How many years after Eta Carinae was first discovered to be unusual did it brighten to magnitude −1.5?
return Eta Carinae was  first discovered ;return year of #1 ;return Eta Carinae did brighten to magnitude −1.5 ;return year of #3 ;return difference of #2 and  #4
['select', 'project', 'select', 'project', 'arithmetic']