Sky & Telescope - February 2021 - 19

Hone your imaging techniques on these
fascinating beauties.

lanetary nebulae are among the most rewarding celestial targets for observers and astrophotographers alike.
They're expanding gaseous shells ejected in the ﬁnal
stages of an intermediate-mass star's life and present intriguing, small-scale details not found in other emission or dark
nebulae. And while the majority are on the small side, they
typically have high surface brightness, which allows them
to punch through light pollution - something that makes
planetaries particularly appealing for urban astro-imagers
shooting under bright skies.

erﬂies

I think of planetary nebulae as the butterﬂies of the celestial ecosystem, produced in a metamorphosing process that
transforms a Sun-like star into a brightly hued treasure. Most
appear spherical, though some display non-symmetrical
shapes. As these stars run out of fuel, they expel their outer
layers of hydrogen, oxygen, sulfur, and other, more exotic
gases into space to eventually become the raw material for
other stars. The remaining hot stellar core of the dying star,
known as a white dwarf, emits strong ultraviolet radiation
that excites the atoms in the expanding shell, causing it
to glow. Due to their typically small sizes and shapes, they
appeared similar to planets to early observers. Today, we
know that these attractive celestial nebulae have little or
nothing in common with orbiting bodies.
These little butterﬂies are relatively short-lived. Over the
course of several thousand years, they continue to expand and
thin out, while the central white dwarf star cools, producing
less ultraviolet energy, and the nebula fades from view.
Planetary nebulae are distributed fairly evenly across the
sky, with a higher concentration towards the galactic center.
That means a few good ones are visible most any clear
night. They come in a range of brightness and sizes that suit
a variety of imaging setups. They're one of the few types
of objects beyond the solar system that display color in the
eyepiece. But their true extent and vivid hues are revealed
only in deep photographs.
As emission nebulae, planetaries display a wonderful
range of colors when shot in either broadband " natural "
color or when imaged through narrowband ﬁlters. In natural
color, the pinkish areas correspond to ionized hydrogen (Hα)
and sulfur emissions (S II). Teal hues are mainly produced by
excited oxygen (O III) atoms, which emit both blue and green
light. Because planetary nebulae are made up primarily of
ionized gasses, they respond particularly well to imaging with
narrowband ﬁlters. This makes them especially good targets
for imagers shooting through strong light pollution.
While many planetary nebulae require moderately large
apertures and long focal lengths to be adequately resolved,
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Sky & Telescope - February 2021

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