Star notes (from what I understand) are 'replacement notes'. The mint has destroyed the original (whether it was defaced or torn or an error while printing,etc.) Therefore they must create a new bill with the same serial number with the exception that the last digit (usually a letter) is replaced with a star (*). While quite often the bill could be worth a bit more than face value, it depends on the condition of the bill (as others have stated before me), and in some cases the serial number. ie. say a $10 bill from 1950 with the serial number BF4012582D was so badly damaged while it was still in production, it would be destroyed by the mint. It would then be replaced with BF4012582*.