Relativistic Doppler Effect

The above simulation represents part of the absorption spectrum from the bright star Vega in the constellation Lyra located about 25 light years from Earth.  (The wavelength, in nm, may be obtained by a mouse-down in the spectrum.)  For simplicity only the H-alpha line in the Balmer series is shown.  In general, there are a lot of stars in our galaxy that are blue-shifted. However,  the great majority of galaxies are red-shifted.  This leads astronomers to the conclusion that the universe is expanding.    

  1. Determine the velocity of Vega relative to Earth as a fraction of the speed of light.

Side note:  Since Earth's axis is slowly changing its orientation, Vega will be the North Star in ~ A.D. 14,000!

  1. The galaxy called Ursa Major I, located a little over a billion light years from Earth, is moving at a much higher speed.  Calculate the relative velocity of this galaxy.  Note the change in wavelength scale.   Is the galaxy moving away from or toward the Milky Way?

Read the sections in your text on cosmology and the Hubble parameter to discover more about the recession of objects in our universe.

Laboratory Spectrum Vega Spectrum Ursa Major I Spectrum