1 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
Dec 19, 2015
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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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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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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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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
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
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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
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
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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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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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QuickTime™ and aTIFF (Uncompressed) decompressor
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Milky Way
Significant mass exists outside radius of most stars!
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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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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.