A cloud of interstellar gas and dust which glows because of the effect on it of ultraviolet radiation from nearby stars. The term H II region is now preferred. The description ‘diffuse’ dates from earlier times when nebulae were defined according to their optical appearance. A diffuse nebula was one which retained its fuzzy appearance under magnification through a large telescope, as opposed to one which could be resolved into stars.
Subjects: Astronomy and Astrophysics.