The Butterfly Planetary Nebula . ... Sometimes a highly symmetric gaseous planetary nebula is created, as is the case in M2-9 , also called the Butterfly. Most planetary nebulae show this bipolar appearance, although some appear nearly spherical . ... Much remains unknown about planetary nebulae , including why some appear symmetric , what creates the knots of emission (some known as FLIERS ), and how exactly stars create them. ... NASA Web Site Statements, Warnings, and Disclaimers . ...