10/25/2010AST210, Fall 2010 1Measuring Motion, Doppler Effect—25 Oct• Review Doppler effect – Doppler effect is the primary method for measuring speed of astronomical objects.• Pickering’s spectra of Mizar– What did Pickering discover?• Modern cosmology begins in the “Realm of Nebulae” (galaxies)– Modern cosmology starts on Wed with Hubble’s Law• Homework 6– Due Mon, Nov 1.– If you downloaded from angel, check that the date is 2010, not 2009.• Astronomical Horizons– Thurs, October 28, 7:30, Abrams Planetarium– Mars Meteorites: Rock Messengers from the Red Planet– Prof. Michael Velbel10/25/2010 AST 210, Fall 2010An element’s fingerprint• Spectral lines are an element’s finger print. (5.4 in textbook)• Eg, in the visible part of the spectrum, hydrogen emits and absorbs light at 656.2, 486.1, 434.0, 410.1nm.10/25/2010 AST 210, Fall 2010SodiumHydrogenCalciumMercuryNeonEmission spectra of hot gasses10/25/2010AST210, Fall 2010 2Doppler eff ect: Summar y• Doppler effect: Motion is encoded in the wavelength of light– Observe wavelength observedof a spectral line from a star.– Measure wavelength rest of same line in the lab, where the source is not moving.observed/ rest =1+v/c– v is speed, positive if star is moving away from us.– c is speed of light.• If motion is perpendicular to the line of sight, there is no change in wavelength.– In the formula, v is the component of the velocity towards or away from the observer.1. Is it possible for a star to be moving and not show a Doppler shift? ___, and this is an example.A. YesB. No10/25/2010 AST 210, Fall 2010Pickering’s discovery• We are interpr eting E. C. Pickering’s spectra of Mizar (a star in the Big Dipper) in 1889.– Spectra showing the H line of hydrogen.– These are absorption spectra: The amount of light is high except at wavelengths where hydrogen absorbs.• Describe the changes in the spectra. (Spectra repeat.)– A single wavelength is seen on 1 Oct.– Two wavelengths are seen on 2 Oct– Separation of two wavelengths grows larger, then smaller.10/25/2010 AST 210, Fall 201010/25/2010AST210, Fall 2010 3Pickering’s discovery1. How can the spectral line of hydrogen appear at different wavelengths?A. The star is moving.B. Hydrogen emits at different wavelengths at different times.C. There was something wrong with Pickering’s spectrometer.D. Some other reason.10/25/2010 AST 210, Fall 2010Pickering’s discovery1. Devise a model for Mizar that explains the data. How can a star move at two speeds?10/25/2010 AST 210, Fall 201010/25/2010AST210, Fall 2010 4Pickering’s discovery• Provisional model: Two stars are in orbit. Test against evidence.1. How can the two stars move so as to show the same wavelength, for example, as on Oct 1? (If possible, you want explanations that do not depend on special accidents.)A. The stars move in the same direction at the same speed on Oct 1.B. The Doppler effect is insensitive to the orbital motion on Oct 1.C. One star hides the other on Oct 1.10/25/2010 AST 210, Fall 2010Pickering’s discovery• Provisional model: Two stars are in orbit. Test against evidence.1. What is the wavelength of the H line of hydrogen emitted by star A on 12 Oct?A. 4860.18Å.B. 4862.17Å.2. What is the relative speed of stars A & B on 12 Oct? Hint: only one has the right units.A. (1.99Å)/(4861Å)B. (1.99Å)/(4861Å)(300,000km/s)C. (1.99Å) (300,000km/s)• Relative speed is 120km/s.10/25/2010 AST 210, Fall 201010/25/2010AST210, Fall 2010 5Pickering’s discovery1. What evidence tells you whether the binary star system is moving toward or away from us?2. What is v/c, where v is the velocity of the binary star system? A. 0.17 / 4861.13 = 3.5105.B. 0.17 / 4861.3 = 3.5105.C. 4861.13 4861.3 = 0.17D. 4861.3 4861.13 = 0.17E. 0.17 / 4861.3 = 3.5105• Pickering discovered the first binary star where the evidence was in the spectra, called a “spectroscopic binary star.”10/25/2010 AST 210, Fall
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