Section 3 review

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Section 3 review

• 2950
• Dr Bryce

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Class notices

• Last homework due 5pm today
• Only two office hours left this semester
• Exam 3 on 12th
• Final Exam on Tuesday 18th

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Important

• It is your responsibility to check that your exam
  and homework scores are present and correct
  before the final
• Finals are very close to the break, so there will
  not be much opportunity to change information
  before the grading deadlines

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Studying tips

• Read through your notes (from class and from the
  book)
• Review the homework
• Online quizzes
• Speak with your friends about problems
• Astronomy tutorial sessions

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The Milky Way

• What does the MW look like?
• Basic components
• Orbits
• Gas recycling
• Star formation
• Galaxy formation
• Galactic centre

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We see our galaxy edge-on Primary features
disk, bulge, halo, globular clusters
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Mapping Globular clusters
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(No Transcript)
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Suns orbital motion (radius and velocity) tells
us mass within Suns orbit 1.0 x 1011 MSun
Sun is about 8kpc from the galactic centre
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Stars appear to be orbiting something massive but
invisible a black hole? Orbits of stars
indicate a mass of about 4 million MSun
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Interstellar medium

• HII regions
• X-ray bubbles
• Molecular clouds
• 21cm hydrogen line

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• Spiral arms are waves of star formation
• Gas clouds get squeezed as they move into spiral
  arms
• Squeezing of clouds triggers star formation
• Young stars flow out of spiral arms

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Halo Stars 0.02-0.2 heavy elements (O, Fe,
), only old stars
Halo stars formed first, then stopped
Disk Stars 2 heavy elements, stars of all
ages
Disk stars formed later, kept forming
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Galaxies

• Types of Galaxies
• Measuring distance
• Hubbles law
• Distance and age

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Hubbles galaxy classes
Spheroid Dominates
Disk Dominates
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The distance ladder
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Hubbles Law velocity H0 x distance
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Important points

• We observe the effects of expansion in virtually
  all galaxies
• Measurements of the rate of expansion tell us
  that it started 14 billion years ago
• Astronomers use many techniques to verify this
  rate, do not depend on just one set type of
  observation
• The Universe (Spacetime) is expanding and the
  galaxies are being carried along

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Galactic evolution

• How did galaxies form?
• Differences between galaxies
• AGN
• Quasars
• Supermassive black holes
• Rotation curves and dark matter

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• The highly redshifted spectra of quasars indicate
  large distances
• From brightness and distance we find that
  luminosities of some quasars are gt1012 LSun
• Variability shows that all this energy comes from
  region smaller than solar system

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Quasars powerfully radiate energy over a very
wide range of wavelengths, indicating that they
contain matter with a wide range of temperatures
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Characteristics of Active Galaxies

• Luminosity can be enormous (gt1012 LSun)
• Luminosity can rapidly vary (comes from a space
  smaller than solar system)
• Emit energy over a wide range of wavelengths
  (contain matter with wide temperature range)
• Some drive jets of plasma at near light speed

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Spiral galaxies all tend to have flat rotation
curves indicating large amounts of dark matter
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• What is the evidence for dark matter in galaxies?
• Rotation curves of galaxies are flat, indicating
  that most of their matter lies outside their
  visible regions
• What is the evidence for dark matter in clusters
  of galaxies?
• Masses measured from galaxy motions, temperature
  of hot gas, and gravitational lensing all
  indicate that the vast majority of matter in
  clusters is dark

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Two Basic Options

• Ordinary Dark Matter (MACHOS)
• Massive Compact Halo Objects
• dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
• Weakly Interacting Massive Particles
• mysterious neutrino-like particles

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Time in billions of years
2.2
5.9
8.6
13.7
0.5
35
70
93
140
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Size of expanding box in millions of lt-yrs
Models show that gravity of dark matter pulls
mass into denser regions universe grows lumpier
with time
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Basic Observations

• When considering the Universe as a whole, what
  are our basic observation?
• Stars exist in galaxies and galaxies exist in
  clusters and clusters exist in super clusters
• Galaxies are receding from each other
• The further we look away from the Milky Way the
  younger the galaxies we see (blue light/low
  metalicity) and the oldest galaxies are closest
  to us
• The oldest stars we see are about 12 billion
  years old

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The standard model

• When we put these observations together we come
  up with the standard model
• The Universe is expanding
• The Universe had a starting point (i.e. the
  Universe has a finite age)
• The laws of physics are the same everywhere in
  the Universe
• The Universe is all made of the same matter
  (stars, gas, dust, photons etc)
• We are not located at a special point in the
  Universe

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• Assumes that the expansion rate of the Universe
  doesnt change
• Called the Hubble time
• Large values of H give us a young Hubble time and
  small values give us old Hubble time
• For example if early expansion happened at a
  slower rate than we observe currently the age of
  the Universe will be older than the Hubble time

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Critical density

• The density of a flat Universe, i.e. the division
  between positive and negative curvature
• A slightly denser Universe would be positively
  curved and slightly less dense would be
  negatively curved
• rc10-26kg/m3
• About 10 hydrogen atoms per cubic meter

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Accelerating universe is best fit to supernova
data
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Dark Matter vs Dark Energy

• What is the similarity?
• Not the same thing at all, dark matter can be
  observed by gravitational interactions but not
  via light, dark energy is inferred by the motion
  of the Universe on large scales, not directly
  observed via light

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Dark Matter vs Dark Energy

• We want to add up all the components of the
  Universe to calculate the density
• From density we get the curvature of the
  Universe, open, flat, closed
• Remember that mass is a type of energy according
  to Einstein Emc2

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The early universe must have been extremely hot
and dense
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• Photons converted into particle-antiparticle
  pairs and vice-versa
• E mc2
• Early universe was full of particles and
  radiation because of its high temperature

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Primary Evidence

• We have detected the leftover radiation from the
  Big Bang.
• The Big Bang theory correctly predicts the
  abundance of helium and other light elements.