The crucible of star formation lies within giant molecular clouds scattered throughout the spiral arms of galaxies. In our own galaxy some of the best known star forming regions lie in a chain of HII clouds located in the Perseus spiral arm of our galaxy. From west to east the chain of giant HII regions are known as W3, W4, and W5, which are also catalogued as IC 1795, IC 1805, and IC 1848. These nebulae are physically linked (radio observations show a bridge between IC 1805 and IC 1848 confirming a physical connection between the two nebulae) and together extend some 490 light years across the massive Perseus spiral arm. The glowing emission clouds are thin blisters formed at the surface of several giant molecular clouds. The clouds of initially cold neutral gas have fragmented and condensed in many areas, forming clusters of bright young stars collectively known as the Cas OB6 association. The bright O and B type giants of Cas OB6 illuminate the chain of giant HII clouds. Star formation within these HII regions has occurred sequentially over time as young OB stars have triggered bursts of new star formation as the HII clouds they illuminate expand into and compress the surrounding denser gaseous medium. The oldest clusters formed first in IC 1805 some 4 million years ago. The ultraviolet radiation and stellar winds of the older stars later forced the expansion of the emission cloud IC 1805 into its neighbor IC 1795 causing the most recent burst of star formation there only 100,000 years ago. Although small compared to its neighbors, IC 1795 borders one of the largest molecular clouds known in the Milky Way. The invisible cloud (detected by CO radio emission) extends some 200 light years across and contains the mass of 100,000 suns.
Two dominant groups of OB stars illuminate the HII region IC 1848. The emission cloud has several bright rims along its perimeter. These represent points of contact where the ionization front meets the denser neutral gas of the adjacent molecular cloud. Triggered bursts of star formation are occurring in the bright rims of IC 1848 as the HII cloud expands into the cold surrounding gas.
Observations at radio wavelengths show that centered on IC 1805 is a massive 300 light year cavity of excavated gas blown out by the stellar winds and ultraviolet radiation of the hot OB stars at its center. The central cluster has the same designation of its parent cloud IC 1805, but is also known as Melotte 15, object number 15 in the catalogue of star clusters by the British-Belgian astronomer Philibert Jacques Melotte (1880-1961). Melotte 15 is an extremely young cluster of massive OB stars with a mean age of only 1.5 million years. It is one of the core clusters of the Cas OB6 association and contains several O type giants with more than 15 solar masses. The cluster is positioned some 50 light years in front of the nebula it illuminates.