Sky & Telescope - April 2023 - 21

possibly consisted of a colliding pair. However, new research
indicates that the smaller galaxy is much more distant and
not physically associated with the larger of the duo.
William Herschel likely encountered this target twice in
the same sweep on February 23, 1784, recording it as different
objects, which in turn led to two listings in the New General
Catalogue of Nebulae and Clusters of Stars (NGC). But there's
nothing at the position of his second observation, and astronomy
historian Harold Corwin concludes that the two objects
are equivalent. John Herschel observed the object that became
NGC 4496 and noted it as double. (Hereafter, to differentiate
father and son, I'll identify each Herschel by his first name.)
However, the NGC doesn't list two designations for it even
if it's identified as a double nebula. Some references, though,
label the two galaxies either NGC 4496A and NGC 4496B or
NGC 4496 and NGC 4505. Observing with my 18-inch Dobsonian
at 197× (my standard setup unless otherwise noted)
and under rural Arizona skies, I described this interesting
object as faint (magnitude 11.4) but having visible structure
and a close companion that appears as a knot.
From NGC 4496, let's swing 45′ west-southwest to
NGC 4457. This 10.9-magnitude lenticular galaxy lies 63 million
light-years from us. William described it in his standard
shorthand as " pB, mbM " (pretty bright, with a much brighter
middle). In my 18-inch, I see it as almost round, with only a
slight northeast-southwest elongation. It has a very obvious
core with a 13th-magnitude star just west of center. The galaxy
also sports a faint outer ring, but that feature is probably
not visible in backyard telescopes. I failed to detect any trace
of it in my 30-inch Dob. NGC 4457 likely had a recent minor
merger (when a significantly larger galaxy absorbs a smaller
one), but I didn't notice any disruption through the eyepiece.
Continuing southeast a little less than 2° will put two
remarkable galaxies into the field of view of a low-power
eyepiece. The cores of NGC 4527 and NGC 4536 are 28′
apart and, at distances of 44 and 47 million light-years,
respectively, the pair are actually fairly close neighbors.
Despite their proximity, William discovered the two objects
on separate sweeps about a month apart.
NGC 4527 and NGC 4536 are both large, bright galaxies.
I see 10.5-magnitude NGC 4527, a nearly edge-on spiral,
as very elongated with a bright core and a mottled appearance.
Neighboring 10.6-magnitude NGC 4536 is a somewhat
S-shaped spiral, the structure of which is hinted at in the
eyepiece by two sweeping bright patches that curve around
the core. I detected faint extensions at the northwestern and
southeastern ends. Astronomers using Spitzer Space Telescope
infrared data in 2011 unearthed evidence of an active galacGALACTIC
TROMPE L'OEIL As with several targets discussed here,
NGC 4496 has gone through several iterations of designations. Astronomy
historians comb through the logbooks of bygone observers to
disentangle the target identification process. Even as this duo of galaxies
appears to be interacting, NGC 4496A (the upper one of the pair) lies at
a distance of 48 million light-years, much closer than its smaller companion,
NGC 4496B, which is more than 200 million light-years distant.
Of Sweeps and Discoveries
The great German-born British astronomer William Herschel
(1738-1822) discovered all the objects discussed
here in but a few nights of observing using his renowned
" sweeping " technique.
1784
1786
January 24
Sweep 120
NGC 4536
February 22
Sweep 154
NGC 4517
NGC 4592
NGC 4666
February 23
Sweep 158
NGC 4496
NGC 4457
NGC 4527
NGC 4636
April 22
Sweep 204
NGC 4684
NGC 4691
April 24
Sweep 205
NGC 4593
NGC 4697
April 25
Sweep 207
NGC 4731
December 29
Sweep 674
NGC 4546
March 3
Sweep 536
NGC 4958
NGC 4699
March 25
Sweep 546
NGC 4759se
NGC 4759nw
NGC 4700
tic nucleus (AGN) in NGC 4536. This was an unexpected
finding since the galaxy doesn't have a prominent bulge at
visible wavelengths. Scientists have long assumed that AGNs
are associated with galactic bulges, and so this result may
contribute significantly to our understanding of how black
holes and their associated AGNs affect galaxy evolution.
NGC 4636 is an elliptical galaxy of magnitude 9.5 located
2.2° east of NGC 4527. William described it in his notes as
pretty bright and pretty large but nevertheless listed it as
Class II (Faint Nebulae) in his classification scheme. In my
10-inch Dob, it's bright and round with a prominent core,
while my 18-inch shows a fainter, slightly elongated halo.
Poking Around the Middle
William placed NGC 4517 in his Class IV (Planetary Nebulae).
Corwin speculates that William classified it as such due
to the supposed interaction with a nearby star (possibly the
one sitting north of the galaxy; see the image on page 23).
Corwin also suggests that this demonstrates that William's
classification scheme wasn't yet fully developed at this point.
Forty-four years after his father's discovery, John observed
the object on April 14, 1828, and, due to a 5′ error in right
ascension, his discovery was assigned the separate designation
NGC 4437. There is no object at John's recorded position,
and his description matches his father's perfectly.
NGC 4517 is a very large, 10.4-magnitude edge-on spiral
located 3.6° southwest of NGC 4636. It has rather low surface
brightness but is indeed impressive in the eyepiece. I noted a
dust lane when viewing it with my 18-inch Dob. In a preliminary
2019 study, Korean researchers found that NGC 4517
and nearby NGC 4592 comprise the core of a galaxy group
that lies in front of the main Virgo Cluster at a distance of
sk yand tele scope .o r g * APRIL 2023 21
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Sky & Telescope - April 2023

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