Sky & Telescope - August 2023 - 59

POSS-II / STSCI / CALTECH / PALOMAR OBSERVATORY
and his colleagues studied Ho 12 at
X-ray, radio, and visible wavelengths
and determined it to be 10″ wide. By
examining the POSS image, I estimated
its magnitude to be 19-19.5. In August
2008 I tried to spot it with my 15-inch
reflector from very dark skies in northern
Minnesota but was unsuccessful.
It took the increased aperture of my
32-inch from the Okie-Tex Star Party
the following month to see it. I noted it
to be borderline detectable with direct
vision and slightly nebulous. There was
a double star ½′ southwest of Ho 12
and a single star ½′ to the northeast.
Kong pointed out that Ho 12 was the
only known supernova remnant in the
galaxy at the time.
Deep imaging in the first decade of
this millennium revealed previously
unknown faint structures associated
with galaxies, such as tidal loops
around NGC 5907 in Draco and the tail
extending east-northeast of NGC 3628
in Leo. In 2006 a group led by Serge
Demers (University of Montreal) found
what they interpreted as a " polar ring "
BETWEEN THE
ARCHER AND THE
EAGLE You'll find
Barnard's glorious
galaxy in northeastern
Sagittarius about
3° south of the border
with Aquila. The
object's dimensions
are 15.5′ × 13.5′.
20h 30m
20h 00m
19h 30m
AQUILA
α
β
36′ in diameter,
seen at right
angles to the main
bar of NGC 6822.
I spotted its western
portion visually
at Okie-Tex in
2008 using my 32-inch.
In the last few decades, surveys of the
CAP
M75
Star magnitudes
2 3 4 5 6 7
π
ο
outer halos of nearby galaxies have identified
a new type of star grouping called
extended star clusters. These are larger
than typical globular clusters, relatively
metal-poor, and between 2 to 10 billion
years old. Their properties can reveal
information about the stellar structure
and evolution of their parent galaxies,
as in those associated with M31's stellar
streams, or those in NGC 6822 aligned
with its old stellar halo perpendicular to
the H I disk. I used my 32-inch reflector
from western Oklahoma in 2017 and
2020 to observe all four of the galaxy's
known extended star clusters. At a distance
of 1.6 million light-years, they're
the closest ones to us. C1 was the
brightest at magnitude 16.1 and C3 the
faintest at 18.2. I saw C1, C2, and C4 as
nonstellar and C3 as stellar (though for
C4 I was observing in poor conditions
through wildfire smoke).
It has taken more than a century to
H I
H VIII
H V
D10
H II
PN 7
D23
H VII
Ho 12
H VI
unlock these secrets from the galaxy
Barnard discovered. NGC 6822's position
within our Local Group allowed
it to avoid interaction with the three
large spirals, so it's relatively pristine
and unperturbed in its development,
something astronomers value. Due to
its open structure and lack of thick layers
of dust (such as those found in M33
and M31) we can - as with an X-ray of
a broken arm - see through its body to
observe its many parts. Next time you're
hunting between the Eagle and the
Archer, take a look at one of the most
fascinating galaxies in the sky.
D16/17/18
D13
¢DAVE TOSTESON gets no deeper joy
from observing than to understand and
share our visible universe.
BRIMMING WITH TARGETS Barnard's Galaxy, also known as Caldwell 57, has a plethora of
targets that you can snag with your telescope. You'll need aperture, but these are all within reach
of amateur equipment. The extended star clusters are outside the field. Image size is 24′ × 20′.
FURTHER READING: For a concise
history of the discovery and subsequent
early observations of NGC 6822, see
Richard Jakiel's fine article at https://is.gd/
NGC6822_Jakiel.
sk yand tele scope .o r g * AUGUST 2023 59
6822
SAGITTARIUS
ρ1
-20°
-15°
19h 00m
-10°

https://is.gd/NGC6822_Jakiel https://is.gd/NGC6822_Jakiel http://skyandtelescope.org

Sky & Telescope - August 2023

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