Sky and Telescope - October 2016 - 56

OBSERVING

Deep-Sky Wonders

Abell 71

NGC 6826

Left: The physical structure of Abell 71 becomes more apparent thanks to astrophotography. J. P. Metsävainio imaged the planetary
nebula with a series of ﬁltered exposures and mapped the emission of the ionized elements to diﬀerent colors: the vibrant blue
center shows the interior oxygen emission. Right: Niels Christensen stacked a series of raw frames captured through a 16-inch ACF
Schmidt-Cassegrain. The halo around NGC 6826 represents hydrogen and oxygen data passed through H-alpha and O III ﬁlters.

within the southwestern edge. Stars about one magnitude dimmer rest slightly west of the nebula's center
and at its northeastern edge. Very faint stars occupy the
southeastern quadrant and the northern and northwestern edges. Those last two are the southernmost of a little
bunch of stars that crowns the nebula. The nebula itself
is roundish, not quite as dim in the sector that runs
east-southeast clockwise to north-northeast, and very
diﬃcult to detect. I estimate a diameter of somewhere
around 2¼′. After seeing Abell 71 in the 15-inch scope, I
tried to observe it a number of times in the 10-inch. With
considerable eﬀort or imagination, I'm honestly not
sure which, I occasionally thought I could glimpse the
"brighter" sector. What do you think?
It's noised about on the internet that Abell 71 is not
truly a planetary nebula, but rather an H II region (an
emission nebula whose hydrogen is ionized by the radiation of massive stars). This idea stems from a single
paper that was published in 1991 (Paris Pis
¸mis
¸ et al.,
Publications of the Astronomical Society of the Paciﬁc). As
of this writing, at least 30 papers that include Abell 71
have been published in the ensuing years, all of them
treating Abell 71 as a planetary nebula.
A telling testament to the nature of Abell 71 are
composite images that use ﬁ lters to capture its various emission lines and combine them in a color-coded
image. For example, the magniﬁcent image by Finnish
astrophotographer Jukka-Pekka Metsävainio clearly
shows the interior oxygen emission (blue) indicative of
a planetary nebula. Lowell Observatory's Brian Skiﬀ
56

October 2016 sky & telescope

writes that images combining hydrogen-alpha and other
emission lines in a complete imaging survey along the
galactic plane would be a signiﬁcant asset to professional
astronomers. Interested parties may contact Skiﬀ at
bas@lowell.edu.
Since we've attempted one exceptionally diﬃcult
planetary nebula, let's close with one that's quite a
showpiece. NGC 6826 rests 28′ east of 16 Cygni, a wide,
pretty double composed of two 6th-magnitude yellow
stars. In my 105-mm refractor at 47×, the planetary is
a tiny, blue-grey disk. At 127× its slightly oval form is
about 25″ across and brighter around the nebula's central
star. Under some observing conditions, the outer oval
seems to dim when you look directly at the central star
but brightens again if you direct your gaze a little to one
side. This accounts for NGC 6826's popular nickname -
the Blinking Planetary.
NGC 6826 is gorgeous in my 15-inch reﬂector. At 133×
the nebula shines a beautiful colonial blue punctuated
with a bright central star. At 247× the planetary looks
oval, tipped a bit east of southeast. There is a bright,
diﬀuse halo surrounding a very bright interior that has
dark patches close to the central star. The entire structure
seems to be embedded in a large, round, exceedingly
faint, outer halo. As judged by the distance between ﬁeld
stars, it's roughly 2′ across. An O III ﬁlter can help make
this ethereal glow detectable. At 345× the oval, inner halo
spans about ½′. I didn't notice the somewhat brighter
patches within the oval's ends that have been logged by
other observers. Can you spot them? ✦

Sky and Telescope - October 2016

Table of Contents for the Digital Edition of Sky and Telescope - October 2016