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Doppler Effect and Dark Matter What Can You Learn from Spectra?

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Doppler Effect and Dark Matter What Can You Learn from Spectra? 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 Frequency Time Time Volume of car Frequency of car horn over time* 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” 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 Red end of spectrum 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” = 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 = 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? = 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 | | | || 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 | | | || 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. 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 • Kepler’s laws didn’t work • Based on Kepler’s and do not apply to galaxies laws of motion • Instead, she applied the Doppler Effect to detect • Like solar system speeds at different places in the galaxies http://astro.berkeley.edu/~gmarcy/women/rubin.html http://nssdc.gsfc.nasa.gov/image/astro/hst_ngc4414_9925.jpg 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.
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