Sky & Telescope - July 2022 - 24

Stellar Conglomerates
Another very nice globular-planetary pair resides in Sagittarius
a bit less than 2.5° west-southwest of the open cluster
M23. NGC 6440 fits in the same low-power field of view
as the wonderful bipolar planetary nebula NGC 6445. My
favorite sight is in the 18-inch Dob at 88×. The two objects
are 22′ apart and appear about equal in brightness though the
globular is larger. The cluster contains a moderately bright,
unresolved core surrounded by a diffuse halo that appears
mottled on the edges.
NGC 6717 is of interest, in part, because of its multiple
identities. Herschel first discovered the cluster on August 7,
1784, and logged it as three small stars with nebulosity,
even as he was uncertain of its character. It's sometimes
conflated with IC 4802 (discovered by Guillaume Bigourdan
in 1884), but that object more correctly refers to a nebulous
patch on the northeastern edge of the globular. Swedish
astronomer Per Collinder was the first to establish the object
as a globular cluster in 1931, but then George Abell " discovered "
it again in 1952 while examining photographic plates
of the first National Geographic Society - Palomar Observatory
Sky Survey, bestowing it the designation Palomar 9. It's
generally considered the easiest of the 15 Palomar globulars
to observe visually. The center of NGC 6717 lies about 2′
south of the 5th-magnitude star Nu2 (ν2) Sagittarii. I see the
object as an unresolved amorphous disk with several stars
superimposed on it.
Globular clusters typically comprise very old stars identified,
in part, by their low metallicity. NGC 6723 is therefore
unusual for containing a large fraction of younger stars with
enhanced metallicity and is an important natural laboratory
for the study of these objects. It's located on the SagittariusCorona
Australis border about 30′ northeast of the 5th-magnitude
star Epsilon (ε) Coronae Australis. NGC 6723 shares a
low-power eyepiece field of view with a number of reflection
nebulae, including NGC 6729, the bright fan-shaped glow
surrounding the star R Coronae Australis. The nebula is part
of the larger Corona Australis Molecular Cloud, one of the
closest star-forming regions at a distance of about 420 lightyears.
This exceptional globular is large, bright, and fairly
easy to resolve. Scottish astronomer James Dunlop discovered
NGC 6723 in June of 1826, while surveying the southern
skies from Australia.
Planetary in a Globular
Dunlop also discovered NGC 6441 in Scorpius. The globular
sits just 4.3′ east of the 3.2-magnitude star G Scorpii. The
star tends to overwhelm the cluster but makes an interesting
contrast with the dense, unresolved globular. Also a little
lost in the glare of the star is the planetary nebula mimic
Haro 2-36 (PN G353.5-04.9). This small object is easily overlooked,
but it's apparent through an O III filter. Haro 2-36 is a
symbiotic star, a binary system usually comprising a red giant
ABOVE THE TEAPOT'S SPOUT NGC 6522 (at right) and NGC 6528 are at a similar distance (around 24,000 and 25,500 light-years, respectively)
from Earth. With only 16′ separating the pair, that also means they're physically fairly close to each other in space. The clusters are located in Baade's
Window, a relatively dust-free region along the line of sight toward the center of the galaxy. The star at the bottom of the frame is 3rd-magnitude
Gamma2 Sagittarii, also known as Alnasl, and lies 25′ from NGC 6528.
24 JULY 2022 * SKY & TELESCOPE
TEAM CHAMELEON / WOLFGANG PAECH / FRANZ HOFMANN

Sky & Telescope - July 2022

Table of Contents for the Digital Edition of Sky & Telescope - July 2022