NGC 7635 - Bubble Nebula

The Bubble Nebula was discovered in 1787 by William Herschel in the constellation of Cassiopeia.  The nebula is being shaped by fierce stellar winds emanating from the inner hot stars, one in particular that has generated the bubble itself.  An extremely bright, massive and short-lived O-type star only 4 million years old has lost most of its outer hydrogen layer and is fusing helium into heavy elements.  As the energized gases escape from the star at over 4 million mph, a spherical shock front forms as collision occurs with cooler  surrounding nebular gases.  Oxygen gas is heated enough nearest the star to emit blue light (doubly ionized oxygen) while the remainder of the nebula glows predominantly in red and yellow, colors mapped here to HII and SII emission.  The star appears off center in the bubble (12 O'clock position) as the shock front encounters greater gas density in one direction than the other.