A protoplanetary nebula (PPN) is a short-lived deep sky object on its way to becoming a fully-fledged planetary nebula. The key difference is that the nebulosity surrounding its white dwarf star has not yet reached ignition temperature for ionization. What we see is merely the reflected starlight from the white dwarf (a reflection nebula). Most protoplanetary nebulae are fairly recent discoveries; none are Messier/NGC/IC objects.
- CRL 2688 (Egg Nebula), in Cygnus
- IRAS 09371+1212 (Frosty Leo Nebula)
- M 1-92 (Minkowski’s Footprint), in Cygnus