Title Bar - Cosmology

Activity: Everyday Doppler Shift

For a simple outdoor activity which analyzes everyday Doppler Shift devised by Kara C. Granger see http://www.astrocappella.com/activities/dopp_lp.html.

Fortunately for astronomers, the information that they receive from stars in space is in the form of electromagnetic radiation footnote17 , which is a fancy word for what people experience as light. Like sound, light changes "pitch" when the object that emits it moves towards the observer. This change in "pitch" is actually a change in color, so that as a star, moves towards an observer, the light it emits becomes bluer and as it moves away from the observer it becomes redder. This change in color is called a "shift": a "blue shift" when the object is moving towards the observer, a "red shift" when the object is moving away. In order to explain why this is called a shift, we must look at a spectrum. A common example of a spectrum is the rainbow that comes out of a prism, in which the white light spreads out into a rainbow so that we can see each color. Similarly, the light emitted by a star can be split into component colors so that we can examine each part. Each star has a specific set of features, like swirls or loops in finger prints, that occur at certain wavelength (colors). These features are unique to each star. When the spectrum is shifted, however, the features appear to the right or left of their expected location depending whether the shift is red or blue.

There are several different ways in which a spectrum can be represented. The Absorption Line Spectrum looks like a rainbow in which the "features" appear as black lines at certain frequencies. In the Emission Line Spectrum, the features appear in color and the rest is dark. The Spectral Line diagram is a sort of squiggly line that shows the amount of energy at each particular wavelength. Here are examples of each of these types of spectra:

Absorption Lines
Absorption lines

Emission Lines
Emission Lines

Spectral Lines with Absorption Lines
Spectral Lines and how they correspond to Absorption lines

These spectral diagrams may be complicated but once a student understands electromagnetic radiation and the spectrum it produces, a red shift of this spectrum can easily be used to measure distances. For an online introduction to light and spectrum in astronomic measurement see http://violet.pha.jhu.edu/~wpb/spectroscopy/spec_home.html.

As mentioned above, the red shift occurs when the light emitting object is moving away from the observer; in this case, the star is moving away from Earth. A famous astronomer, Edwin Hubble (for whom the Hubble Space Telescope is named), discovered footnote21 that the red shift is related to the distance that the moving object is from the observer in the following way:

(Redshift * Speed of light)/(Hubble Constant) = Distancefootnote22.

To calculate the red shift, one must measure the distance that the spectrum has been shifted. Usually this is done by comparing the observed spectrum to one a standard. The formula for red shift is:

Redshift = (The Shift in Wavelength)/(The Original Wavelength).

For example, if a feature on a spectrum is normally measured at 393.3 x 10-9 m and then that feature in a galaxy is measured at 401.8 x 10-9 m , the red shift of that galaxy would be:


and the speed at which the galaxy was moving away from Earth would be:

6480 km/s


Then the distance to the galaxy would be:

130 Mpc

Back | Next

Back Next Table of Contents Click Here for details about distributing the Materials in the Teacher's Guide.