An immense interstellar, diffuse cloud of gas and dust from which a central Self-luminous object held together by its own self-gravity. Often refers to those objects which generate energy from nuclear reactions occurring at their cores, but may also be applied to stellar remnants such as neutron stars. and surrounding planets and Hypothetical solid celestial body that accumulated during the last stages of accretion. These bodies, from ~1-100 km in size, formed in the early solar system by accretion of dust (rock) and ice (if present) in the central plane of the solar nebula. Most planetesimals accreted to planets, but many – condense and accrete. The properties of nebulae vary enormously and depend on their composition as well as the environment in which they are situated. Emission nebula are powered by young, massive stars and emit their own light, reflection nebulae shine by reflecting light from nearby massive stars, and dark nebulae, as the name suggests, are dark and can only be seen when silhouetted against a bright background.
Some or all content above used with permission from J. H. Wittke.