“The bright stars in NGC 604 are extremely young by astronomical standards, having formed a mere 3 million years ago.”
Nebulas are giant clouds of dust and gas where infant stars are born. These are known as star nurseries.
In some cases, nebulas are cast out into space when a dying star goes supernova.
Most of the brightest and hottest stars in a nebula form a cluster within a cavity near the cloud’s centre.
Stellar winds churned out by these blue stars, together with supernovas, are responsible for these cavities.
source: express.co.uk