Soul Nebula

IC 1848 in Cassiopeia is the other half of the Heart and Soul pair, an emission nebula catalogued as Sh2-199 and known to radio astronomers as W5, lying around 7,500 light-years away in the same star-forming complex as IC 1805. Its shape comes from two adjacent cavities, each hollowed out and lit by a handful of hot O-type stars — HD 17505 and HD 17520 on one side, HD 18326 on the other — with the bright rims marking the ionisation fronts where that radiation is still eating into denser gas. The ridges and pillars standing along those rims are what the erosion has not yet cleared, and Spitzer’s infrared survey of W5 found the youngest stars concentrated at the rims while older ones sit in the emptied interiors, making this one of the better cases anywhere for star formation being triggered by the generation before it rather than proceeding on its own. The older name for it, the Embryo Nebula, describes the outline rather better than the one it is usually filed under.