Open-Faced Spirals
32 JUNE 2019 • SKY & TELESCOPE
M49M60 M59 M58M613521
40305147524862φσιυνβογ ηδεθζVIRGOBOÖTESLEO13 h 30 m 12 h 30 m 12 h 00 m 11 h 30 m+10°+5°0 °13 h 00 m –5°ing Handbook and Catalogue of Deep-Sky Objects. It perfectly
evokes the image of the dense, starlike knots that populate
some nebulous objects. The arms of NGC 3310 are short,
barely more than stubs, but studying the galaxy until they
become detectable will establish the technique you’ll employ
for the rest of this tour. Owners of large-aperture scopes
might pull even more out of this small galaxy, as some very
faint and wispy outer arms extend out to the northwest.
Often one has the unmistakable impression of stellar struc-
ture even without being able to trace out and separate the indi-
vidual arms of an object. NGC 3521 in Leo is a good example.
Located in the same fi eld as 6th-magnitude 62 Leonis, this is
a rather large spiral tilted at an oblique angle to our line of
sight. NGC 3521 is often described as a fl occulent galaxy — its
multiple branching arms give the halo a fl eecy appearance.
The arms seem to blend together, becoming distinct only at
their extreme ends. This is a remarkable object with a great
deal to offer the persistent observer, so give it some time. A
very subtle dust lane can be detected on its western edge with
the faintest hint of nebulosity beyond it.
We might think of the spiral arms as a population of
stars orbiting a galaxy as a structured group, but that’s not
the case. One theory proposes that the arms are essentially
density waves that orbit the galactic core independent of the
stars within them. Stars move through the arms as they orbit
the galaxy. Orbiting gas clouds also move through the arms,
where they get squeezed and compressed by the denser grav-
ity there, leading to star formation. Indeed, most of a spiral
galaxy’s starbirth occurs in the arms, and it’s this starburst
activity that highlights them.
Also situated below the bowl of the Dipper, NGC 3631 is
a bright face-on spiral with a small bright core and loosely
wound spiral arms. At least two major arms wrap around
the core counterclockwise. One arm fl attens out into a
straight band that stretches to the east. The arms can be
traced out rather easily by the numerous stellarings that
highlight their mottled form. These dense knots suggest
regions of star formation where we are seeing the combined
light of illuminated or radiating clouds of gas and dust and
the hot young stars within them. Regions of star formation
in our own neighborhood, like M42 or M17, would likely
appear similar to an observer in a far-off galaxy.
About 1° northeast of Chi
(χ) Ursae Majoris, NGC 3893
isn’t quite face-on to us. It
has one large arm that wraps
around its southern edge and
stands out prominently to the
east of the core. NGC 3893 is
smaller than our Milky Way
and about 49 million light-
years distant. It’s likely inter-
acting with tiny NGC 3896,
which is in the same fi eld
of view. Photographs of thepWOOLLY GALAXY Some galaxies, such as NGC 3521 in Leo, are
members of a class of objects known as fl occulent galaxies. They are so
named for their soft, woolly appearance.pTAKE A LONG LOOK The graceful spiral NGC 3938 in Ursa Major re-
quires deeper examination to tease out its structure — but you’ll be richly
rewarded by the galaxy’s delicate spiral arms with their brighter knots.NGC^352
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