SEEN 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 realise they're shaped by the multitudinous tugs and nudges among billions of stars.
These simple individual interactions add up to farreaching forces that have proven difficult to untangle. Astronomers noticed 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. Live-large-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: , in which the inner parts rotate more quickly than the outer parts, sees to that.
