Q:  What is the radial velocity of Arcturus?
      Is it coming towards us, or going away from us?


         Look at the line which appears in the Sun at about 882.4 nm.
         In the spectrum of Arcturus, it appears at about 882.55 nm.

            shift (Arcturus - Sun)  =  882.55 nm - 882.4 nm  

                                    =  0.15 nm


                                  0.15 nm
            radial velocity  =  ------------- * (300,000 km/s)
                                  882.4 nm
     

                             =  50 km/s


         So Arcturus was moving AWAY from us at about 50 km/s
         when the spectrum was taken.