Explanation : Old photographs show no evidence of the above nebula. In 1992, a white dwarf star in Cygnus blew off its outer layers in a classical nova explosion : an event called Nova Cygni 1992 . Light flooded the local interstellar neighborhood, illuminated this existing gas cloud , excited the existing hydrogen , and hence caused the red emission. The only gas actually expelled by the nova can be seen as a small red ball just above the photograph's center. Eventually, light from the nova shell will fade, and this nebula will again become invisible!
Media Credits & Attribution
Credit: P. Garnavich (CfA), 1.2-m Telescope, Whipple Observatory