Nebularis

Nebularis is a term used in speculative astronomy and in science fiction to describe a proposed class of extremely diffuse, large-scale interstellar gas clouds. The name derives from nebula, reflecting its cloud-like nature. In theoretical discussions, Nebularis are envisioned as vast, filamentary structures extending from tens to hundreds of light-years, with very low particle densities and predominantly atomic gas, often with subsolar metallicity. They are imagined as cold to moderately warm, stabilized by a balance of turbulence, magnetic fields, and weak self-gravity.

Observationally, Nebularis would be difficult to detect; they would produce faint 21 cm hydrogen emission and weak optical emission lines, and could be more readily identified through absorption lines against bright background sources or via indirect effects on surrounding gas dynamics. They are often framed as transitional reservoirs between the intergalactic medium and the denser galactic gas, potentially acting as sites of delayed or inefficient star formation.

Compared with known components such as diffuse ionized gas and high-velocity clouds, Nebularis remain unconfirmed as a distinct class. No formal catalog entries exist, and the concept is used mainly in theoretical exploration and in science fiction as a descriptive label for large, tenuous gas structures.

The term reflects the Latin origin of nebula, emphasizing the cloud-like character of these hypothetical structures and their place in discussions about the distribution of baryons and the conditions for star formation in faint, diffuse gas phases.