Sky & Telescope - December 2023 - 59

POSS-II / STSCI / CALTECH / PALOMAR OBSERVATORY
CLUSTERS AND KNOTS Use this guide to
lead you to the various star clusters and H II
regions that pepper the galaxy.
the 36-inch at 273×. I couldn't detect
knot D (my designation, at left) at all.
TG 17
IC 10D
IC 10C
IC 10A
TG 46
TG 48
IC 10B
TG 24
12.7mag.
star
TG 20
71/2′ × 9′
skies. If I didn't know exactly where
to look, it would be essentially lost in
the field of several hundred stars and
2.3-magnitude Beta Cassiopeiae riding
the edge. Increasing the magnification
to 56×, it's a soft, thumb-shaped glow
just 3′ by 1.5′ with a 12.7-magnitude
star in its lower half.
Nothing about this dwarf galaxy
comes easy, what with the brightest
H II star-forming region IC 10A being
a challenge itself in my 10-inch SCT.
At 300×, I've managed to see IC 10A
7″ east of a 14.5-magnitude star and
glimpsed IC 10B lying 1.1′ northnorthwest
of the 12.7-magnitude star.
In my 16-inch f/4.5 Dobsonian (which,
coincidentally, has the same light-gathering
capability as the telescope Swift
used to discover IC 10), the galaxy is
a faint, diffuse, oblong glow buried
in a rich star field at 70×. At 150×,
IC 10A shows enhancement when I
add a narrowband filter. Upping the
magnification to 440× and losing the
filter reveals a small, round glow with
no central brightening barely detached
from the star 7″ away.
The H II knot IC 10B is also visible
with averted vision at 440× as a small,
nonstellar glow with a brighter center,
while IC 10C, which lies 18″ farther
northeast, isn't quite visible. At 600×, I
found that the galaxy still easily fits in
the 11′ field thanks to a 110° apparentfield-of-view
eyepiece. Scrutinizing
IC 10A, the edge opposite the 14.5-magnitude
star appears soft and fans out.
Using the motor-driven 36-inch Dobsonian
at Whispering Pine Observatories
in northwestern Arkansas, I was able to
see a noticeable stellar core at 273×. This
corresponds to TG 54, the 5.5-millionyear-old
star cluster that Deidre Hunter
(Lowell Observatory) found using the
HST. While not visible in my 16-inch
even at 600×, I could glimpse IC 10C in
Clusters Ahoy!
Using the Whispering Pines 36-inch, I
managed to see several of IC 10's star
clusters one night in October 2022
when my Sky Quality Meter (lensed)
reading near the zenith was 21.20. At
664×, the cluster that my eye found
most easily was TG 24, which lies 30″
west-northwest of the 12.7-magnitude
star. It barely beat out TG 20, though,
some 40″ farther to the west-southwest.
Swinging northward and well outside
the galaxy's glow, I hunted down TG 17.
To my surprise, with a lot of time and
concentration, I was able to detect it
even though a 15.0-magnitude star lies
a mere 4″ northwest! Other clusters
eluded me, except for the combined
light of TG 48 and TG 46, two 18thmagnitude
objects.
If you choose to tackle this dwarf
galaxy on a chilly night, take your time
at the eyepiece and remember that its
brightest star clusters should be within
reach of a quality 24-inch telescope
under dark skies.
¢SCOTT HARRINGTON is grateful to his
friend Victor van Wulfen for digging up
Kohoutek's paper.
Buried Treasures in IC 10
Object
Type
IC 10
IC 10A
IC 10B
IC 10C
TG 24
TG 20
TG 17
TG 48
TG 46
Galaxy
H II region
H II region
H II region
Globular cluster
Globular cluster
Open cluster
Star cluster
Star cluster
Mag(v)
10.4
-
-
-
17.6
17.6
17.3
18.0
18.4
Size
6.3′ × 5.1′
-
-
-
2.8″
2.6″
1.5″
0.9″
1.0″
Age
-
-
-
-
4.8 Gyr
4.6 Gyr
450 Myr
26 Myr
18 Myr
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 * DECEMBER 2023 59
https://skyandtelescope.org/

Sky & Telescope - December 2023

Table of Contents for the Digital Edition of Sky & Telescope - December 2023

Contents
Sky & Telescope - December 2023 - Cover1
Sky & Telescope - December 2023 - Cover2
Sky & Telescope - December 2023 - 1
Sky & Telescope - December 2023 - Contents
Sky & Telescope - December 2023 - 3
Sky & Telescope - December 2023 - 4
Sky & Telescope - December 2023 - 5
Sky & Telescope - December 2023 - 6
Sky & Telescope - December 2023 - 7
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