A giant star; (loosely) any giant star, bright giant, supergiant, or hypergiant.
The greater an object's mass, the greater its gravitational force. If a star could attain a certain mass --- say a million to 100 million times the mass of our own sun --- then its gravity would become 100 times as powerful as the forces generated by the strongest known nuclear reactions. Such a super superstar, in a normal course of its evolution, would contract.
Toward the centre of the cluster, where they were more strongly congested, the whole region glowed as though as it were a monstrous superstar.