#nebula.cloud  an immense cloud of gas (mainly hydrogen) and dust in interstellar space
  supertype:  cloud  any collection of particles ($('<' #physical_phenomenon replaced by #natural_object because #cloud.atmospheric_phenomenon already exists and a clous a color, a mass, ... like all natural objects)$ e.g., smoke or dust) or gases that is visible
  subtype:  diffuse_nebula__diffusenebula__gaseous_nebula__gaseousnebula  a cluster of stars within an intricate cloud of gas and dust
     instance:  Orion.constellation__orion__thehunter
  subtype:  planetary_nebula__planetarynebula  a nebula that was once thought to be a star with its planets but is now thought to be a very hot star surrounded by an expanding envelope of ionized gases that emit a fluorescent glow because of intense radiation from the star

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