Sky & Telescope - November 2023 - 63

Then, last year I sold the 28-inch optics and replaced
them with a 30-inch f/2.73 meniscus mirror. I modified the
structure of the 28-inch to fit the new optics, and I now have
a scope with a significantly wider true field of view - so I just
had to sketch M33 all over again. I started with a preliminary
rendering one night when it was too spectacular to ignore:
Oh my goodness, M33 is so BIG! The usually difficult to see
outer spiral arms are surprisingly distinct and easy to follow as
they wrap around the galaxy. [My] rough sketch [is] mostly to
show the extant and width of these arms. 140× gives the best
view. 21.65 SQM
That was a great night, and I had several more in the late
summer of 2022, but I saw way too much in M33 to describe
it all. Instead, here are some of my favorite highlights.
Spiral Arms
The northern and southern spiral arms form the backbone
S of M33's spiral arms and dominate the overall structure
of the galaxy. The northern arm is the most prominent and
curves northward like a crooked finger from the core region.
About halfway along its extent, there's a noticeable kink,
which reminds me of the sharp bend in M51's northern spiral
arm. You may have to look for it carefully because it can get
lost with everything else going on. It's possible this kink is a
result of M33's flocculent (having multiple arms) spiral structure
forming and reforming over time.
The main southern spiral arm is punctuated with three
large star clouds that originate from what may be a very
short central bar near the origin of the much shorter eastern
spiral arm. But then, the appearance of this area may just
be where these two arms meet the core. A recent study by
astronomers using the Hubble Space Telescope showed that
stars older than one billion years might actually form a central
bar that's masked by the distribution of younger stars.
Did I see some of that older-population bar? Probably not,
because what I saw is much shorter than what would constitute
a central bar.
There are faint extensions of the northern, southern, and
eastern arms that continue wrapping around M33 as well as
several equally faint features that seem only casually connected
to the main arms. I noted arm segments as well, but
seeing any of them is very much dependent on observing
conditions and the size of your telescope.
Several areas of dark gas and dust lanes underline the
shape of M33's arms, with the most prominent ones being
where the northern spiral meets the core. Just about as visible
is a small, dark patch on the inside bend in the kink of the
northern arm. There are many more scattered throughout the
galaxy, but they're generally fairly diffuse.
The Many Marvels of M33
In a fairly large amateur scope, an observer can detect most
of these internal details: H II regions, stellar associations
(collections of OB stars in star-forming regions), and even a
smattering of globular clusters. Some telescopes show significant
structure at high magnification on nights with steady
seeing and even reveal a couple of M33's brightest individual
stars. This galaxy is absolutely loaded with stuff to observe if
you have a dark enough sky and a scope 8 inches or larger.
Foremost is NGC 604, a star-forming H II region so large
that it displays a wonderful irregular shape at high power.
It's about 1,500 light-years in diameter, which is also about
how far away we are from the Great Orion Nebula in our
own galaxy - imagine a star-forming region that stretches
from here to M42! NGC 604 and the galaxy's core are M33's
brightest features.
DIVERSE NEBULOSITIES NGC 604 is on the right side of this sketch as I saw it through my 28-inch scope at 695×. I usually perceive this H II
region as two overlapping triangles of nebulosity, and on the best nights I see a stubby tail. The faint stars within NGC 604 are huge O and Wolf-Rayet
stars. The stellar association A85 is at left in the sketch. The star between the two nebulae is 11.0-magnitude TYC 2293-642-1, and just outside the
upper left of this closeup is the globular cluster C27. North is to the upper right.
sk yand tele scope .o r g * NOVEMBER 2023 63

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Sky & Telescope - November 2023

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