In the 1930's, Edwin Hubble discovered that all galaxies have a positive redshift. In other words, all galaxies were receding from other galaxies. We live in a dynamic, expanding Universe.

Suppose the galaxy was not moving away or toward  the earth and emitting light.  If the galaxy started moving we could check the wavelength of the light and the wavelength will appear longer than usual.  The longer wavelength is known as redshifted because the visible light, their wavelengths get closer to the wavelength of red light.

The mathematical value that relates the speed at which the galaxy is moving away to the amount of increase we see in a particular wavelength of light include the redshift.
 

    After we determine the redshift, we ca do the following calculation to find the radial velocity of the galaxy. We will use 300,000 km/sec for the speed of light
 

Scientists look for a wavelength of light that they know is emitted by a certain type of atom.  They know what the wavelength would be if the atoms of this type were not moving.  They measure the wavelength of the light from the atoms of this type in the moving galaxy.
                1.  measure the increase in wavelength
                            increase in l = measured wavelength - original wavelength
            2.  calculate the redshift
                            redshift   =    increase in wavelength
                                                        original wavelength
                3.  Find the radial velocity
                            radial velocity  =    redshift X speed of light

Measurements are often reported as redshifts of a certain value.  An astronomer knows that the wavelength of light from a certain atom is 500 nanometers. She measures the wavelength of light from the that type of atom in a moving galaxy to be 510 nm.  What is the radial velocity of the galaxy?
 
 

                        increase in wavelength = 510 nm - 500 nm
                                                             = 10 nm

                        Redshift = 10nm /500nm
                                          = 0.02

                        Radial velocity = 0.02 X 300,000 km/sec
                                                 = 600 km/sec

See if you can find the radial velocity of several galaxies.

1.  Suppose a wavelength of light is 600 nm and the measured wavelength for that light from a moving galaxy is 605nm.  What is the radial velocity of that galaxy?
 
 
 

2.  Suppose a wavelength of light is 388.9 nm and the measured wavelength for that light from a moving galaxy is 390.4 nm.  What is the radial velocity of that galaxy?
 
 

3.  Suppose a wavelength of light is 154.9 nm and the measured wavelength for that light from a moving galaxy galaxy is 159.4 nm.  What is the radial velocity of that galaxy?
 
 

4.  An astronomer reports a galaxy with a redshift of 0.025.  What is the radial velocity of that galaxy?
 
 
 

5.  An astronomer reports a galaxy with a redshift of 0.001.  What is the radial velocity of that galaxy?
 
 
adapted from Mark Twain Media, Inc., Publishers