Sky & Telescope - August 2021 - 58

AUGUST 2021 OBSERVING
Going Deep
+20°
PEGASUS

70 Peg

55 Peg


PISCES
2
3
4
5
6
7
0h 00m

Circlet



0°
23h 30m
23h 00m
AQR
Pegasus 1
+10°

at Mount Wilson
Observatory,
fi rst measured the
radial velocity of
NGC 7619. Edwin
Hubble's iconic
1929 paper that
established a relationship
between
recessional velocity
and distance noted
that this was the
fi rst reliable result
consistent with the
linear relationship
that's now known
as the HubbleLemaître
Law.
In X-rays,
NGC 7631
NGC 7626
PGC 197705
UGC 12535
MCG 1-59-58
15′ × 15′
ELUSIVE TARGET Can you detect the faint companion to the
distant galaxy UGC 12535?
in a friend's 25-inch. It has since become
a favorite galaxy-hunting ground.
Starting Point
At the center of the group are the two
dominant members of the cluster,
NGC 7619 and NGC 7626. They're a
pair of large, similar-looking ellipticals
with bright cores, visible in an 8-inch
telescope. Milton Humason, observing
58 AUGUST 2021 * SKY & TELESCOPE
NGC 7619 is quite
bright and sports
an intriguing, long
tail that streams
off to the southeast.
Astronomers
study this enigmatic
feature in order
to understand the
physical properties
of the hot interstellar
medium and
the dynamics and
evolution of galaxy
clusters. Researchers
fi nd NGC 7626
equally fascinating
due to its kinematically
peculiar core.
At its heart, separate
coexisting cores
rotate with different
velocities and
axes of rotation,
revealing details of
the galaxy's merger
history. NGC 7626 was one of the fi rstknown
examples of such a peculiar core.
Moving about 11′ north of the pair,
we fi nd NGC 7623 forming the apex of
an isosceles triangle with NGC 7626
and NGC 7619. Larger apertures reveal
a north-south elongation in the small
and round bright core. Herschel didn't
note nearby NGC 7621, 2.2′ to the
southwest; German astronomer Albert
Marth discovered it 79 years later. Since
NGC 7621 is rather faint and very elongated,
I used averted vision to pick it up.
NGC 7619 also has a companion:
NGC 7617. It's located 2.8′ to the
southwest and is small and faint with
a slightly brighter core. The NGC entry
for 7617 credits both Heinrich d'Arrest
and Marth - who both observed it in
1864 (two months apart) - with the
discovery. However, Bindon Stoney,
while serving as assistant to William
Parsons, the Third Earl of Rosse, at
Birr Castle may have fi rst detected the
galaxy in 1851. Unsure of his observation,
he noted that the nebulous object
may have been stars instead, and so
Dreyer didn't credit him with the
discovery. But, either Bindon Stoney
or his older brother George Johnstone
Stoney discovered NGC 7631 in August
1851 at Birr Castle, while d'Arrest
independently noted it 11 years later.
Sitting about 11′ east of NGC 7626,
this 13.1-magnitude galaxy is elongated
roughly in the east-west direction,
brightening gradually toward the center.
Nudging North
Look 15.6′ northwest of NGC 7623 to
fi nd NGC 7612. This relatively small,
moderately faint lenticular galaxy
has a brighter core and a challenging
companion. With averted vision, I see
PGC 71091 as a very small, very faint
spot of nebulosity 2.8′ east-southeast
of NGC 7612.
About 8′ west of the NGC 7623/7621
pair, we come to NGC 7615, a small spiral
galaxy with a smooth brightness distribution.
John Herschel discovered it in
1830; however, d'Arrest, as well as German
astronomer Wilhelm Tempel and
Swedish astronomer Herman Schultz,
failed to recover this galaxy during their
investigations of the group in the 1860s.
At 197 million light-years away, this is
the most distant object in the collection.
About 10′ farther west-southwest
you'll fi nd slightly fainter NGC 7608.
With averted vision, it appears very
elongated and a little brighter toward
the center. Nearby UGC 12510 manifests
as a small, nebulous spot 7.8′
southeast of NGC 7608.
Star magnitudes
POSS-II / STSCI / CALTECH / PALOMAR OBSERVATORY
Sky & Telescope - August 2021

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