Sky & Telescope - December 2023 - 58

DECEMBER 2023 OBSERVING
Going Deep
Shajn and Vera Gaze misclassified it as a
nebula (Simeis 20) in their 1951 catalog
listing. Meanwhile, Western astronomers
puzzled over how to categorize
IC 10 - many wondered if it wasn't a
large, late-type spiral only a portion of
which was visible through a " hole " in
the galactic obscuration. Morton Roberts
of Harvard College Observatory resolved
this in 1962: Radio observations showed
that neutral hydrogen (which is unaffected
by galactic obscuration) aligned
with the optically brightest part of the
galaxy. Hence, no major portion of it was
obscured, and Roberts confirmed that
IC 10 was indeed an irregular galaxy.
Local at Long Last
With that mystery resolved, the question
of IC 10's distance loomed
large. In the early 1960s, astronomers
knew little more than that
it was fairly nearby, based on
resolved features in images
Fritz Zwicky obtained
with the 200-inch Hale
Telescope at Palomar
Observatory. Over the
next few decades,
various measurements
indicated that Hubble had indeed
been prophetic in thinking it a member
of the Local Group. Abhijit Saha (at
the time at the Space Telescope Science
Institute) and colleagues published an
article in 1996 placing IC 10 securely in
the Local Group by way of the Cepheid
variables they'd discovered.
However, even today distance estimates
vary wildly. This is due to IC 10's
light not only undergoing strong
extinction from gas and dust in our
galaxy, but also from the intrinsic gas
and dust in IC 10 itself! In fact, it has
the most foreground extinction (about
3 magnitudes) of any of the Local
Group galaxies.
Today, we understand IC 10 is not
only an irregular galaxy but has ongoing
star formation sparked by infalling
gas from the aforementioned neutral
hydrogen cloud. This fact was revealed
primarily by the finding reported in
1992 of an unusually high number of
extremely hot, blue stars known
as Wolf-Rayet stars (S&T:
Feb. 2023, p. 12). In
fact, IC 10 has
such energetic
star
formation that it's actually blue
when we correct for foreground reddening
- and it's the fifth most luminous
member of the Local Group.
In 1972, just one year before he discovered
his namesake comet that would
go on to disappoint so many, Czech
astronomer Luboš Kohoutek reported
sweeping up IC 10 in a hydrogen-alpha
survey while seeking new planetary nebulae.
He detected the same two compact
H II regions that Mayall had identified
three decades earlier and cataloged them
as IC 10A and IC 10B, plus a fainter one
he designated IC 10C. Almost 20 years
later, Paul Hodge and Myung Gyoon Lee
(both then at the University of Washington)
performed the most comprehensive
survey of the galaxy's ionized hydrogen
distribution and discovered 144 individual
regions - an unusually high number
for such a small galaxy. Their complex
#111, which corresponds to Kohoutek's
region IC 10A, was the brightest and
most intriguing. But it wasn't until
the Hubble Space Telescope imaged the
galaxy in the late 1990s that we learned
that at the heart of IC 10A lies a possible
super star cluster of 17th magnitude.
Russian astronomers identified seven
IC 10C
IC 10B
star cluster candidates in the early
1990s with the 6-m Bolshoi Teleskop
Azimutal'nyi in the Caucasus Mountains.
But it was only in 2008 that
they unveiled the vast majority of the
66 clusters we currently know of. In
addition, IC 10 also counts some star
clusters in its halo, with the farthest
some 9.8′ out.
IC 10A
VIGOROUS STAR FORMATION
This Hubble
Space Telescope image
highlights glowing gas
clouds in green. Astronomers
today refer to IC 10
as either a starburst or blue
compact dwarf galaxy.
58 DECEMBER 2023
Digging Deep
I haven't had a very long history with
IC 10 considering my first observation
was in 2015 with my 10-inch SCT. I
was excited at the time to log my ninth
member galaxy of the Local Group
and was surprised by how bright it
appeared. Since then, I've learned that
Danish amateur Thomas Jensen managed
an amazing sighting with his Zeiss
63-mm refractor at 34×! Using an eyepiece
yielding 39× and boasting a 100°
apparent field of view in my 6-inch f/5
reflector, I can detect the galaxy as a
faint, ill-defined glow under my rural
NASA / ESA / F. BAUER
Sky & Telescope - December 2023

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