In strong aggregation a part entity cannot exist without the containing whole entity in the whole-part relationship.
For example, university has multiple departments. This a composition relationship between the university and department entity. A department cannot exist on its own with out the university.
Composition is used for aggregations where the life span of the member object depends on the life span of the aggregate.
The aggregate not only has control over the behavior of the member, but also has control over the creation and destruction of the member. In other words, the member object cannot exist apart from the aggregate.
This greater responsibility is why composition is sometimes referred to as strong aggregation. Composition is a subset of aggregation, just as aggregation is a subset of association.