Sky & Telescope - June 2023 - 59

Lessons Learned
Three planetary nebulae are found
within the boundaries of M7. The
14.0-magnitude planetary nebula Hf 2-1
is large enough to detect as more than
a starlike object if the seeing is steady
enough to discern its 9″ disk. The other
two planetaries are M 1-30 and Cn 2-1,
which are respectively magnitude 14.7
and 12.2 with apparent sizes of 5″ and
2″. All three planetaries lie far in the
background of M7, and so - if you spot
them - you're peering at them through
the cluster.
Interestingly, these three objects are
arrayed in a nearly straight line through
M7: Cn 2-1 is located about 10′ north
of the 6th-magnitude star HD 162817,
and the other two are lined up to the
southwest, spaced about 25′ apart.
Their locations are all easy star-hops,
but care is needed to identify them. I
used my 30-inch one night in August
2022 during poor seeing - star images
were huge, shimmering blobs. I blinked
the fields of each planetary by holding
an O III filter in front of the eyepiece
and moving it in and out of view to see
which " star " stayed bright. Real stars
usually get dimmer with the O III filter,
while tiny planetary nebulae will suddenly
stand out. Usually. I made careful
sketches of all three fields and, considering
the strong O III response I got in
the right spots, I was certain that I'd
seen them all and didn't double-check
until I got home.
Oops! I went 0 for 3. Dang. It turns
out there's a star near each planetary
that responds well to an O III filter, so
beware. And now, as I write this article,
it's too late in the season to try again.
Globular Cluster
NGC 6453 appears on M7's western
border but has no relation to the open
cluster other than being in the same
line of sight. Recent data place the
globular at a distance of approximately
33,000 light-years on the opposite side
of the galactic center from the Sun. And
NGC 6453, like most galactic globular
clusters, is in the halo and lies about
2,700 light-years below the galactic
plane, putting it outside the Milky
Way's disk. Somehow, in spite of the
intervening gas and dust, it still manages
to muster a visual magnitude of
9.2, making it accessible to most amateur
telescopes. Even if you have little
interest in M7, this improbably observable
globular cluster is a must-see.
With the 8-inch, NGC 6453 was a
small, fuzzy glow without any resolution
into individual stars. Even the
30-inch could resolve only a few stars in
the cluster, and for all I know they may
even be in the foreground.
Depending on the source consulted,
the apparent diameter of NGC 6453 is
anywhere from 21.5′ to 7.6′ across, and
based on my observations, I favor the
smaller diameter. If you have trouble
finding the globular, you can pinpoint
its position by using the 5.9-magnitude
stars HD 162374 and HD 162391.
NGC 6453 is about 17′ due west of a
line connecting these two prominent
stars; the cluster serves as the third
point of an equilateral triangle.
Which Scope Is Best?
My 30-inch provided easier access to
the faintest parts of the cluster - like
the three " planetaries " and NGC 6453
M7's Delights
Object
Type
M7
HD 162587
B287
B283
NGC 6444
NGC 6455
Trumpler 30
Hf 2-1
M 1-30
Cn 2-1
HD 162817
NGC 6453
HD 162374
HD 162391
Open cluster
Star
Dark nebula
Dark nebula
Open cluster
Star cloud
Open cluster
Planetary nebula
Planetary nebula
Planetary nebula
Star
Globular cluster
Star
Star
BALL OF STARS This drawing shows the
globular cluster NGC 6453 as I saw it through
the 30-inch at 183×. Do any of the four stars
arrayed over the glow of NGC 6453 belong to
the cluster? North is up.
- while still being able to comfortably
fit M7 into its 1° field. However, the
views through the 8-inch were the most
breathtaking. Beholding M7 suspended
within this magnificent part of the
Milky Way through a rich-field 8-inch
scope was just wow!
¢Contributing Editor HOWARD BANICH
loves this stuff, even the oops. You can
reach him at hbanich@gmail.com.
Mag(v)
3.3
5.6
-
-
-
-
8.8
14.0
14.7
12.2
6.1
9.2
5.9
5.9
Size
75′
-
25′ × 15′
90′ × 60′
12′
58′
20′
9″
5″
2″
-
7.6′
-
-
RA
17h 53.8m
17h 53.4m
17h 54.4m
17h 51.3m
17h 49.6m
17h 51.8m
17h 56.4m
17h 51.2m
17h 53.0m
17h 54.5m
17h 54.5m
17h 50.9m
17h 52.2m
17h 52.3m
Dec.
-34° 47′
-34° 54′
-35° 12′
-33° 52′
-34° 49′
-35° 10′
-35° 19′
-34° 55′
-34° 38′
-34° 22′
-34° 28′
-34° 36′
-34° 48′
-34° 25′
Angular sizes are from recent catalogs. Visually, an object's size is often smaller than the cataloged
value and varies according to the aperture and magnification of the viewing instrument. Right ascension
and declination are for equinox 2000.0.
sk yand tele scope .o r g * JUNE 2023 59

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Sky & Telescope - June 2023

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