“What is art but life upon the larger scale, the higher. When, graduating up in a spiral line of still expanding and ascending gyres, it pushes toward the intense significance of all things, hungry for the infinite?” -Elizabeth Barrett Browning
I don’t mean to ask why the Milky Way is a spiral in an existential sort of way. You see, many galaxies, like our own, and also like Andromeda and the Triangulum Galaxy (below) — our nearest galactic neighbors — are spiral galaxies.
They’re what I think of when I think of galaxies, and they’re probably the first thing that pops into your head, too. But not all galaxies are spirals; a huge fraction of them, in fact, look vastly different. In fact, they look more like diffuse blobs than anything else.
They’re known as elliptical galaxies, and my favorite picture of one is this guy: Centaurus A.
(You can thank the Canada-France-Hawaii telescope for this shot; it’s easily the prettiest picture of Centaurus A I’ve ever seen!)
But why are we a spiral? Here we are, with our stars living in a plane, with great, dense spiral arms, and just a small bulge at our center. We find plenty of spiral galaxies edge-on, like this one:
And what do we see? Well, if we take a picture of our galaxy from within it, it looks almost exactly the same!
(And you can thank the old COBE satellite’s infrared eyes for being able to see through our galaxy’s dust!)
So why are we a spiral instead of an elliptical galaxy? Believe it or not, we think that nearly every galaxy started off as a spiral! Why’s that? Well, before we were a galaxy, we were just a giant blob of matter, waiting to collapse under its own gravity.
However we started off, one side was inevitably shorter than the other two, and that direction collapsed first. The other two directions were large, however, which means we went from an ellipsoid down to a pancake-shaped object!
This is why most galaxies that we find in isolation — known as field galaxies — are spirals!
But what happens when these galaxies meet up with other large galaxies? They don’t just pass like two ships in the night; gravity is not nearly so quiet when you get two massive objects close to one another. Take a look at the heart of Stephan’s Quintet, and look particularly at the upper two galaxies in the center of the image.
Let’s take a closer look, but this time, let’s superimpose X-ray light on top of this visible light image. Why? X-ray light comes from hot gas, and hot gas shows up when you have galaxy collisions! So if we see X-rays, it tells us that there was some part of these galaxies that just collided with another one!
So we look, and what do we see?
Awesome! There’s totally evidence of a collision there! In fact, we can look out at all sorts of points in the Universe, and find galaxies in different stages of a major collision. You know what we find?
It looks like colliding spiral galaxies form elliptical galaxies! We call this hierarchical mergers, and this helps explain why we find elliptical galaxies mostly in dense clusters, and spirals most out in the sparse “field”.
But perhaps a video simulation of two merging galaxies can show you this better than my words can.
This is your future, folks! Andromeda, our closest galaxy, is not only about the same size as us (more or less), but she’s coming right for us! In a few billion years, the Milky Way and Andromeda will merge, and do you know what happens then? That’s right, we’ll be one giant elliptical galaxy, too!
So enjoy our spiral galaxy while we have it; it won’t last forever!