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Charles HakesFort Lewis College 1
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Chapter 15-16
The Milky Way
Dark MatterExtending 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 Newton’s modification to Kepler’s 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 Newton’s modification to Kepler’s 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 Newton’s modification of Kepler’s 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 16.8Head–Tail 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 galaxy’s 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 star’s 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 star’s distance to be 76 parsecs. You now find out that the star isn’t 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 star’s distance to be 76 parsecs. You now find out that the star isn’t 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: Can’t 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: Can’t 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 today’s topics?
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