velocity = Hubble’s Constant x Distance V = HDIf you are twice as far away,you are moving away twice as fast, so you started moving away at the same time!

How long ago was that?

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Age of the Universe

• V=HD

• D = VT = V/H

• T = D/V = 1/H Age if expansion not accelerated or decelerated

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Fig 22.18

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Questions:

How did galaxies form?

Why are there different types of Galaxies?

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Galaxy Formation

Similar to star formation• H & He gas filled space almost uniformly• Where density slightly greater, gravity slightly

greater• Matter falls into gravitational potential well,

increases gravity• Matter pulled in by more gravity, density

excess grows• Densest cores became 1st generation massive

stars

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Assumptions:

• Matter originally filled all of space almost uniformly

• Gravity of denser regions pulled in surrounding matter

Galaxy Formation Models

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Galaxy Formation:Gravitational Instability

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Clues to Galaxy Formation

Halo stars are old, have randomly oriented orbits

Disk has young stars with orbits nearly in plane Initially gravity pulled in matter from all

directions. Stars formed during this stage have random orbits passing close to center

Later, rotation made any remaining gas flatten into disk. Stars forming after this have orbits in disk.

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Spiral or Elliptical Galaxy?

Possible explanations

1. Rate of star formation

2. Amount of Rotation

3. Collisions & mergers

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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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Collisions & Mergers

Question 1: If the Milky Way were the size of a grapefruit, where would the Andromeda galaxy (nearest comparable size galaxy) be?

A. About 1 cm awayB. About 1 m awayC. About 1 km awayD. About San FranciscoE. About the MoonF. About the Sun

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Collisions & Mergers

Question 1: If the Milky Way were the size of a grapefruit, where would the Andromeda galaxy (nearest comparable size galaxy) be?

A. About 1 cm awayB. About 1 m away (~3 m)C. About 1 km awayD. About San FranciscoE. About the MoonF. About the Sun

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Galaxies are Close togethersimulation of galaxy interaction via gravity

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Galaxies are close together

Evidence of galaxy interactions via gravity

Burst ofstarformation

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Evidence of Galaxy mergers

Super massive galaxies in the centers of clusters of galaxies

See also Fig 21.11

Merged galaxies

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Galaxy Merger Simulation

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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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How do we know? When we look farther out in space we are looking farther back in time. See galaxies at different stages in their evolution

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Expansion of the Universe

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Problem!

• In an expanding universe, gravity takes longer to pull matter together.

Need stronger gravityNeed more massDon’t see any more massPostulate existence of DARK MATTER

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Other evidence for Dark Matter

1. Rotation of galaxies

2. Motions in clusters of galaxies

3. Hot x-ray emitting gas in clusters of galaxies

4. Gravitational lensing

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A plot of orbital velocity versus orbital radius

Solar system’s rotation curve declines because Sun has almost all the mass & gravity gets weaker withIncreasing distance

Rotation Curve

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of Milky Way stays flat with distance

Mass must be more spread out than in solar system

Rotation Curve

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Milky Way

Significant mass exists outside radius of most stars!

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Mass within Sun’s orbit:

1.0 x 1011 MSun

Total mass:

~1012 MSun

-> Dark Matter

Mass of Milky Way

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Motions of Galaxies in Clusters

• Galaxies are moving too fast in clusters of galaxies to be held together by the gravity of the visible stellar material

Total Mass ~ 10x visible mass

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Hot Gas in Galaxy Clusters

• Galaxy clusters are filled with hot gas that emits x-rays. Temperature ~ 100 million K

• Pressure of hot gas must be balanced by gravity to hold it together in the cluster.

Total Mass ~ 10x visible mass

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Gravitational Lensing

• Mass produces gravity

• Gravity bends light

• Gravity can distort the image of an object behind the mass

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Galaxy cluster acts as

gravitational lens.

Focuses image of galaxy behind it into

blue arcs.

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What is Dark Matter?

• We DON’T KNOW!!!

• We only know what it is not It is not ordinary matter composed of

protons, neutrons, electrons, etc.

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Large Scale Structure of the Universe

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Galaxies are

arranged like soap bubbles

with voids

devoid of galaxies inside