A star cluster shines in this stellar image from the European Southern Observatory (ESO). The region in question can be found approximately 5,500 light-years from Earth in the Serpens constellation.
Called NGC 6604, the cluster lurks within close proximity to the Eagle Nebula, which is, by far, the larger and more famous of the two. In fact, some might say NGC 6604 is pretty obscure. Contrarily, it contains nearly one hundred very bright stars, seen toward the upper left, that hang near the top of of the size spectrum.
The cluster is certainly impressive in its own right, but it is truly set apart by the surrounding nebula. It forms when the hot, blue-white stars expel large amounts of ultraviolet radiation via stellar winds; when this radiation comes in contact with hydrogen gas content, electrons are knocked clear of their atoms for a brief span of time, before they rejoin. This process is called ionization, and it ultimately generates emission nebulae, which are often accompanied by dark nebulae as well.
According to the ESO,
See a larger image here.