Sky & Telescope - November 2023 - 59

Messier 33
An Observer's Guide
Get the best out of this wonderfully detailed spiral galaxy.
I
n which face-on spiral galaxy can you see spiral arms, H II
regions, stellar associations, globular clusters, patches of
dark interstellar gas and dust, and maybe even a couple
of its brighter stars? And all this without needing a monster
telescope to boot?
From the Northern Hemisphere, look no further than
M33. Famous for both its remarkable wealth of galactic
detail and its visual subtlety, the Triangulum Galaxy, for far
too many observers, is equally infamous for being effectively
invisible. Minimal light pollution or a slightly hazy sky can
mask or completely hide the wonders of M33. But the good
news is that even a small scope can show a lot of satisfying
structure under an unspoiled, dark sky.
How small? An 80-mm will do for a start. But as is usually
the case, the bigger the scope and the better the sky, the more
you'll see. But don't let less-than-optimal sky conditions stop
you from having a look.
I made sketches of M33 while observing with my 28-inch
scope between 2009 and 2013 and with my 30-inch in
2022. I also sketched it with 8-inch and 80-mm scopes on
the same nights I used the 30-inch. Each instrument gave
memorable views, and even though I'm partial to what I saw
in the 28- and 30-inch, I was rather amazed at how much
I detected in the smaller apertures. But then, M33 is rather
nearby as galaxies go.
M33 IN ALL ITS GLORY This sketch illustrates how M33 appeared
through my 30-inch f/2.73 alt-az Newtonian. This view approximates
what I observed at 140× over seven nights in 2022 and highlights all the
objects discussed in the text - and many more. It requires clear, dark,
and transparent skies to see this much even in a large telescope, but
much smaller instruments can nevertheless capture remarkable detail.
Aside from spiral arms and dark nebulae, you can observe H II regions,
stellar associations, clusters, and even individual stars in M33. North is
up in all images unless otherwise noted.
uIN THE TRIANGLE M33 is located close to the western boundary
of the tiny constellation Triangulum, from which it gets its common
name. You should find the galaxy quite readily a bit more than 4° westnorthwest
of 3.4-magnitude Alpha (α) Trianguli. On nights with excellent
observing conditions, you might even see it without optical aid.
To my surprise, we still don't know exactly how far away
the Triangulum Galaxy is. Based on different methods of
measurement, distance estimates have ranged from 2.2 to
3.2 million light-years. Most recent studies place it in the
middle of this range, indicating that it's approximately 2.8
million light-years from us.
M33 may be a gravitational companion to M31, the
Andromeda Galaxy, and studies place it as far away as some
770,000 light-years from M31. The Triangulum spiral is also
the third-largest member of the Local Group (after M31 and
the Milky Way), and at 60,000 light-years in diameter, it's a
bit more than half the size of our galaxy.
It's classified as an SA(s)cd galaxy, where SA denotes it's an
ordinary spiral galaxy without a central bar; the (s) indicates
it has prominent S-shape arms emanating from its central
core; and the cd means the spiral arms are loosely wrapped.
That's a pretty good overall description of M33, but it really
2h 00m
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TRIANGULUM
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ARIES
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M31
M110
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Sky & Telescope - November 2023

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