Download Doppler Effect

Objectives
 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.
 Explain why the Doppler Effect occurs.
 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?
Doppler Effect
and
Dark Matter
What can you learn from spectra?
•
•
•
•
•
Speed
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
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.
What happened to the VOLUME of the horn?
1. sounded louder and louder as the source
approached and sounded fainter and fainter as
the source receded
2. stayed at a constant loudness as the source
approached then dropped to a fainter but
constant loudness as the source receded
3. stayed at the same constant loudness
throughout the motion
4. varied too much to tell what the volume was
doing
What happened to the VOLUME of the horn?
1. sounded louder and louder as the source
approached and sounded fainter and fainter as
the source receded
2. stayed at a constant loudness as the source
approached then dropped to a fainter but
constant loudness as the source receded
3. stayed at the same constant loudness
throughout the motion
4. 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?
What happened to the PITCH (frequency) of the horn?
1. became higher and higher as the source approached
and became lower and lower as the source receded
2. stayed at a constant high pitch as the source
approached and then dropped to a constant lower pitch
as the source receded
3. stayed at the same constant pitch throughout the motion
4. stayed at the same pitch except at the moment the
source passed
5. varied too much to tell what the pitch was doing
What happened to the PITCH (frequency) of the horn?
1. became higher and higher as the source approached
and became lower and lower as the source receded
2. stayed at a constant high pitch as the source
approached and then dropped to a constant lower pitch
as the source receded
3. stayed at the same constant pitch throughout the motion
4. stayed at the same pitch except at the moment the
source passed
5. varied too much to tell what the pitch was doing
Frequency
Volume
Time
Volume of car
horn over time
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
(
(
(
←wave moves
Source
( * )
vsource = 0
)
)
)
)
wave moves→
If source is stationary
(
(
(
←wave moves
Source
( * )
vsource = 0
)
)
)
)
wave moves→
If source moves
Source

(
(
*
Longer λ
Lower f
Red Shift
) ) ) ) )
Shorter λ
Higher f
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
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”
Standard Emission Spectra for comparison
Absorption Spectra lines of star
Earth 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”
Red shift
Blue shift
Moving away from Earth = Red shift
Ex: “Earth’s Orbital Speed”
Is “b” red shifted or blue shifted?
Standard
Emission
Spectra
Standard
Emission
Spectra
Ex: “Earth’s Orbital Speed”
Absorption Spectra lines of star
Earth is at “B”, so star is given designation “b”
Red shift
Blue shift
Ex: “Earth’s Orbital Speed”
Spectrum of element Xo (at rest)
Spectrum of star A (at rest)
 = 546 nm

|
||
|
||
 = 643 nm

||
From the spectra above, you can conclude that star A
1. Contains the element Xo and only that element
2. Contains the element Xo and at least one more element
3. Does not contain the element Xo
4. There is not enough information to determine the composition
Spectrum of element Xo (at rest)
Spectrum of star A (at rest)
 = 546 nm

|
||
|
||
 = 643 nm

||
From the spectra above, you can conclude that star A
1. Contains the element Xo and only that element
2. Contains the element Xo and at least one more element
3. Does not contain the element Xo
4. There is not enough information to determine the composition
Spectrum of element Xo (at rest)
Spectrum of star A
 = 546 nm

|
||
|
||
 = 643 nm

||
From the spectra above, you can conclude that star A
1. Contains the element Xo and only that element
2. Contains the element Xo and at least one more element
3. Does not contain the element Xo
4. There is not enough information to determine the composition
Spectrum of element Xo (at rest)
Spectrum of star A
 = 546 nm

|
||
|
||
 = 643 nm

||
From the spectra above, you can conclude that star A
1. Contains the element Xo and only that element
2. Contains the element Xo and at least one more element
3. Does not contain the element Xo
4. There is not enough information to determine the composition
Spectrum of element Xo (at rest)
Spectrum of star A
 = 546 nm

|
||
|
||
 = 643 nm

||
From the spectra above, you can conclude that Earth and star A
1. Are moving toward each other
2. Are moving away from each other
3. 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
Spectrum of element Xo (at rest)
Spectrum of star A
 = 546 nm

|
||
|
||
 = 643 nm

||
From the spectra above, you can conclude that Earth and star A
1. Are moving toward each other
2. Are moving away from each other
3. 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
Spectrum of element Xo (at rest)
Spectrum of star A
Spectrum of star B
 = 546 nm

|
||
|
||
|
||
 = 643 nm

||
||
From the spectra above, you can conclude that
1. Star A is moving faster away from element Xo than star B
2. Star B is moving faster away from element Xo than star A
3. 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.
Spectrum of element Xo (at rest)
Spectrum of star A
Spectrum of star B
 = 546 nm

|
||
|
||
|
||
 = 643 nm

||
||
From the spectra above, you can conclude that
1. Star A is moving faster away from element Xo than star B
2. Star B is moving faster away from element Xo than star A
3. 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.
Doppler Effect Summary
• https://www.youtube.com/watch?v=h4OnB
YrbCjY
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
• 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.html
http://nssdc.gsfc.nasa.gov/image/astro/hst_ngc4414_9925.jpg
Vera Rubin – 1960s
• Instead, 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
http://teacherlink.ed.usu.edu/tlnasa/reference/ImagineDVD/Files/imagine/d
ocs/features/news/grav_lens.html
Gravitational Lensing – 1980s
• Gravity bends light
• Creates multiple images or distorts into arcs
• Confirms dark matter
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, not baryons, not antimatter,
not black holes
Possibly
• MACHOs or WIMPS
Soudan Underground Mine State Park in
Tower-Soudan, MN – neutrino experiments
with Fermilab (outside Chicago)
Cosmos: A Spactime Odyssey (2014)
with Neil deGrasse Tyson
Episode 13: Unafraid of the Dark
Journey into the unknown forces of the
universe.
Homework & Observations
• Observations
• Moon Phases – Due Thursday, Mar. 2
• Telescopes, Star Gazing & Moon Craters – See Calendar
• Jackson, Eisenhower, Bell/UMN, Baylor, Bell & Baylor
• Keep up with Objectives at the beginning of each lecture
• Only use the textbook as a reference to answer objectives
• No MCTC Classes: (does NOT affect astronomy lecture
or lab)
• Wednesday, Feb. 22 for Faculty Development
• No Astronomy Office Hours this day
• LAB Dimensional Analysis & Significant Figures Quiz
• Feb. 21 or 23 depending on your lab section & day
• Do prep work for Lenses & Telescopes