Mother sudoku explained

I presented a puzzle on 26/01/2013 titled mother sudoku  .http://puzzlebrainex.blogspot.in/2013/01/mother-sudoku.html. The puzzle asked to establish relation between magic square and sudoku. Now I present the solution. Put the digital roots of the numbers in each cell. See what comes!

 

addition 6

In the addition below all figures have been replaced by asterisks.Given that each figure is one of 1,3,7 and 9.

Also, each figure appears thrice.

    *  *  *

+  *  *  *

+  *  *  *

=   *  *  *

 Can you find the sum?