Spectra, Absorption, and the Red Shift


    The image above or at left is the spectrum of light coming from the sun. The spectrum begins in the upper left corner of the image, goes across to the right, resumes on the left and goes to the right, and so on down through the image. Thus the spectrum that you're looking at would be about 15 feet long if we could attach all the horizontal strips end to end. It extends from longer wavelengths (redder colors) to shorter wavelengths (bluer colors).

     Note that the spectrum is not continuous - it has dark gaps in it (white arrows mark two of those gaps). The gaps occur because there are gases around the sun, and those gases have absorbed certain wavelengths of light. In fact, we can tell what elements are in gases around distant stars or in distant nebulae because different elements absorb different wavelengths of light.

     When we observe the light coming from distant galaxies, we find that the gaps caused by adsorption aren't in the same places as they are in the spectrum above. Instead, the gaps have the same spacing relative to each other, but they are shifted in the red direction (to the left in our example). This is what we call a "red shift", because the wavelenghts of the adsorption bands (the gaps) and of the surviving light (the colored intervals in the spectrum) are shifted toward the red end of the spectrum. Take a look at an animation of the red shift.

     In the two illustrations above, spectra are shown as bands of color. We can also depict spectra as a series of peaks, as in the third illustration. In this case, the peaks represent wavelengths adsorbed by elements. The lower part of the third illustration shows the spectrum after a red shift of the spectrum in the upper part.

     This red shift is one of our keys to understanding the motion of galaxies. A red shift is an apparent lengthening of wavelength, which happens when the light source moves away from the observer. In a sense, the light is "stretched" as the source moves away from the observer, so that its apparent wavelength is longer (or shifted toward the red end of the spectrum). The red shift that we observe in light from distant galaxies tells us that those galaxies are moving away from us.



The image above is from NOAO, and it was featured on the Astronomy Picture of the Day for August 15, 2000.  

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