Study Points▪ Describe an example of the Doppler Effect that involves sound.

Describe the pitch, frequencies, and wavelengths.

▪ Describe an example of the Doppler Effect that involves light.

Describe the frequencies and wavelengths.

▪ What is the Doppler Effect?

▪ Describe how the Doppler Effect is used to measure the speed of a

star or planet relative to the Earth.

▪ What is meant by a red shift or a blue shift?

▪ Given the spectrum of a star and a reference spectrum, identify if

the star's spectrum is red or blue shifted, whether the Earth and the

star are moving toward or away from each other, and whether the

Earth and the star have large or small relative speeds.

▪ What did Vera Rubin and her colleagues measure with the Doppler

Effect? What did they discover about a galaxy’s rotation? About a

galaxy’s mass?

▪ What is gravitational lensing and why is it helpful?

Doppler Effect

and

Dark Matter

What can you learn from spectra?

• Temperature (energy)

• Density (from type of spectra)

• Composition (from lines)

• Moving toward or away (Doppler)

Demo: Sound

What is that sound?

Demo: Car horn blaring as it passes (example)*

Car moving at nearly constant speed

Listen to pitch and volume.

What changes in the car sound?

Does the volume change?

Note: Volume can change but this is NOT

the Doppler Effect.

1. What happened to the VOLUME of the

horn?

a. sounded louder and louder as the source

approached and sounded fainter and fainter as

the source receded

b. stayed at a constant loudness as the source

approached then dropped to a fainter but

constant loudness as the source receded

c. stayed at the same constant loudness

throughout the motion

d. varied too much to tell what the volume was

doing

1. What happened to the VOLUME of the

horn?

a. sounded louder and louder as the source

approached and sounded fainter and fainter as

the source receded*

b. stayed at a constant loudness as the source

approached then dropped to a fainter but

constant loudness as the source receded

c. stayed at the same constant loudness

throughout the motion

d. varied too much to tell what the volume was

doing

What changes in the car sound?

Does the volume change?

YES, but not due to the Doppler Effect.

The volume increases on approach and

decreases as it moves away.

Does the pitch (frequency) change?

2. What happened to the PITCH (frequency) of the horn?

a. became higher and higher as the source approached and became lower and lower as the source receded

b. stayed at a constant high pitch as the source approached and then dropped to a constant lower pitch as the source receded

c. stayed at the same constant pitch throughout the motion

d. stayed at the same pitch except at the moment the source passed

e. varied too much to tell what the pitch was doing

2. What happened to the PITCH (frequency) of the horn?

a. became higher and higher as the source approached and became lower and lower as the source receded

b. stayed at a constant high pitch as the source approached and then dropped to a constant lower pitch as the source receded*

c. stayed at the same constant pitch throughout the motion

d. stayed at the same pitch except at the moment the source passed

e. varied too much to tell what the pitch was doing

Volume

Time

Volume of car horn over time*

Frequency

Time

Frequency of car horn over time*

Doppler Effect

Shift in frequency (wavelength) due to

motion of source or observer or both.*

Used to measure:

• Motion toward or away

• Speed

Side Note:

Discovered by Christian Doppler in the mid-1800’s

Sydney Harris

“I love hearing that lonesome wail of the train whistle as the frequency of the wave changes due to the Doppler Effect.”

Visual of waves from moving source:

http://www.acs.psu.edu/drussell/Demos/do

ppler/doppler.html

Drawing waves from moving source

Drawing waves…

If source is stationary

Source

( ( ( ( * ) ) ) ) )

←wave moves vsource = 0 wave moves→

Drawing waves…

If source is stationary

Source

( ( ( ( * ) ) ) ) )

←wave moves vsource = 0 wave moves→

If source moves

Source

→

( ( * ) ) ) ) )

Longer λ Shorter λ

Lower f Higher f

Red Shift Blue Shift

If source is stationary

Source

( ( ( ( * ) ) ) ) )

←wave moves vsource = 0 wave moves→

If source moves

Source

→

( ( * ) ) ) ) )

What if source moves faster?

( ( *)))))

Stretched more Compressed more

Higher speeds Bigger shifts*

Same results if source or observer

or both move

Approach Shift to shorter λ Blue shift*

(moving toward) higher f

Recede Shift to longer λ Red shift*

(moving away) lower f

Bigger shift (change) in λ Bigger speed*

Doppler Effect on Spectra

• Watch this spectra shift animation: http://physics.bu.edu/~duffy/HTML5/EM_Doppler.html

Ex: Earth’s Speed of Revolution or Orbital Velocity

Ex: “Earth’s Orbital Speed”

Predict: If Earth is at A, will the star’s

spectrum be red-shifted or blue-shifted?

Ex: “Earth’s Orbital Speed”

Predict: If Earth is at A, will the star’s

spectrum be red-shifted or blue-shifted?

Moving away

Red shifted*

Ex: “Earth’s Orbital Speed”

Ex: “Earth’s Orbital Speed”

Standard Emission Spectra for comparison

Absorption Spectra lines of starEarth is at “A”, so star is given designation “a”

Ex: “Earth’s Orbital Speed”

What color are these lines?

Ex: “Earth’s Orbital Speed”

What color are these lines?

Violet (bluish)

Ex: “Earth’s Orbital Speed”

Where is the red end of the spectrum?

Ex: “Earth’s Orbital Speed”

Blue end of spectrum

Red end of spectrum

Ex: “Earth’s Orbital Speed”

Is “a” red shifted or blue shifted?

Is the star (“a”) moving toward or away from Earth when compared to the standard spectra (top)?

Ex: “Earth’s Orbital Speed”

Blue shift

Red shift

Moving away from Earth = Red shift*

Ex: “Earth’s Orbital Speed”

Standard Emission Spectra

Is “b” red shifted or blue shifted?

Absorption Spectra lines of starEarth is at “B”, so star is given designation “b”

Standard Emission Spectra

Ex: “Earth’s Orbital Speed”

Blue shift*

Red shift*

= 546 nm = 643 nm

Spectrum of element Xo (at rest) | | |

Spectrum of star A (at rest) | | | ||

1. From the spectra above, you can conclude that star A

a. Contains the element Xo and only that element

b. Contains the element Xo and at least one more element

c. Does not contain the element Xo

d. There is not enough information to determine the composition

= 546 nm = 643 nm

Spectrum of element Xo (at rest) | | |

Spectrum of star A (at rest) | | | ||

1. From the spectra above, you can conclude that star A

a. Contains the element Xo and only that element

b. Contains the element Xo and at least one more element

c. Does not contain the element Xo

d. There is not enough information to determine the composition

= 546 nm = 643 nm

Spectrum of element Xo (at rest) | | |

Spectrum of star A | | | ||

2. From the spectra above, you can conclude that star A

a. Contains the element Xo and only that element

b. Contains the element Xo and at least one more element

c. Does not contain the element Xo

d. There is not enough information to determine the composition

= 546 nm = 643 nm

Spectrum of element Xo (at rest) | | |

Spectrum of star A | | | ||

2. From the spectra above, you can conclude that star A

a. Contains the element Xo and only that element

b. Contains the element Xo and at least one more element

c. Does not contain the element Xo

d. There is not enough information to determine the composition

3. From the spectra above, you can conclude that Earth and star A

a. Are moving toward each other

b. Are moving away from each other

c. There is not enough information to determine the relative direction of motion of Earth and the star.

Blue Shifted or Red Shifted?

= 546 nm = 643 nm

Spectrum of element Xo (at rest) | | |

Spectrum of star A | | | ||

3. From the spectra above, you can conclude that Earth and star A

a. Are moving toward each other

b. Are moving away from each other

c. There is not enough information to determine the relative direction of motion of Earth and the star.

Blue Shifted or Red Shifted?

Moving Away = Red Shifted

= 546 nm = 643 nm

Spectrum of element Xo (at rest) | | |

Spectrum of star A | | | ||

4. From the spectra above, you can conclude that

a. Star A is moving faster away from element Xo than star B

b. Star B is moving faster away from element Xo than star A

c. There is not enough information to determine the relative speed of each star to element Xo.

Both star A and B are moving away from Xo and are red shifted.

Astronomers can calculate the speed from the wavelength difference.

= 546 nm = 643 nm

Spectrum of element Xo (at rest) | | |

Spectrum of star A | | | ||

Spectrum of star B | | | ||

4. From the spectra above, you can conclude that

a. Star A is moving faster away from element Xo than star B

b. Star B is moving faster away from element Xo than star A

c. There is not enough information to determine the relative speed of each star to element Xo.

Both star A and B are moving away from Xo and are red shifted.

Astronomers can calculate the speed from the wavelength difference.

= 546 nm = 643 nm

Spectrum of element Xo (at rest) | | |

Spectrum of star A | | | ||

Spectrum of star B | | | ||

Doppler Effect Summary

https://www.youtube.com/watch?v=h4OnBYrbCjY

Apply the Doppler Effect to Galaxies

What can we learn?

http://nssdc.gsfc.nasa.gov/image/astro/hst_ngc4414_9925.jpg

Fritz Zwicky – 1930s

• Galaxies are moving too fast

• Something holding them together

• Proposed dark matter to explain galaxy motion

http://ned.ipac.caltech.edu/level5/Biviano2/Biviano4_2.html

Vera Rubin – 1960s

• Studied rotational speeds of galaxies*

• Galaxies have bright centers

• Expect most mass at center

• Expect inner stars to move faster and outer stars to move slower

• Based on Kepler’s laws of motion

• Like solar system

http://astro.berkeley.edu/~gmarcy/women/rubin.htmlhttp://nssdc.gsfc.nasa.gov/image/astro/hst_ngc4414_9925.jpg

• Kepler’s laws didn’t work and do not apply to galaxies

• Instead, she applied the Doppler Effect to detect speeds at different places in the galaxies

Vera Rubin – 1960s

• Outer stars orbit about same speed as inner ones (galaxy rotation problem)*

• Lots of mass far from center*

• 90% of mass is unseen = Dark Matter*

Figure 12-11. Horizons by Seeds and Backman

Gravitational Lensing – proposed by

Einstein and Zwicky

From NASA: https://www.universetoday.com/wp-content/uploads/2014/11/Gravitational-lensing-galaxyApril12_2010.jpg

Gravitational Lensing – 1980s

• Gravity bends light*

• Creates multiple images or distorts into arcs

• Confirms dark matter

• Able to see objects behind shining mass*

Hubble Sees a Smiling Lens

https://www.nasa.gov/content/hubble-sees-a-smiling-lens Feb.10, 2015

Lots of Gravitational LensingEarly galaxies, 600 million years after Big Bang

https://www.nasa.gov/feature/goddard/hubble-spies-big-bang-frontiersOct. 22, 2015

Bullet Cluster

• 2 clusters of galaxies moving apart after colliding

• Red: X-rays (colliding gas)

• Blue: dark matter (gravitational lensing)

http://antwrp.gsfc.nasa.gov/apod/ap080823.html

http://www.nasa.gov/mission_pages/hubble/news/dark_matter_ring_feature.html

Dark Matter

Matter “seen” by its gravitational pull (lensing)

• Motions of galaxies within clusters (Zwicky)

• Rotation of galaxies (Rubin)

• Bending of distant galaxy light by

intervening clusters (Gravitational Lensing)

• Collision of two clusters of galaxies

• Planck spacecraft: about 5% common

matter, 25% dark matter, 70% dark energy

Dark Matter: What is it?

We are much more certain what dark matter is NOT

than we are what it is. – NASA

– not visible matter, baryons, antimatter, or black holes

Possibly

• MACHOs

– Neutrino experiments with Fermilab

• WIMPS

• Hypothetical axion subatomic particles

WATCH Astronomy Crash Course #41:

Dark Matterhttps://www.pbs.org/video/crash-course-astronomy-41/

• Next lecture

– Tour of the Planets

Test 2

Tuesday, March 24

•40 multiple choice questions

•Planets Writing Assignment for

20 points

–Write about 2 planets

• Continue flashcards of STUDY POINTS • D2L Quiz 6 open. Quizzes 5-8 for Test 2

• Observations:❑ Moon Phases Due Mar. 5 (10 points) Observe 4 phases & record in table❑ Stargazing (SG) Due May 12 (20 points) Go stargazing & write report❑ Telescope (Tel) Due May 12 (20 points) Look through a telescope ❑ Moon Craters (MC) Due May 12 (10 points) Look at moon craters

• No Lab this week due to Student Conference last Tuesday

• Tutor Oskar in T3200 Mon, Wed, Fri – class website has hours

Homework & Observations