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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?