Why would there be cause for concern if a young pregnant mother is rh-, her husband is rh , and this is their second child?

Because of a condition called eythroblastosis fetalis.
In this condition, the mother immune system attack the baby red blood cell destroying them. During her first child is when she is immunized against the red cell, the second child is when the attack going to happen

1.989×10^30 kg is the weight of the sun.