IC 1396 is a large HII region in the constellation Cepheus spanning 3 full degrees of winter sky, the same angular distance of 6 full moons. It formed at the southern edge of an enormous 400 light year bubble of molecular gas known as the Cepheus bubble. Amazingly the IC 1396 complex is illuminated by a single massive star, the class O6 star HD206267, a blue supergiant located in the center of the donut shaped emission cloud. HD206267 is member of the cluster, known as Trumpler 37, believed to be the core of the expansive Cepheus OB2 association. HD206267 is a trapezium type stellar system with HD206267 as the dominant ultraviolet energy source with a smaller UV contribution from three cooler companion B0 type stars. The stars of the entire trumpler 37 cluster are about 7 million years old, although HD206267 formed more recently about 4 million years ago.
A distinct feature of IC1396 is the radial arrangement of several bright rimmed globules that form a loose and slowly expanding ring around the illuminating stars. The ring of loosely arranged dark globules has a radius of about 40 light years with HD206267 at its center. The "comet-like" configuration of some of these dark structures has coined the descriptive term "cometary globule". Although several of the globules are optically conspicuous the most prominent is catalogued as IC 1396A. IC 1396A contains the well known reflection nebula vdB 142. Low and intermediate mass stars appear to be actively forming within the globules. The star formation within the globules has been induced by a process known as "radiation driven implosion" where the ultraviolet flux from a massive star like HD206267 compresses the cold molecular gas within the globules, thus triggering collapse of the cloud and subsequent formation of lower mass stars.
Attempts to trace the history of IC 1396 point to a complex interplay of sequentially triggered star formation and cloud-cloud interactions. Events likely unfolded with an initial burst of star formation that occurred between 13 and 18 million years ago and gave rise to the first generation of stars which includes the existing nearby cluster NGC 7160. The more massive members from that first generation of stars went on to destroy themselves in supernovae explosions and no longer exist today. About 7 to 8 million years ago the effects of the first generation of supernova driven shock fronts and powerful stellar winds from existing stars created a huge 400 light year diameter bubble known as the Cepheus bubble. The expanding bubble compressed and flattened surrounding molecular clouds triggering a second burst of star formation which went on to form the Cepheus OB2 association some 7 million years ago. Also formed in this second generation of star formation were Trumpler 37 and its dominant star, HD206267 some 4 million years ago. Under the influence of the ionizing radiation field from the new star cluster, the HII cloud IC 1396 and its globules formed some 2 to 3 million years ago. Triggered by the expanding Cepheus bubble many well known HII regions have formed along its perimeter including IC 1396, Sh2-129, 133, 134, and 140. As IC 1396 expanded from the stellar winds of HD206267, surviving fragments of molecular clouds in the form of globules formed into an expanding ring around the central exciting star. The third and youngest generation of lower mass stars is currently forming within the dark globules of IC 1396 by the process of radiation driven implosion (see above).
The bright yellow star just to the north of IC1396 is the supergiant mu cephei. It formed during the first generation of stars which created the Cepheus bubble. It has the distinction of being one of the most luminous stars in our galaxy, emitting 350,000 times the power of our sun.