What is "spiral nebula"

Nebula \Neb"u*la\ (n[e^]b"[-u]*l[.a]), n.; pl. Nebul[ae] (n[e^]b"[-u]*l[=e]). [L., mist, cloud; akin to Gr. nefe`lh, ne`fos, cloud, mist, G. nebel mist, OHG. nebul, D. nevel, Skr. nabhas cloud, mist. Cf. Nebule.]

1. (Astron.) A faint, cloudlike, self-luminous mass of matter situated beyond the solar system among the stars. The term was originally applied to any diffuse luminous region. Now, technically, it is applied to interstellar clouds of dust and gases ( diffuse nebula). However distant galaxies and very distant star clusters often appear like them in the telescope, such as the spiral nebula in Andromeda, known now to be a distant galaxy.

2. (Med.)

   1. A white spot or a slight opacity of the cornea.

   2. A cloudy appearance in the urine. [Obs.]

n. 1 (context astronomy English) a spiral-shaped nebula 2 (context astronomy obsolete English) a spiral-shaped white nebula; these were generally determined to be spiral galaxy in the 20th century

n. a galaxy having a spiral structure; arms containing younger stars spiral out from old stars at the center [syn: spiral galaxy]