Sky & Telescope - September 2024 - 30

Celestial Collage
Distance Markers
(million light-years)
Milky Way
NGC 7320
50
tidal tails, it's reasonable to suppose that NGC 7320C has
significant motion toward us and is at a similar distance
as NGC 7319. More than that, it may have also created the
Northern Starburst Region, which is a gigantic area of intergalactic
gas it generated when zooming by NGC 7319 that has
been energized by the more recent passage of NGC 7318B.
The case of this galaxy is perhaps even more fascinating. Its
redshift places it at a similar distance as NGC 7320C, but a
150,000-light-year-long ridge of shocked gas and star formation
tells a different story. NGC 7318B is compressing the
intergalactic medium (composed mostly of hydrogen and carbon
monoxide) to such an extent that it's not only causing star
formation but is also heating the gas so that it glows in X-rays.
The only way for NGC 7318B to do this is to have a significant
component of motion toward us. It does, and it's been
measured at about 900 km/s. This ongoing, high-speed physical
interaction is also evident in visual and infrared images,
and simulations place NGC 7318B at about the same distance
as NGC 7319, NGC 7318A, and NGC 7317.
A close examination of the Hubble and JWST images also
shows that tendrils of gas and dust from the shocked ridge of
gas are in front of NGC 7319. Computer simulations suggest
that as NGC 7318B hurtled toward both NGC 7319 and
NGC 7318A from behind, along the way it may have briefly
interacted with NGC 7318A before crashing into the intergalactic
medium, creating the ridge. It then zipped between
NGC 7319 and NGC 7318A about 300 million years ago.
Another sign of the gravitational game of Twister being
played in Stephan's Quintet is that NGC 7319's stellar disk
and gas disk are nearly perpendicular to each other.
Quintet Plus One
Object
Type
NGC 7317
NGC 7318A
NGC 7318B
NGC 7319
NGC 7320
NGC 7320C
Elliptical
Elliptical
Barred spiral
Barred spiral
Spiral
Barred spiral
100
150
Astronomers expect that in a few billion years NGC 7317,
NGC 7318A, and NGC 7319 will merge and form a single
elliptical galaxy. NGC 7320C may or may not get caught up
in this merger, but NGC 7318B definitely won't - speeding
through at 900 km/s it will escape into intergalactic space.
Cool beans, huh?!
Observations
Can any of these galaxies, let alone the entire compact group,
be seen visually with an amateur telescope? The good news is
" yes, but . . . " The catch, as always, is that you must be under
a truly dark and transparent sky to see Stephan's Quintet well.
Remember, the brightest galaxy in the group is magnitude 12.6.
I've seen the four brightest galaxies through a smoky but
dark sky by putting a 10-inch aperture mask on my 30-inch
scope, temporarily turning it into a 10-inch f/8 unobstructed
reflector. I was unable to split NGC 7318A and NGC 7318B,
but in the spirit of Stephan's original discovery, I'm (ahem)
giving myself, and many other observers, credit for both.
During that observation, NGC 7320 - the closest and
brightest of the group - was the easiest to detect, followed by
the combined glow of NGC 7318A/B, and finally NGC 7319.
I couldn't detect NGC 7317, probably because of the wildfire
smoke. This observation has become motivation for me to try
again on a dark night with better transparency.
Views with my 20-inch, 28-inch, and 30-inch scopes have
all been rather similar - the main difference lay in that the
larger apertures showed the galaxies as progressively more
substantial glows. I've seen precious little internal detail in
any of these targets, except that NGC 7320C is easier to see
Stephan
No.
19
20
20
21
22
-
Surface
Brightness
13.8
13.1
13.2
13.8
13.5
13.4
Mag(v)
13.6
13.4
13.1
13.1
12.6
15.5
Size/Sep
0.4′ × 0.4′
1.2′ × 1.0′
1.6′ × 1.1′
1.4′ × 1.1′
2.2′ × 1.1′
0.6′ × 0.4′
Distance
(Ml-y)
314
317
275
321
37
285
RA
22h 35.9m
22h 35.9m
22h 36.0m
22h 36.1m
22h 36.1m
22h 36.3m
Dec.
+33° 57′
+33° 59′
+33° 58′
+33° 59′
+33° 57′
+33° 59′
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.
30 SEPTEMBER 2024 * SKY & TELESCOPE
BEATRIZ INGLESSIS / S&T

Sky & Telescope - September 2024

Table of Contents for the Digital Edition of Sky & Telescope - September 2024