M8 The Lagoon Nebula Credit Michael Miller Jimmy Walker Course Announcements Quiz 2 will take place Monday 19 September A few hints today Error found in Question 16 in Practice Questions for Quiz 2 Exam 1 Problem corrected If you downloaded yesterday or this morning please download the new version Exam 1 will take place Wednesday 28 Sept Will cover Chapters 1 4 and perhaps Chapter 5 inclusion and how much of depend on how quickly we get through Chapter 4 Assignments Reading Assignments Chapter 4 Sections 4 4 and 4 5 Read for Monday 19 Sept Parallel Lectures No new parallel lectures Mastering Astronomy Chapter 3 Homework Due Monday 19 Sept at 11 59 PM EDT Quiz 2 Hints Know the difference between prograde slightly more eastward relative to the stars night after night and retrograde slightly more westward relative to the stars night after night apparent motion of planets Do the practice problem set I put on Blackboard The Doppler Effect How the frequency wavelength of a wave changes with respect to relative motion Familiar Case Sound waves from an emergency vehicle Toward Waves get compressed Shorter Wavelengths bluer Away Waves get stretched Longer Wavelengths redder Not Moving Case Moving at 70 wave speed Wave fronts in stationary case Object moving to right appears to compress the wave as it is moving in the same direction as the wave The Doppler Effect How the frequency wavelength changes with respect to relative motion If one is moving toward an object the wavelengths seem shorter higher frequency bluer If one is moving away from an object the wavelengths seem longer lower frequency redder The Doppler Effect How it works In this example the object is moving and the observer is not moving The Doppler Effect depends on relative motion Motion needs to be radial motion toward away Side to side motion not toward nor away is called transverse motion No Doppler Effect No Relative Motion Object Moving to Right Measuring Velocities with Doppler Effect Example If an object is receding moving away at Earth s orbital velocity of 30 km s we can calculate the observed change in wavelength change in wavelength recession velocity true wavelength wave speed 30 km s 0 01 percent 300 000 km s If that object is giving off a beam of blue light with a wavelength of 400 00 nm we would measure that light as having a wavelength of 400 04 nm A tiny but easily detectable change Doppler in practice Determining moving toward or away from Earth Consider a person moving holding a green laser pointer 532 nm The case of a non Doppler shifted wavelength of light is called the rest wavelength Example 1 You measure the wavelength of the laser to be 530 nm instead of the rest wavelength of 532 nm Is the person moving toward or away from you Answer Example 2 You measure the wavelength of the laster to be 540 nm instead of the rest wavelength of 532 nm Is the person moving toward you or away form you Answer Bonus High difficulty How fast would the person have to be moving away from you to shift the color of the laser to red light at 650 nm Answer Doppler in practice Determining moving toward or away from Earth Consider a person moving holding a green laser pointer 532 nm The case of a non Doppler shifted wavelength of light is called the rest wavelength Example 1 You measure the wavelength of the laser to be 530 nm instead of the rest wavelength of 532 nm Is the person moving toward or away from you Answer Toward you 530 nm 532 nm blue shifted Example 2 You measure the wavelength of the laster to be 540 nm instead of the rest wavelength of 532 nm Is the person moving toward you or away form you Answer Away from you 540 nm 532 nm red shifted Bonus High difficulty How fast would the person have to be moving away from you to shift the color of the laser to red light at 650 nm Answer Recession Vel c 650nm 532nm 532nm 0 22c or 66 500 km s Measuring Velocities with Doppler Effect Such small changes might seem insignificant but it s how a Radar Gun works and it is how we detect the motions of stars due to extrasolar planets Exoplanets Spectroscopy CHAPTER 4 Spectroscopy What happens when they interact Carbon Atom Red Light The study of the interaction between light and matter Spectroscopy is the study of the interaction between light electromagnetic waves and matter atoms molecules etc This interaction is depends strongly on the type of matter specific atom or molecule and the wavelength or frequency of light So to study it we take part of the Electromagnetic Spectrum i e Visible Light and break it into it s component wavelengths colors The tool we use to measure the Intensity Brightness of each wavelength color is called a spectrometer Brightness as a function of wavelength we saw this with the Blackbody Curve Spectrometer An instrument to measure how bright each wavelength of light is The Tool Spectroscope Spectrograph Spectrometer 6 4 Dispersion Element 3 Narrow Slit 5 Focusing Element 2 1 Star Planet etc 2 Focusing Element 1 In this case Blackbody curve You don t need to memorize the parts of a spectrometer Types of Spectra 1 Continuous Spectra Created by a blackbody A sufficiently dense object e g Star planet light bulb filament hunk of iron very dense gas 2 Emission Spectra Distinct lines of color in emission 3 Absorption Spectra Distinct lines of color removed absorbed A Blackbody Spectrum a continuous spectrum Types of Spectra Emission and Absorption Spectra and Lines Blackbody Emission Lines Absorption Lines Emission Spectra Emission from a hot gas Spectrum from a hot diffuse very low density gas Emission Spectrum Hot Diffuse Gas Emission Spectra Every element has unique signature of emission lines Absorption Spectra Occur when continuous emission passes through a cool gas Spectrum from viewing a continuous source through a cool diffuse gas Hot Continuous Source Absorption Spectrum Cold Gas Absorption Spectra Again every element has a unique set of absorption lines Hydrogen Carbon Oxygen Nitrogen Every element has unique set of lines Look like continuous spectra with specific wavelengths removed Emission and Absorption Spectra Emission and Absorption Lines occur are precisely the same wavelengths Emission top and Absorption bottom spectrum for hydrogen H Every element has a unique set of spectral fingerprints Emission and Absorption Spectra Emission and Absorption Lines occur are precisely the same wavelengths Emission top and Absorption bottom spectrum for hydrogen H Every element has a unique set of spectral
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