Q: What is the angular size of a sun-like
     star at a distance of 10 pc?
     Express your answer in degrees, arcseconds,
     and milli-arcseconds (mas).


       The sun's radius is about 700,000 km.
       At a distance D = 10 pc = 3 x 10^(14) km,
       that would subtend an angle of

                                700,000 km    
         half-angle  = atan ( ---------------- )
                               3 x 10^(14) km


                     =  1.3 x 10^(-7) degrees
  
                     =  3.8 x 10^(-4) arcseconds

                     =  0.00038 arcseconds

                     =  0.38 milli-arcseconds


       Remember that ordinary pictures taken from 
       the Earth's surface have a typical resolution of
       about 1 arcsecond.