Ursa Major Solar has a lookup relationship between a custom Galaxy _c object and a...

The roll-up summary can be achieved by creating a formula field on the Galaxy__c object.

The lookup relationship will need to be converted to a master-detail relationship before a roll-up summary field can be created.

The roll-up summary can be achieved by creating a formula field on the Star__c object.

A roll-up summary field will need to be created on the Galaxy__c object with a field filter that select all related Star_c records