Announcements

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Announcements

• Reading for next class Chapters 21 22
• Cosmos Assignment 3,Due Monday, April 19
• 1. Astronomy Place tutorial Measuring Cosmic
  Distances, review and complete lessons, submit
  exercises
• 2. Astronomy Place tutorial Hubbles Law, do
  all lessons and submit exercises
• Email me questions you would like me to address
  in class

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Questions

• Why do we bother measuring the distance of
  objects in the universe if they are all just
  constantly moving away from us? What is the
  signifigance?
• Differences between types of galaxies

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Questions

• Hubbles law
• Hubbles constant
• Its units
• How to use it
• What it means
• How can the universe expand if its size is
  infinite?
• What is a standard candle?
• Measuring cosmic distances

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MilkyWayCartoon
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Life of a GalaxyGas -gt Star -gt Gas cycle

• Gravity clumps gas, makes stars
• Stars produce heavy elements by fusion
• Stars die and return enriched gas to ISM

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Gas -gt Star -gt Gas cycle
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Halo 0.02-0.2 heavy elements (O, Fe, )
only old stars, only hot no cold gas
Disk 2 heavy elements, gas dust, stars
of all ages, spiral arms
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Where do stars form?
Much of star formation in disk happens in spiral
arms
Whirlpool Galaxy
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Center of the Milky Way

• Stars moved fastest closest to the center
• Similar Solar System
• Speed decreases as 1/?D
• Point mass at Center of Galaxy
• From velocity distance can determine the
  MassM (inside distance D) V2 D / G
• Supermassive Black Hole, M 3-4x106 Msun

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The Larger Universe

• Other galaxies
• Measuring Cosmic Distances
• Hubbles Law
• Age of the Universe

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Space is crowded with Galaxies
Ellipical Galaxy
Hubble Ultra Deep Field
Irregular Galaxy
Spiral Galaxy
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What are galaxies?

• This was a question in the early 20th century
• Are they large collections of stars, like the
  Milky Way?
• Are they small objects, gaseous nebula or small
  collections of stars, that are part of the Milky
  Way?
• Great debate at the National Academy of Sciences
  in 1920.

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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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halo
disk
bulge
Spiral Galaxy
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Disk Component
Spheroidal Component (halo bulge)
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Disk Component stars of all ages, many gas
dust clouds
Spheroidal Component bulge halo, old
stars, few gas clouds
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Disk Component stars of all ages, many gas clouds
Blue-White color indicates ongoing star formation.
Red-Yellow color indicates older stars
Spheroidal Component bulge halo, old
stars, few gas clouds
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Question 1

• Why does the blue-white regions indicate ongoing
  star formation
• Young stars are bluish whitish
• Ionization nebula are bluish whitish
• Bluish stars have short lives
• Reddish stars are old

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Question 1

• Why does the blue-white regions indicate ongoing
  star formation
• Young stars are bluish whitish
• Ionization nebula are bluish whitish
• Bluish stars have short lives. Bluish stars are
  hot, massive, highly luminous main sequence
  stars. Not many, but provide most light because
  so luminous.
• Reddish stars are old

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Question for next class

• Why are there different types of Galaxies?

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Measuring Cosmic Distances

• How do you ESTIMATE distances when you cant use
  a ruler or tape measure?
• Size (compared to expected size)
• Location (in front or behind other objects)
• Brightness (compared to expected brightness)

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Astronomers use mostly BRIGHTNESS,Need to know
actual Luminosity

• For nearby stars in our own galaxy use PARALLAX
  (geometry not brightness)
• For remainder of our galaxy FIT MAIN SEQUENCE
• For nearby galaxies use bright pulsating stars,
  called CEPHEID VARIABLES
• For distant galaxies use white dwarf SUPERNOVA

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Stellar Parallax
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Main Sequence Fitting
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Cepheid Variables
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White Dwarf Super-nova
Why are WD supernova standard candles? All
happen for WD at mass limit of 1.4 Msun, so
properties similar.
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Hubble Diagram

• V. M. Slipher first found all but few nearby
  galaxies are red-shifted.
• Edwin Hubble measured distances and red-shift
  velocity away from us for nearby galaxies (1929)

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Doppler Shift
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RedshiftofGalaxies
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HubblesLaw
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Hubbles Results
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Hubble Law Results (1994)
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What does Hubbles Law Mean?
B
C
A
What happens if it expands?
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Viewgraph Experiment
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Question 2

• If we now measure distances from E, will the
  points in the plot of change in distance vs.
  original distance fall along the same line or
  not?
• Same Line
• Different Line

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Viewgraph Experiment
DDconstant x D
DD/D slope H
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Space is Expanding
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Expansion of the Universe

• The 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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Question 3

• Suppose your friend calls you on their cell phone
  and says, I have been driving at 70 mph straight
  away from you and I am now 140 miles distant.
  How long did your friend take to get there,
  assuming they drove at a constant speed?
• 1 hour
• 2 hours
• 3 hours
• Cant tell how long

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Question 3

• Suppose your friend calls you on their cell phone
  and says, I have been driving at 70 mph straight
  away from you and I am now 140 miles distant.
  How long did your friend take to get there,
  assuming they drove at a constant speed?
• 1 hour
• 2 hours
• 3 hours
• Cant tell how long

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Question 4

• Suppose another friend calls you at the same time
  on their cell phone and says, I have been
  driving straight away from you at 35 mph (broken
  down car) and am now 70 miles distant. When did
  you second friend start?
• Before your first friend
• At the same time as your first friend
• After your first friend
• Not possible to tell when they started

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Question 4

• Suppose another friend calls you at the same time
  on their cell phone and says, I have been
  driving straight away from you at 35 mph (broken
  down car) and am now 70 miles distant. When did
  you second friend start?
• Before your first friend
• At the same time as your first friend
• After your first friend
• Not possible to tell when they started

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Question 5

• Suppose yet another friend with a hyperspatial
  communicator calls you from their galaxy and
  says, I have been traveling away from you at a
  constant speed of 2x1011 km/yr and am now
  2.6x1021 km distant from you. How long have
  they been travelling?
• 13 million years
• 13 thousand year
• 13 billion years
• 13 trillion years

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Question 5

• Suppose yet another friend with a hyperspatial
  communicator calls you from their galaxy and
  says, I have been traveling away from you at a
  constant speed of 2x1011 km/yr and am now
  2.6x1021 km distant from you. How long have
  they been travelling?
• 13 million years
• 13 thousand year
• 13 billion years
• 13 trillion years

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Question 6

• Suppose another friend in a different galaxy
  tells you that they have been travelling twice as
  fast and are twice as far away, how long have
  they been travelling?
• A shorter time
• The same time
• A long time
• We cant tell

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Question 6

• Suppose another friend in a different galaxy
  tells you that they have been travelling twice as
  fast and are twice as far away, how long have
  they been travelling?
• A shorter time
• The same time
• A long time
• We cant tell

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Hubbles Law

• Velocity Hubbles Constant x Distance
  V HDmeans the same thing. If
  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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