Sky and Telescope - August 2018 - 39

SAGITTA

+15°

M 1-74

6803

Altair

χ
γ

+10°

6807

+5°

M 1-70

6804

Star magnitudes

Zeta (ζ) Aquilae and 28′ east of a magnitude-6.7 star. It's
an irregular annular disk that appears ragged on its north
rim and very much brighter and more sharply deﬁned on
the south. A number of stars superposed on the nebula add
immeasurably to its charm. While it responds to nebular
ﬁlters, they are not necessary to see the object. The 16.7-magnitude central star is not seen. It's likely that we see NGC
6781 pole-on and that its true shape is bipolar. A study of
the molecular hydrogen in its halo shows evidence of shock
excitation, suggestive of very intense polar outﬂows.
NGC 6790 is sometimes challenging to detect and appears
stellar at low powers. High power on nights of good seeing
yields a tiny visible disk that is very subtly blue. It is located
on a line connecting Delta and Lambda Aquilae about a quarter of the way from Delta. NGC 6790 seems to be a relatively
young object and is thought to be truly spherical. It's been
suggested as the prototype of a body that's intermediate in
age, density, and evolutionary stage.
If you look some 3° 40′ west of Gamma (γ) Aquilae you
should ﬁnd the small, bright, bluish disk of NGC 6803. The
center of the nebula is bright and hides the 15.2-magnitude
central star. It's stellar at low powers, but an O III ﬁlter helps
to make the small disk visible.
About 50′ directly south of NGC 6803 is NGC 6804. The
14.3-magnitude central star is seen surrounded by a fainter
shell. It responds well to ﬁlters but is much prettier without.
Two other stars are superposed on the disk. This nebula was
discovered by Sir William Herschel, who mistook it for a
cluster of stars and placed it in his Class VI (rich and compressed clusters). NGC 6804 is an example of a multiple-shell
planetary nebula. It has an oval inner shell that is apparently
breaking through the rounder outer structure.
NGC 6807 is easily seen with direct vision as a faint,
vaguely blue star that responds to the O III ﬁlter. It is located
3.5° northeast of Delta Aquilae.
Two degrees northeast of Eta (η) Aquilae, NGC 6852
is seen as a tenuous transparent disk when an O III ﬁlter
is employed. It's 4.5′ northeast of a 7th-magnitude star,
and there's a 14th-magnitude star on the southeast edge
of the nebula. A fainter star lies within the nebula, but the
17.9-magnitude central star is not visible.
Abell 70 is sometimes called the Diamond Ring, a name
popularized by Eric Honeycutt in his May 2002 Sky &
Telescope article, "The Best Abell Planetary Nebulae." However, Honeycutt attributes this nickname to Jay McNeil, of
McNeil's Nebula fame. Find Abell 70 in the southeast corner
of the constellation, where it is the apex of a triangle that
has Epsilon (ε) Aquarii and 3 Aquarii as its base. Abell 70 is a
ghostly disk that beneﬁts from an O III ﬁlter.
Abell 70 is now known to harbor a binary central star and
is the subject of intense study. The binary consists of a main
sequence star seen at optical wavelengths and a hot white
dwarf secondary detected in the ultraviolet. The spectrum
identiﬁes the secondary as a barium star, which is an important object of interest to astronomers. It provides evidence

2
3
4
5
6
7

6781

σ
δ
M 1-69
6852
6790

0°

6741

M 1-66

6778
6772

AQUILA
-5°

λ
κ

-10°

6751

IC 4846
20h 00m

19h 30m

19h 00m

p SOAR WITH THE EAGLE All the targets discussed in the text except
for Abell 70 (in the far southeastern corner of Aquila) appear here.

supporting a hypothesized scenario whereby the secondary
has become polluted by material from the primary. During its
thermal pulse phase of evolution, carbon and heavier s-process
(slow neutron capture process) elements are dredged up from
deep within the larger star and deposited on the white dwarf
through a process known as wind accretion.
Objects like Abell 70 and NGC 6778 illustrate the importance of planetary nebulae as laboratories for the study of
stellar evolution and processes like common envelope interaction. Our current understanding of planetary nebulae as
purveyors of the heavy elements that enrich the interstellar
medium points to their extreme importance in the cycle of
creation that makes things like planets and people possible.
It's remarkable to contemplate that if we are, in the words of
Carl Sagan, "star stuff," it's planetary nebulae that are in part
responsible for making that star stuff available.

¢ Contributing Editor TED FORTE observes planetary nebulae
and other faint fuzzies from his home observatory near Sierra
Vista, Arizona. He coordinates the Astronomical League's Planetary Nebula Observing Program.
sk yandtelescope.com * AUGUST 2 018

http://www.skyandtelescope.com

Sky and Telescope - August 2018

Table of Contents for the Digital Edition of Sky and Telescope - August 2018