nebular

Nebular is an adjective relating to a nebula, a diffuse cloud of gas and dust found in interstellar space. Nebulae vary in scale and appearance, and they are important in both the birth and death of stars.

Nebulae are categorized by their appearance and physical processes. Emission nebulae glow because their gas is ionized by nearby hot stars, producing characteristic spectral lines. Reflection nebulae shine by reflecting nearby starlight, often appearing blue. Dark nebulae are dense, opaque regions that obscure background light. Planetary nebulae are glowing shells ejected by aging stars, while supernova remnants can also take on a nebular form as the explosion expands and enriches the surrounding medium.

The term nebular is also used in the Nebular hypothesis, a historical model of solar system formation. Proposed in the 18th and 19th centuries by scientists such as Kant and Laplace, it posited that the solar system formed from a rotating cloud of gas and dust that cooled and contracted into a central sun surrounded by a protoplanetary disk from which planets emerged. Modern theories refine this view, incorporating cloud dynamics, turbulence, and disk physics, but the basic idea of a solar system forming from a collapsing nebula remains influential.

In observational astronomy, nebular refers to phenomena associated with nebulae, including nebular spectroscopy, where emission and absorption lines reveal composition, temperature, density, and motion. Studying nebulae provides insights into star formation, galactic chemical evolution, and the end stages of stellar life. The word itself derives from Latin nebula, meaning cloud or mist.