Variable nebulae are reflection nebulae that change in brightness because of changes in their illuminating star.
See also McNeil's Nebula NGC 1555 (Hind's Variable Nebula) NGC 2261 (Hubble's Variable Nebula) NGC 6729 (R Coronae Australis Nebula)
References
External links Astrobiscuit: Seeing The Speed Of Light fun and educational video about variable nebula and the amateur community observing them