I'm LinkedIn and Google-Plussed.

Mail and packages, use maildrop:
Norman Sperling
2625 Alcatraz Avenue #235
Berkeley, CA 94705-2702

cellphone 650 - 200 - 9211
eMail normsperling [at] gmail.com

What is a Galaxy 'Spiral Arm'?

© 2002 Norm Sperling, excerpted from What Your Astronomy Textbook Won't Tell You

The Milky Way, and many other disc-shaped galaxies, are said to have "spiral arms". The term comes from early drawings and photographs, which show an overall spiral impression in the bright parts.

Humans tend to "connect the dots". When you carefully inspect photos of real galaxies, hardly any have arms so smooth you can actually trace them all the way around.

The very few that do seem to result from recent galactic encounters. M 51 is currently encountering NGC 5195, and M 81 has just passed M 82.

Most other so-called "spirals" only show overall impressions of a spiral-like theme, but notice:
* The "arms" look very patchy. It's easy to notice the bright spots, but make a point of noticing the faint spots and places where no arm appears at all. Only by connecting the bright patches do people perceive the continuous spiral arms. The actual "arms" are almost always very discontinuous.
* Color photos reveal the bright patches to be blue, meaning they are OB associations. Of course, wherever O and B stars form, every other kind of main sequence star forms too, but the blue giant O and B stars outshine all the others, making the area look blue. O and B stars shine brilliantly but gobble up their fuel much faster than lower-mass types. No blue giant seems to live longer than a few million years. So, what the spiral-arc patches marked by blue giants show is where star-birth happened very recently, and often where it happens right now. Blue giants never live long enough to drift very far away.
* Dimmer stars keep shining long after waves of star-birth sweep over their nests. They fill the disc, including the spaces between the "arms", with lots of stars. While the small patches that trace the "arms" are brighter than the big places between them, lots of light also comes from between arms, especially compared to places way beyond the galaxy. The disc's A, F, G, K, and M stars put out quite a lot of light.
* Color photos reveal arcs of hydrogen-pink nebulae, paralleling the blue tracery of OB associations. Arcs of dark, thick nebulosity parallel those, too. Radio-telescope traces of molecular clouds also reveal segments of spiral-like arcs.

However, each of those (blue, pink, black, etc.) lies in a different location! The disc is full of segments of spiral-like arcs, but no single one of them constitutes a physical arm, because they all lie in different places. Arms are not physical structures. Arms are illusions. The physical structure is a disc. On the disc, there are many segments of various sorts, mostly following overall spiral-like arcs.

Some galaxies have spiral segments that appear to be wound much looser or tighter than others. Sometimes the looser-wound segments are nicknamed "spurs". But in some galaxies, like NGC 7217 and NGC 1398, there are large zones where the spiral segments are wound much differently than elsewhere in those same galaxies.

The Journal of Irreproducible Results
This Book Warps Space and Time
What Your Astronomy Textbook Won't Tell You

Your Cart

View your shopping cart.