The Medusa Nebula . ... Also known as Abell 21, this Medusa is an old planetary nebula some 1,500 light-years away along the southern border of the constellation Gemini . ... The planetary nebula phase represents a final stage in the evolution of low mass stars like the sun , as they transform themselves from red giants to hot white dwarf stars and in the process shrug off their outer layers. ... The Medusa Nebula is estimated to be over 4 light-years across. ... NASA Official: Jay Norris. ...