The Ghost Nebula was a nebula and a sector in the Expansion Region that blocked light from most of the stars beyond and within it, isolating it from the galaxy and inspiring its name. Ultraviolet radiation from massive stars at its center ionized the surrounding gases to form plasma, which bathed the planets within the nebula in their ultraviolet light. It surrounded the inhabited planets of Umbara and Atoa, respective homeworlds of the near-Humans Umbaran and Atoan peoples.