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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 What are Galaxies?

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Announcements

  • Reading for next class: Chapters 22.6, 23

  • Cosmos Assignment 4,Due Wednesday, April 21, Angel Quiz

  • Monday, April 26Quiz 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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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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Galaxy Formation Models

  • Assumptions:

  • Matter originally

  • filled all of space

  • almost uniformly

  • Gravity of denser

  • regions pulled in

  • surrounding

  • matter


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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

  • Rate of star formation

  • Amount of Rotation

  • 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?

  • About 1 cm away

  • About 1 m away

  • About 1 km away

  • About San Francisco

  • About the Moon

  • 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?

  • About 1 cm away

  • About 1 m away (~3 m)

  • About 1 km away

  • About San Francisco

  • About the Moon

  • 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 of

star

formation


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Evidence of Galaxy mergers

Super massive galaxies in the centers of clusters of galaxies

Merged galaxies

See also

Fig 21.11



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Elliptical galaxies are much more common in huge clusters of galaxies

Denser cloud

More collisions

(hundreds to thousands of galaxies)


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How do we know? galaxies

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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Problem! galaxies

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

  • Need stronger gravity

  • Need more mass

  • Don’t see any more mass

  • Postulate existence of DARK MATTER


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Other evidence for Dark Matter galaxies

  • Rotation of galaxies

  • Motions in clusters of galaxies

  • Hot x-ray emitting gas in clusters of galaxies

  • Gravitational lensing


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Rotation Curve galaxies

A plot of orbital velocity versus orbital radius

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

Increasing distance


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Rotation Curve galaxies

of Milky Way stays flat with distance

Mass must be more spread out than in solar system


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Milky Way galaxies

Significant mass exists outside radius of most stars!


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Mass of Milky Way galaxies

Mass within Sun’s orbit:

1.0 x 1011MSun

Total mass:

~1012MSun

-> Dark Matter


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Motions of Galaxies in Clusters galaxies

  • 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 galaxies

  • 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 galaxies

  • Mass produces gravity

  • Gravity bends light

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


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Galaxy cluster galaxiesacts as gravitational lens. Focuses image of galaxy behind it into blue arcs.


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What is Dark Matter? galaxies

  • 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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