charles hakes fort lewis college1. charles hakes fort lewis college2

Charles HakesFort Lewis College 1


Chapter 15-16

The Milky Way

Dark MatterExtending the Distance Scale


Mapping the Milky Way


Spiral Galaxies

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


Figure 14.1Galactic Plane



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



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

• Orderly rotation in the plane.• Random orbits in the halo.


Figure 14.12Stellar Orbits in Our Galaxy


Figure 14.10Observations of the Galactic Disk


Mass of the Milky Way

• Recall Newton’s modification to Kepler’s third law:


Figure 14.18Galaxy Rotation Curve


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.


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


Figure 14.19Gravitational 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


Galaxy Masses


Figure 16.4Galaxy Rotation Curves


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.


Figure 16.5Galaxy Masses

• from Newton’s modification of Kepler’s law


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.


Figure 16.6Dark Galaxy?


Figure 16.7abGalaxy Cluster X-Ray Emission

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


Figure 16.7cGalaxy Cluster X-Ray Emission


Figure 16.8Head–Tail Radio Galaxy - Could this be a “wake” through intergalactic clouds?


Extending the Distance Scale

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


Figure 14.7Variable Stars on Distance Ladder

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


Figure 15.12Local Group


Tully-Fisher Relationship


Figure 15.9Galactic “Tuning Fork”

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


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


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.


Figure 15.11Extragalactic Distance Ladder


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.


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).


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


Figure 15.11Extragalactic Distance Ladder


Review Questions


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


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


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


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.


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.


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


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


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?

charles hakes fort lewis college1. charles hakes fort lewis college2

Documents

galaxy slide

milky way slide

galaxy rotation curve

galactic plane slide

galactic disk slide

gravitational lensing

distance scale slide

galaxy rotation curves