Collier's New Encyclopedia (1921)/Nebula

NEBULA, plural NEBULÆ, in astronomy, a slight cloudy patch of light, retaining its form unchanged except under keen and long-continued observation. More than 8,000 nebulæ, or star-clusters, closely resembling them, have been found in both hemispheres, and in nearly every constellation. A few, as the great nebulæ of Orion, Argo Navis, and Andromeda, are visible on very clear nights to the naked eye; the rest are telescopic. When greatly magnified some are found to be composed of many thousand remote stars, others remain only as diffused masses of light.

The great nebula of Orion surrounds a multiple star, Theta Orionis, consisting of six, apparently revolving round their common center of gravity. It has been found to alter its form very slightly. The spectroscope has shown that many of the nebulæ are merely gaseous. The nebulæ in Andromeda has a bright ball in the center and is spindle-shaped. In pathology, a slight speck on the cornea; also a mist or cloud suspended in the urine.