spectroscopy

# spectroscopy - Lecture Outline Spectroscopy(Ch 4 NOTE These...

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Lecture Outline: Spectroscopy (Ch. 4) NOTE: These are just an outline of the lectures and a guide to the textbook. The material will be covered in more detail in class. We will cover nearly all of the material in the textbook, but in a somewhat different order. First, we consider a property of wave motion, the Doppler effect , that allows us to determine how fast something is moving. If you had to pick one phenomenon associated with light that has allowed us to learn about the universe, it is the Doppler effect. Late in the course we’ll see how it reveals that the entire universe is in a state of expansion, and allows us to obtain the distance to a galaxy billions of light years away simply from the wavelength of a spectral line that has been affected by the Doppler effect.

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Doppler effect The Doppler effect is one of most useful and important techniques used in all of astronomy. We will encounter it again and again. It basically recognizes that The wavelength (or frequency) of a wave, as measured by an observer, depends on the relative radial speed of the source and observer. Radial motion means: motion towards or away; along the line of sight. The Doppler effect involves only this component of motion. Moving away: wavelengths increase (“ redshift ”) Moving toward: wavelengths decrease (“ blueshift ”) Shift in λ radial velocity this is how we get speeds of cosmic objects, stars, galaxies, even expansion of universe. Actual formula is: λ (apparent)/ λ (true) = 1 + (vel./speed of light) For most objects in the universe, this relative shift is tiny (think about it while looking at the formula!), so we can’t detect it using the “shift” of the whole spectrum. But we can use places in the spectrum whose wavelengths are precisely known spectral lines (the subject of Chapter 4)
Spectral lines—very narrow, well-defined (in wavelength) wavelength/frequency regions in the spectrum where excess photon energy appears ( emission lines ) or else where photons are missing ( absorption lines ). Often these lines are superimposed on a smooth, “continuous” spectrum, which is the near-blackbody emission of a heated object that we have been discussing so far (ch. 3, Wien, Stefan-Boltzmann). The “continuous spectrum” of an object has properties that are controlled only by its temperature (recall Wien’s, Stefan-Boltzmann laws). Look again at this plot of the use of the continuous spectrum as an “astronomical thermometer:” Now compare to the spectrum of an actual star, or planet, or comet, or just about anything. The first thing you notice are spectral absorption lines in stellar spectrum. These are wavelenths in the spectrum where light is “missing.” It will turn out that the answer to the question “What could be causing the photons to be missing?” will lead us to ways to diagnose a star’s properties, in particular the abundances of the elements.

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## This note was uploaded on 04/20/2008 for the course AST 301 taught by Professor Harvey during the Fall '07 term at University of Texas.

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spectroscopy - Lecture Outline Spectroscopy(Ch 4 NOTE These...

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