By now, we’ve spent a heck of a lot of time exploring spiral galaxies.
It makes sense–they’re certainly the most photogenic. Seriously. Do me a favor and do a quick Google search for galaxies. When I did, nearly all the results were spirals…even though spirals are not the most common galaxies in the universe.
There is, of course, another reason we’re so familiar with spirals right now. We dipped our toes in the waters of studying galaxies by exploring our own home galaxy–a reasonable starting point. Our Milky Way just happens to be a spiral.
Well…it doesn’t “just happen” to be a spiral. But we’ll get to the reasons for that…
For now, let’s take a dive into all the different types of galaxies.
Alright, people…time to finish off our exploration of the Milky Way Galaxy, our home in the cosmos!
For the past nine weeks, we’ve covered everything from how our galaxy was “discovered” to how it may have formed. But there’s so much more to explore–and, starting next week, we’ll begin covering the vast universe of galaxies beyond our own!
But before we do that…I want to wrap up our discussion of our own galaxy with an overview to tie the last nine posts together.
(By the way, has anyone noticed I actually managed to chug out a post a week for the entire Milky Way “module”? I’m a bit impressed with myself for that!)
Probably the most spectacular feature of our Milky Way galaxy is its spiral arms.
We can’t get a probe far enough out yet to take a galactic selfie, but astronomers arereasonably sure that we live in a spiral galaxy. Observations of other spiral galaxies offer clues to what kind of objects can help us trace out the shapes of spiral arms, called spiral tracers. Using those spiral tracers, we’ve been able to map out patterns within our own galaxy that appear to be spiral arms.
Over the years, astronomers have tested the spiral arm hypothesis against the evidence again and again, and there is now a great deal of confidence that the Milky Way is a spiral galaxy.
More than that–star formation, which we know is limited to the disk of the galaxy (rather than its central bulge or halo), appears to be specifically found in the spiral arms.
But why? And for that matter…what even are spiral arms?
The Milky Way–our home galaxy–is a spiral galaxy, a classification I often describe as pinwheel-shaped.
The main difference between a spiral galaxy’s shape and a pinwheel’s shape is that spiral galaxies, like the Milky Way, only have two main arms. For the Milky Way, those are the Scutum-Centaurus arm and the Perseus arm. If you study the image above, you’ll notice that all the other arms are a bit wispier, and most branch off from the main arms.
There’s just one problem, though…
How do we even know that this image is an accurate depiction of our galaxy? How do we know that the Milky Way has spiral arms?