THE DOPPLER EFFECT

# THE DOPPLER EFFECT - companion Note that this method does...

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THE DOPPLER EFFECT It is understood that this absorption of light occurs because of gases in the outer atmosphere of the star. Each element in the atmosphere absorbs a particular set of wavelengths, and the exact value of these wavelengths (for hydrogen, helium, nitrogen, even for elements like iron and sodium that are vaporized at star temperatures) is known. Now if the star is moving toward the earth these lines are blue shifted; if the star is moving away the lines are red shifted. From the amount of the shift we can determine the speed of that motion. Figure 1 shows the geometry of this method. T he star's straight line path is along the y-axis. The line from the earth to the star is along the x-axis. And the path wobbles left and right, toward and away from the earth. From the variation in the wavelength, changing periodically from the blue shift to the

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red shift and back, we deduce the orbit of the star, and hence the mass of the invisible
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Unformatted text preview: companion. Note that this method does not depend on light from the star going in an absolutely straight line as it comes to the observer, and so it works for an observer based on the earth's surface. Note that the Doppler method and the astrometric method allow you to look at two different kinds of orbital motion. Suppose in the same figure the straight path of the star is still in the +y direction, but the wobble is not from side to side, but up and down. (Let there be a z-axis coming up out of the paper, and suppose the wobbly motion is alternating plus z to minus z to plus z, etc. In this kind of motion the star would not be moving closer or further from the observer, and so there would be no Doppler shift. But the observer (at least above the atmosphere) would "see" a wobbly path. Figure 2 shows the same thing from another perspective:...
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THE DOPPLER EFFECT - companion Note that this method does...

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