|A Hubble Space Telescope sampler of planetary nebulae.|
NASA / ESA
The planetary nebula phase is brief, lasting only around 10,000 years before the cast-off cloak became so distended it slowly fades from view. Only the lonely white dwarf and whatever planets it might still possess soldier on. Such will be the fate of the Sun, one of the reasons that observing planetaries gives pause to reflect on the future of our own Solar System.
There are an estimated 10,000 planetary nebulae in our galaxy alone, of which roughly 1,500 have been cataloged to date. Many are very small and can be mistaken for stars. The only way to tell them apart is to "blink" them with a nebula filter such as an Oxygen III filter. Nebula filters pass the light of ionized oxygen, prominent in planetary nebulae while suppressing skyglow and manmade light pollution. To "blink" a planetary, slide the filter back and forth between your eye and eyepiece while gazing at the nebula. The filter will cause the object to sharply become brighter compared to the neighboring field stars, immediately identifying it as the nebula.
An O III filter blocks natural and human-made light pollution while allowing emissions of doubly ionized oxygen in planetary nebulae to pass through. The filter darkens the sky background and increases the nebula's contrast and visibility.
Read more on hunting and observing planetary nebula by Bob King on Sky and Telescope magazine's website, "Hunting Giant Planetary Nebulae".