NGC 7822 (W1) (S 171) (C 214)
Distance: 2750 Light Years

Right Ascension: 00 : 03.6 (hours : minutes)
Declination: +68 : 37 (degrees : minutes)

The young stellar association Cepheus OB4 lies in a region of bright nebulosity known as NGC 7822 and S 171. The fainter northern arch of nebulosity is designated NGC 7822 and the brighter southern cloud is designated S 171 (Sharpless, 1959) or alternatively Cederblad 214. The Molecular cloud complex which gave rise to the visible nebula is known as the radio source W1 (Westerhout 1), one of the largest molecular cloud complexes in the Milky Way. The molecular cloud is known to have a northern and southern component which corresponds well to the structure of the visible HII region.

The Cepheus OB4 association is a young stellar association with the oldest stars forming some 5 million years ago. Stellar winds and supernovae driven shock fronts produced by the hot OB stars of the association have expanded an immense 200 light year diameter shell of molecular gas which contacts the surrounding cold molecular cloud. The HII regions (NGC 7822, S 171) have formed as a thin ring-like ionization front along the interface of the expanding shell and molecular cloud. The HII regions are illuminated by over 40 OB stars grouped in several distinct clusters imbedded within the nebulosity. Many of the bright stars are obscured and reddened by the foreground dust clouds of W1 which represent the leading edge of the shell expanding towards the observer. The most prominent cluster is Berkeley 59 (Be59) located in the center of the nebulosity. Be59 is one of the dominant ionization sources for the HII region and is located along the northern border of S 171(C 214). Be59 is likely a second generation cluster whose formation was triggered by the expanding shell of gas. NGC7762 is an older cluster imbedded in the western arc of the nebula complex.

Several prominent bright rimmed globules exist in the S 171 complex. The formation and evolution of these structures involves the interplay of gas, dust and ionizing energy fields. The ionizing radiation field and stellar winds of young massive stars tend to evaporate and destroy the parent molecular cloud from which they formed over a time scale of about 10 million years. During this process triggering of further star formation may occur as the molecular cloud becomes compressed and altered. The lower density gas in the cloud is swept away or ionized, however higher density clumps of gas and dust will resist being boiled away and will emerge as dark globules protruding into adjacent HII regions where they are subject to the ultraviolet radiation of nearby OB stars. The surface of the dense globules becomes ionized forming the characteristic bright rim of the globule. The expansion of the higher density ionized gas into the lower density HII region causes a persistent mass loss from the globule termed photoevaporation. The contents of a globule may be accelerated away from the UV source as it evaporates, forming a comet shaped structure known as a "cometary globule". Ultimately a new generation of low mass stars can form within the globule as it becomes compressed by the radiation and winds of the nearby stars. This process is known as radiative implosion. The lifetime of the globule is relatively short as they tend to evaporate away within 500,000 years.