NGC 7538 is an active star forming dense nebula located in the constellation Cepheus approximately 9,000 light-years from the Earth. This remarkable nebula is located very near the more famous Bubble Nebula. Perhaps this is the reason many amateur astronomers do not observe or image it as often as it should be. This is not the case for professional astronomers as NGC 7538 is the object of much attention and research. (One such research paper may be viewed here.) The nebula is the home to many actively forming very massive stars. One of the largest, yet discovered, protostars in the Milky Way is located in the nebula. This protostar region is about 300 times the size of our solar system and is thought to be the birth place for several super massive stars. Many shock waves permeate the nebula and they are thought to be the ignition triggers for new stars and clusters of stars.
The image above is known as a mapped, or false, color image and was acquired using narrowband filters. It was assembled using the standard Hubble palette with SII mapped to Red, Ha mapped to Green and OIII mapped to Blue. The stars were overlaid with color data from a separate RGB image.