Sky & Telescope - August 2023 - 58

AUGUST 2023 OBSERVING
Going Deep
COMING FULL CIRCLE Three-quarters of
a century after Edwin Hubble first identified
H II regions in NGC 6822, the Hubble Space
Telescope captured Hubble V. This giant starforming
region is about 200 light-years across.
American astronomer Paul Hodge
(University of Washington) studied the
galaxy several times in 1977 and noted
16 OB associations as well as open clusters,
globular clusters, and H II regions.
In 1991 he counted 360 stars in the
galaxy brighter than 18th magnitude,
including 100 between magnitudes 13.5
and 16.5. For a dedicated observer with
a medium-size reflector, most of these
objects are accessible.
In the microcosm of the universe
that is our Local Group, we are fortunate
to have a representative variety of
galaxy types. Their relative proximity
allows detailed study of their structure
and content. NGC 6822 is classified
as a barred irregular galaxy similar to
Barnard's Treats
Object
Type
NGC 6822
Hubble I
Hubble II
Hubble V
D10
D13
D16/17/18
D23
PN 7
Hubble VI
Hubble VII
Hubble VIII
Ho 12
C1
C3
C2
C4
Galaxy
H II region
H II region
H II region
H II region
H II region
H II region
H II region
Planetary nebula
Globular cluster
Globular cluster
Globular cluster
Supernova remnant
Extended star cluster
Extended star cluster
Extended star cluster
Extended star cluster
the Magellanic Clouds, particularly the
Small Magellanic Cloud. It's about 7,000
light-years in diameter and has a mass of
around 2 billion Suns, with an absolute
magnitude of -16. NGC 6822 sits 1.6
million light-years from us and is isolated
from other Local Group galaxies.
In a 2001 Astronomical Journal article,
Caltech astronomer Ted Wyder noted
that the galaxy is 12 to 14 billion years
old and, excepting a dip 3-5 billion years
ago, has been making stars steadily since
its birth. He found evidence of a recent
increase in the rate of star formation in
the last 100-200 million years and discovered
young stars in the outer halo's
neutral hydrogen disk. This contrasts
with other Local Group galaxies, such
as the Ursa Major and Draco Dwarfs,
which comprise mostly old stars, and
Leo A, which consists largely of newborn
stars. The microenvironments of these
smaller structures, particularly their
gas content and tidal interactions with
other galaxies, can trigger waves of new
star and cluster formation.
Myriad Observations
Lowell Observatory's Brian Skiff has
spotted NGC 6822 with 7×35 binoculars,
and the central bar is well within
reach of 4- to 6-inch telescopes (as
evidenced by Barnard's and Hubble's
observing notes). As aperture increases
to the 8- to 12-inch range, H II regions
and some of the stellar associations
become visible. Let's take a tour of some
of the highlights of this remarkable
object visible in larger apertures.
I first observed Barnard's Galaxy
Mag(v)
8.8
-
-
-
-
-
-
-
20.8
16.1
15.3
17.2
~19-19.5
15.8
18.2
17.4
17.4
Right ascension and declination are for equinox 2000.0.
RA
19h 45.0m
19h 44.5m
19h 44.5m
19h 44.9m
19h 44.8m
19h 44.8m
19h 44.9m
19h 45.0m
19h 45.1m
19h 44.9m
19h 44.9m
19h 45.0m
19h 44.9m
19h 40.2m
19h 45.7m
19h 43.1m
19h 47.5m
Dec.
-14° 48′
-14° 42′
-14° 45′
-14° 43′
-14° 44′
-14° 53′
-14° 53′
-14° 49′
-14° 48′
-14° 49′
-14° 49′
-14° 43′
-14° 48′
-15° 22′
-14° 49′
-14° 58′
-14° 27′
at the 1993 Texas Star Party with my
25-inch reflector. I saw it as moderately
faint, diffuse, 15′ long by 4′ to 5′ wide,
and with a scattering of stars across it. I
also noted three bright H II regions (H I,
H II, H V) on its northern side. In 2007
I used my 32-inch reflector from the
same site and noted " many " H II regions
(including D10, D13, D16/17/18, and
D23), along with PN 7 (the brightest
planetary nebula in an article that
Pierre Leisy and collaborators published
in 2005). I needed a magnification of
361× to bag it (see the finder image on
page 59 to locate the planetary and other
targets). Many of the galaxy's stellar
associations I've observed are within
and around its H II regions. The galaxy's
open structure makes them easier targets
for visual observing, with the largest
spanning 35″ to 100″. From the southwestern
U.S. with my 32-inch I spotted
several of the globulars Hubble discovered,
including H VI, H VII (the largest
and brightest of the lot), and H VIII.
In 1969 Hodge found an emissionline
object in NGC 6822, now called
Ho 12. Several years later narrowband
imaging and optical spectroscopy confirmed
it to be a supernova remnant. In
2004 Albert Kong (Harvard University)
58 AUGUST 2023 * SKY & TELESCOPE
NASA / ESA / THE HUBBLE HERITAGE TEAM / STSCI / AURA
Sky & Telescope - August 2023

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