Definition : a dark or light hazy patch which cannot be resolved; a cloud of interstellar matter, gas mixed with dust found frequently in galaxies. Nebulas exist within other galaxies as well as in our own Milky Way galaxy. They are classified as planetary nebulas, supernova remnants, and diffuse nebulas, including reflecting, emission, and dark nebulas.
Planetary nebulas, or planetaries, are so called because many of them superficially resemble planets through telescopes. They are actually shells of material that an old average star sheds during a late, red giant stage in its evolution, before becoming a white dwarf.
Diffuse nebulas are extremely large structures, often many light-years across, that have no definite outline and a tenuous, cloudlike appearance. They are either luminous or dark. The former shine as a result of the light of neighboring stars.
Spectral studies show that light emanating from luminous nebulas consists of reflected light from stars and also, in so-called emission nebulas, of stimulated radiation of ionized gases and dust from the nebulas themselves. Dark, diffuse nebulas are observed as nonluminous clouds or faintly luminous, obscuring portions of the Milky Way and too distant from the stimulation of neighboring stars to reflect or emit much light of their own.
Both dark and luminous nebulas are considered likely sites for the processes of dust-cloud condensation and the formation of new stars.
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