Sky and Telescope - July 2017 - 35

A BELL 43: ED WA LENDOWSK I / A DA M BLOCK / NOAO / AUR A / NSF;
A BELL 50: STEFA N BINNE WIES / JOSEF PÖPSEL

But why had William and John Herschel, along with other
keen-eyed 19th-century visual observers who scoured the
heavens for nebulae, missed the remaining objects? Many of
the Abells are highly evolved planetaries with bloated spherical shells, anemic surface brightnesses, and dim central
stars. The collection poses a formidable observing challenge,
regardless of aperture or experience, and tracking down over
half the list will require careful planning, excellent sky conditions, and dogged persistence.
The stars that center planetary nebulae spent part of
their lives as red giants. As these stars transitioned from
the Asymptotic Giant Branch (AGB) to a white dwarf, their
ultraviolet radiation ionized the surrounding gaseous shells.
As the electrons in the shells recombined, they emitted visible light, primarily in the greenish wavelength of doubly
ionized oxygen (500.7 nm). The planetary nebula stage is
ﬂeeting, lasting only a few tens of thousands of years, for the
shell gradually expands to several light-years and disperses
into the interstellar medium.
Fortunately there's a powerful observing tool for planetary
nebulae - the O III line ﬁlter, which selectively transmits the
wavelength of doubly ionized oxygen while suppressing background light pollution and skyglow. Because of the contrast
boost, O III and UHC-style narrowband ﬁlters are essential
tools in chasing ghostly planetaries.
I've selected 13 summer favorites that will test your
observing skills using a 10-inch or larger scope (though all
have been spotted through an 8-inch aperture under pristine
skies). As a general guideline, start with fairly low magniﬁcation (~70× to 100×) and an O III ﬁlter to identify the target.
Unﬁltered observations at higher power may reveal elusive
details including the central star. I used my 18-inch reﬂector
for these descriptions, as well as detailed computer-generated
ﬁnder charts to pinpoint the locations.
Abell 39 is a beautifully symmetric gaseous bubble
located 4.7° southwest of Zeta (ζ) Herculis, the southwestern star of the distinctive Keystone asterism. Shining at
13th-magnitude, its light is spread over a 3′ diameter and the
surface brightness is quite low. It was faintly visible unﬁltered
at 115× as a round glow over 2.5′ in diameter with a 14thmagnitude star just off the western side. The disc was crisply
deﬁned with an O III ﬁlter, and the rim contained a couple
of slightly brighter narrow arcs, most noticeably along the
eastern edge. The 15.7-magnitude central star occasionally
winked at 283× in moments of steady seeing.
Distances and physical diameters of planetaries have long
been notoriously difﬁcult to pin down, but a new technique
developed by astronomers David Frew and Quentin Parker
(University of Hong Kong) using H-alpha surface brightness
and angular size yields an impressive diameter of 5.5 lightyears and a distance of 5,500 light-years for Abell 39 with an
uncertainty of only 18%.
Abell 43 lies in northern Ophiuchus, roughly midway
between 2nd-magnitude Alpha (α) Ophiuchi (Rasalhague)
and 3.7-magnitude 72 Ophiuchi. In the eyepiece ﬁeld of view,

the planetary lines up with a 9th-magnitude star 4′ northwest and two 10.5- and 11th-magnitude stars at a similar
distance southeast.
Viewing at 175× with a narrowband ﬁlter, I found a
moderately large circular glow, perhaps 70″ in diameter,
with a fairly smooth surface brightness. I easily spotted the
14.7-magnitude central star without a ﬁlter; two dimmer
stars appeared at the southern and eastern limb. I saw no
sign of the planetary's intricate network of woven ﬁlaments.
Abell 43's unusual nucleus is a pulsating hydrogen-deﬁcient white dwarf. An investigation published in Astronomy
& Astrophysics in 2007 identiﬁed pulsations at six different
vibration frequencies with periods from 40 to 100 minutes.
This instability is driven by the ionization of carbon and
oxygen in the star's envelope in a process called the Kappa
mechanism (κ-mechanism).
Abell 50 was ﬁrst discovered by William Herschel while
sweeping for nebulae on July 8, 1788, and later cataloged as

ABELL

p PULSATING WHITE DWARF Small changes in density within the
ionization zone of the star at the center of Abell 43 produce associated
changes in opacity. When density increases, the ionization zone becomes more opaque and absorbs more energy from the stellar interior.
The ionization layer subsequently heats and expands, then drops back
to its previous density. These expansion and collapse cycles set up an
oscillation pattern, causing the white dwarf's outer layers to pulsate.

ABELL

p CYAN CIRCLE With a low-power eyepiece, Abell 50 will appear stellar,
but you might be able to detect a slight blue-green color. Use an O III filter
and higher magnification to reveal the nebula's circular structure.
s k y a n d t e l e s c o p e .c o m

* J U LY 2 017

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Sky and Telescope - July 2017

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