NGC 7635 is commonly known as the Bubble Nebula and is located approximately 7,800 light-years distant in the constellation Cassiopeia.  The nebula was formed by a very massive star designated BD +60•2522.  This star is estimated to be about 40 times as massive as our Sun.  Its huge energy emissions and prodigious stellar winds have blown a bubble of ionized gas approximately 6 to 10 light-years in diameter.  This very active star is of a stellar class known as Wolf-Reyet stars.  These stars are in the final stage of their lives and are rapidly expelling their outer layers in an extremely powerful and high velocity stellar wind that can exceed 1500 kilometers per second.  When this wind rams into a surrounding interstellar gas cloud it creates and expanding shock front we observe as a ring or bubble.  This process will continue until the star finally ends its violent life in a supernova explosion.

Located in the upper left of the image is the open cluster M52.  It is actually a foreground object and not associated with the Bubble Nebula.  The cluster is approximately 5,000 light-years from the Earth.

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.

A near true color version (Ha, OIII, OIII) of the image may be viewed here.

¹http://www.robgendlerastropics.com/Bubbletext.html