Minimum number of supernodes such that all other nodes are connected by supernodes

Let's say I have graph, in that graph I have to mark some nodes as supernodes such that all other nodes can have direct access this supernodes (nodes which are supernodes have access to it's own).

Let's say based on image my supernodes are 1,3,5 as all other nodes are connected by it.

My question is how to find minimum number of such supernode?

Thanks.