Charles Hakes

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Chapter 15-16

• The Milky Way
• Dark Matter
• Extending the Distance Scale

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Mapping the Milky Way
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Spiral Galaxies

• A view of spiral galaxies from face-on and
  edge-on.

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Figure 14.1Galactic Plane
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Mapping the Milky Way

• Radio observations can determine much of the
  structure and rotation rates.

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Mapping the Milky Way

• Radio observations can determine much of the
  structure and rotation rates.
• Orderly rotation in the plane.
• Random orbits in the halo.

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Figure 14.12Stellar Orbits in Our Galaxy
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Figure 14.10Observations of the Galactic Disk
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Mass of the Milky Way

• Recall Newtons modification to Keplers third
  law

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Figure 14.18Galaxy Rotation Curve
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Mass of the Milky Way

• There is apparently more mass than can be seen.
• Unseen mass out to 50 kpc.
• Recall radius of observable Milky Way is 15
  kpc.
• Dark Matter
• Can detect gravitational effects
• Cannot detect any other way.

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Dark Matter

• Is not atomic or molecular clouds - we would
  detect those using spectroscopy.
• Could be brown dwarfs or white dwarfs - very
  difficult to see.
• MACHOs - MAssive Compact Halo Objects
• Could be exotic subatomic particles
• WIMPs - Weakly Interacting Massive Particles

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Figure 14.19Gravitational Lensing
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What observations suggest the mass of the Galaxy
goes much farther out than its visible disc?

• A) the orbits of the open clusters in the disc
• B) x-ray images of other galaxies' discs from
  Chandra
• C) the rotation curve beyond 15kpc
• D) 21 cm maps of the spiral arms
• E) infrared observations of distant brown dwarfs

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What observations suggest the mass of the Galaxy
goes much farther out than its visible disc?

• A) the orbits of the open clusters in the disc
• B) x-ray images of other galaxies' discs from
  Chandra
• C) the rotation curve beyond 15kpc
• D) 21 cm maps of the spiral arms
• E) infrared observations of distant brown dwarfs

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Galaxy Masses
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Figure 16.4Galaxy Rotation Curves
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Galaxy Masses

• Galaxy masses determined from Newtons
  modification to Keplers third law.
• Within the visible spiral, radial velocities (and
  masses) can be measured directly.
• Outside the visible spiral, observe multiple
  galaxy systems.
• Only radial velocity determined with Doppler
  shift.
• Reliable statistical information from lots of
  observation.

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Figure 16.5Galaxy Masses

• from Newtons modification of Keplers law

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Galaxy Masses

• Galaxies apparently have invisible halos similar
  to the Milky Way.
• All contain 3-10 times the visible mass.
• Mass discrepancy is even greater for clusters of
  galaxies.

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Figure 16.6Dark Galaxy?
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Figure 16.7abGalaxy Cluster X-Ray Emission

• Intergalactic space is filled with superheated
  gas in this cluster.

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Figure 16.7cGalaxy Cluster X-Ray Emission
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Figure 16.8HeadTail Radio Galaxy - Could this
be a wake through intergalactic clouds?
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Extending the Distance Scale

• Variable Stars
• Tully-Fisher Relationship
• Supernovae
• Cosmological Redshift

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Figure 14.7Variable Stars on Distance Ladder

• Greater distances can be determined than
  typically available through spectroscopic
  parallax, because these variables are so bright.

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Figure 15.12Local Group
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Tully-Fisher Relationship
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Figure 15.9Galactic Tuning Fork

• Galaxies are classified according to their shape
  (Hubble classification)
• Elliptical
• Spiral
• Irregular

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Figure 15.10Galaxy Rotation

• Rotation rates can be determined using Doppler
  shift measurements
• Blue shift indicates moving towards you
• Red shift indicates moving away from you

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Tully-Fisher Relationship

• Rotation speed can be used to determine a
  galaxys total mass.
• A close correlation between rotation speed and
  total luminosity has been observed.
• Comparing (true) luminosity to (observed)
  apparent brightness allows us to determine
  distance
• Distance scale can be extended to 200 Mpc.

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Figure 15.11Extragalactic Distance Ladder
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Supernovae

• Type II Supernovae
• Are a result of a very massive stars core
  collapse
• Can vary in brightness, since the cores can vary
  in size.
• Therefore, they are not a good distance
  indicator.

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Supernovae

• Type I Supernovae
• White dwarf, carbon detonation
• Are a result of a white dwarf exceeding its
  Chandrasekhar limit (1.4 Msolar).
• They are all about the same size.
• They are very good distance indicators (Standard
  Candles).

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Standard Candles

• Standard Candles are easily recognizable
  astronomical objects whose luminosities are
  confidently known.
• Term usually only refers to very luminous objects
• Type I supernovae
• Other objects might include
• Rotating spiral galaxies
• Cepheid variables
• Main sequence stars

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Figure 15.11Extragalactic Distance Ladder
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Review Questions
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Which of these does not exist?

• A) a .06 solar mass brown dwarf
• B) a 1.3 solar mass white dwarf
• C) a six solar mass black hole
• D) a million solar mass black hole
• E) a 3.3 solar mass neutron star

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Which of these does not exist?

• A) a .06 solar mass brown dwarf
• B) a 1.3 solar mass white dwarf
• C) a six solar mass black hole
• D) a million solar mass black hole
• E) a 3.3 solar mass neutron star

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A star has an apparent magnitude of 1.0 and an
absolute magnitude of 1.0. If the distance
between Earth and the star increases, the
apparent magnitude would _____, and the absolute
magnitude would _____.

• A) increase decrease
• B) decrease increase
• C) increase not change
• D) decrease not change
• E) not change increase

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A star has an apparent magnitude of 1.0 and an
absolute magnitude of 1.0. If the distance
between Earth and the star increases, the
apparent magnitude would _____, and the absolute
magnitude would _____.

• A) increase decrease
• B) decrease increase
• C) increase not change
• D) decrease not change
• E) not change increase

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A star has apparent magnitude of 8.0 before it
goes nova and increases its luminosity by 10,000
times. Its apparent magnitude after it goes nova
is.

• A) 8.0
• B) 18.0
• C) -8.0
• D) -2.0
• E) 3.0

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A star has apparent magnitude of 8.0 before it
goes nova and increases its luminosity by 10,000
times. Its apparent magnitude after it goes nova
is.

• A) 8.0
• B) 18.0
• C) -8.0
• D) -2.0
• E) 3.0

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Using spectroscopic parallax, you find a stars
distance to be 76 parsecs. You now find out that
the star isnt a main sequence star, but is a red
giant. Your distance estimate is

• A) too large
• B) too small
• C) fine - no significant change in estimate is
  needed.

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Using spectroscopic parallax, you find a stars
distance to be 76 parsecs. You now find out that
the star isnt a main sequence star, but is a red
giant. Your distance estimate is

• A) too large
• B) too small
• C) fine - no significant change in estimate is
  needed.

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Which is correct?

• 1 - The new moon rises at noon.
• 2 - The first quarter moon rises at noon.
• 3 - The full moon rises at noon.
• 4 - The third quarter moon rises at noon.

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Which is correct?

• 1 - The new moon rises at noon.
• 2 - The first quarter moon rises at noon.
• 3 - The full moon rises at noon.
• 4 - The third quarter moon rises at noon.

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In Paris, France (50 degrees north latitude),
what is the longest day of the year?

• 1 March 21
• 2 June 21
• 3 September 21
• 4 December 21

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In Paris, France (50 degrees north latitude),
what is the longest day of the year?

• 1 March 21
• 2 June 21
• 3 September 21
• 4 December 21

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Where along the horizon does the Sun rise on June
21 in Paris, France?

• 1 Due east
• 2 North of east
• 3 South of east
• 4 Cant tell with information given

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Where along the horizon does the Sun rise on June
21 in Paris, France?

• 1 Due east
• 2 North of east
• 3 South of east
• 4 Cant tell with information given

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Three Minute Paper

• Write 1-3 sentences.
• What was the most important thing you learned
  today?
• What questions do you still have about todays
  topics?