Planetary nebulae are the cloudy remnants of smaller stars that have shed their outer layers of gas, but haven't exploded into supernovas. ... The expanding shell of gas forms another type of nebula: a supernova remnant. The Crab Nebula (M1) is a good example. read more
A planetary nebula, abbreviated as PN or plural PNe, is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. read more