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Charles Hakes
Fort Lewis College
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Charles Hakes
Fort Lewis College
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Chapter 15-16
The Milky Way
Dark Matter
Extending the Distance Scale
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Fort Lewis College
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Mapping the Milky Way
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Fort Lewis College
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Spiral Galaxies
• A view of spiral
galaxies from faceon and edge-on.
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Figure 14.1
Galactic Plane
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Fort Lewis College
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Mapping the Milky Way
• Radio observations can determine
much of the structure and rotation
rates.
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Fort Lewis College
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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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Fort Lewis College
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Figure 14.12
Stellar Orbits in Our Galaxy
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Figure 14.10
Observations of the Galactic Disk
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Mass of the Milky Way
• Recall Newton’s modification to
Kepler’s third law:
total mass (solar) =
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Fort Lewis College
orbit size (A.U.)3
orbit period (years)2
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Figure 14.18
Galaxy Rotation Curve
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Fort Lewis College
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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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Fort Lewis College
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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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Fort Lewis College
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Fort Lewis College
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Figure 14.19
Gravitational
Lensing
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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Fort Lewis College
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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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Fort Lewis College
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Galaxy Masses
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Figure 16.4
Galaxy Rotation Curves
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Fort Lewis College
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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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Fort Lewis College
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Figure 16.5
Galaxy 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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Fort Lewis College
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Figure 16.6
Dark Galaxy?
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Figure 16.7ab
Galaxy Cluster X-Ray Emission
• Intergalactic space is filled with
superheated gas in this cluster.
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Figure 16.7c
Galaxy Cluster X-Ray
Emission
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Figure 16.8
Head–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 14.7
Variable 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 15.12
Local Group
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Tully-Fisher Relationship
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Figure 15.9
Galactic “Tuning Fork”
• Galaxies are classified according to their
shape (Hubble classification)
• Elliptical
• Spiral
• Irregular
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Figure 15.10
Galaxy 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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Fort Lewis College
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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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Fort Lewis College
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Figure 15.11
Extragalactic Distance Ladder
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Fort Lewis College
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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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Fort Lewis College
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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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Fort Lewis College
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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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Fort Lewis College
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Figure 15.11
Extragalactic Distance Ladder
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Review Questions
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Fort Lewis College
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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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Fort Lewis College
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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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Fort Lewis College
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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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Fort Lewis College
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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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Fort Lewis College
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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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Fort Lewis College
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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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Fort Lewis College
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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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Fort Lewis College
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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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Fort Lewis College
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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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Fort Lewis College
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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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Fort Lewis College
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In Paris, France (50 degrees north latitude),
what is the longest day of the year?
1:
2:
3:
4:
March 21
June 21
September 21
December 21
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Fort Lewis College
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In Paris, France (50 degrees north latitude),
what is the longest day of the year?
1:
2:
3:
4:
March 21
June 21
September 21
December 21
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Fort Lewis College
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Where along the horizon does the Sun rise on
June 21 in Paris, France?
1:
2:
3:
4:
Due east
North of east
South of east
Can’t tell with information given
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Fort Lewis College
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Where along the horizon does the Sun rise on
June 21 in Paris, France?
1:
2:
3:
4:
Due east
North of east
South of east
Can’t tell with information given
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Fort Lewis College
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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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Fort Lewis College
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