Start by looking for Thomas Holmgreen. You didn't say if he was in the US or Norway.
I entered the name Thomas Holmgreen at the free databases of the website familysearch.org
It finds Thomas Holmgreen living with his parents in 1880 living in Jackson Co. Wisconsin with his parents. That census gives their ages and their birthplaces. Her name isn't given as Helga but that not surprising for a Norwegian emigrant. People adapted their names to suit the new location.
Try it -- you'll have more information to start the search with.