Sky & Telescope - March 2023 - 15

NGC 6814: ESA / HUBBLE & NASA, ACKNOWLEDGEMENT: JUDY SCHMIDT;
GALAXY ICON: SUPZA / SHUTTERSTOCK.COM
Spirals are ubiquitous among galaxies in the
nearby universe, but we still don't know how
these patterns arise.
S
een through a telescope, spiral galaxies are surprisingly
subtle - faint and fuzzy things. Yet the structure they
possess captures the viewer's imagination. Whether you
glimpse the grand swirls of the Whirlpool (M51) or the frizzy
petals of the Sunflower (M63), the patterns become all the
more incredible when you realize they're shaped by the multitudinous
tugs and nudges among billions of stars.
These simple individual interactions add up to far-reaching
forces that have proven difficult to untangle. Astronomers
observed galactic spirals even before they knew what
galaxies were, but sound ideas explaining the pattern's origin
have come only in recent decades.
Now, even though astronomers agree on the primary
mechanism that propagates a spiral pattern, they still argue
about what sets it all in motion. What is clear, though, is
that spiral arms aren't just pretty. They play an essential role
in shaping galaxies - including our own.
What Are Spiral Arms?
Spirals are easy to see even at great distances because bright
regions of active starbirth trace the arms. But not every spiral
looks the same. The most pleasing to view are those with two
sweeping arms that wrap around the galaxy in a grand-design
pattern. Others with multiple arms - whether three or four
or just a fuzzy mess of swirls - are called flocculent.
We see these spirals, in whatever form, across many wavelengths.
Mid-infrared light spotlights young stellar objects, the
about-to-be stars whose heat shines through the dust. Livelarge-die-young
O and B stars emit blue and ultraviolet light
from their place along the spirals' spines. Radio waves emitted
from warm gas near these young stars, as well as from
cool hydrogen gas across the galaxy, also reveal a spiral shape.
The pattern isn't found only in gas and stars. Dust follows
the spiral, too, though dust arms are often offset from those
traced by stars. Even a galaxy's loose electrons and magnetic
fields align with its arms.
Any small disturbance can create a galactic swirl: Differential
rotation, in which the inner parts rotate more quickly
than the outer parts, sees to that. You can see similar
shapes in your morning coffee when you stir in the
Spiral Surfing
The Milky Way's spiral arms rotate at 210
kilometers per second (470,000 mph),
while the Sun speeds along at 240 km/s
through the galaxy. So our solar system surfs in
and out of our galaxy's arms over time.
sk yand tele scope .o r g * MARCH 2023 15

http://www.SHUTTERSTOCK.COM

Sky & Telescope - March 2023

Table of Contents for the Digital Edition of Sky & Telescope - March 2023

Contents
Sky & Telescope - March 2023 - Cover1
Sky & Telescope - March 2023 - Cover2
Sky & Telescope - March 2023 - 1
Sky & Telescope - March 2023 - Contents
Sky & Telescope - March 2023 - 3
Sky & Telescope - March 2023 - 4
Sky & Telescope - March 2023 - 5
Sky & Telescope - March 2023 - 6
Sky & Telescope - March 2023 - 7
Sky & Telescope - March 2023 - 8
Sky & Telescope - March 2023 - 9
Sky & Telescope - March 2023 - 10
Sky & Telescope - March 2023 - 11
Sky & Telescope - March 2023 - 12
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Sky & Telescope - March 2023 - 14
Sky & Telescope - March 2023 - 15
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Sky & Telescope - March 2023 - Cover3
Sky & Telescope - March 2023 - Cover4
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