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Saturday, December 17, 2016

Observing Planetary Nebulae

Planetary nebulae are very interesting objects to view due to their delicate-distinct shapes and pastel colors. Not only a treat to the eye, they possess subtle details that test the limits of your vision. For example outer rings, darker centers, and those frequently faint central stars that are barely visible from behind the pale veils of nebulosity. Only averted vision could bring out these shy stellar objects.

Image of A Hubble Space Telescope sampler of planetary nebulae. NASA / ESA
A Hubble Space Telescope sampler of planetary nebulae.
NASA / ESA
The planetary nebula, so called because their generally round shapes reminded early observers of planets, represent a late stage in the evolution of Sun-like stars from a red giant to white dwarf. Powerful stellar winds emanating from the star's core blow away its outer layers, creating an expanding shell of gas and dust.

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".

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