What if we were to travel beyond our own Milky Way Galaxy, and foray out into the galactic wilderness of deep space? What do you think we’d expect to find?
Would we find galaxies spread out apart from one another, little islands unto themselves? Or would we find them clustered in groups?
Well…I guess the post title kinda gave it away, huh?
Hang on tight, because it’s time for a tour of galaxy clusters!
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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.
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When the earliest astronomers and philosophers looked up at the night sky, they never could have imagined a sight like this.
What if I told you there are only four single stars in this image?
That’s right. Four.
The rest are whole galaxies, full of billions of stars.
You can tell the foreground stars from the galaxies by the diffraction spikes–astronomer speak for those four bright spikes of light. Can you find them?
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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!)
Anyway…on to the Milky Way!
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Over the course of my last eight posts, we’ve covered just about everything there is to cover about our home galaxy–or, well, at least the basics.
We’ve explored how astronomers first discovered what that incredible, milky stream of dust across the night sky actually is. We’ve followed astronomers like the Herschels and Harlow Shapley as they tried to measure the size of our galaxy.
We’ve covered its structure–a thin disk of spiral arms, surrounded by an enormous, diffuse halo–and how truly massive this great wheel is.
We then explored those spiral arms, where all the youngest stars are–and where stars form in the first place. And we explored the chaotic nucleus at the very center of the galaxy.
Most recently, we delved into the composition of the Milky Way–that is, how much heavy elements its stars contain. We discovered that stellar compositions hint at how old certain parts of the galaxy are.
But there’s one question we haven’t answered yet, and it’s quite possibly the most important one of all.
How did the Milky Way actually become what it is today?
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What is our home galaxy made up of?
In the broadest sense, it’s made up of stars, clouds of dust and gas, and the mysterious dark matter.
We could also get a little more detailed. We could say that it is a great wheel of stars, made up of a thin disk component, a central bulge, and a broader spherical halo that surrounds the disk.
We could even build on that, and say that the thin disk is where all the youngest stars are found. We could say that within the thin disk are spiral arms, where the star formation actually happens. We could say that the oldest stars are found in the central bulge and the halo, where there is very little dust and gas to make new stars.
But…what about its chemical composition? If we could explore our galaxy and bring home test tubes of “star stuff,” what would we find? And what can that tell us about our galaxy’s history?
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Here is an edge-on illustration of our Milky Way Galaxy. (Keep in mind that the disk actually stretches quite a bit farther out from the budge than is apparent in this illustration. Proportionally, its full diameter makes its thickness less than that of a pizza crust.)
What if I asked you to imagine what that central bulge would look like to us–lifeforms living inside the galaxy? What would you imagine?
Perhaps you’d imagine looking inwards toward a glowing ball of light. Perhaps you’d imagine a region of our sky unusually thick with stars and interstellar clouds. Or perhaps you’d imagine something entirely different.
But…would you imagine this?
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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 are reasonably 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?
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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?
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Over centuries of philosophy and research, through the times of the classical astronomers to Galileo’s observations of the Milky Way, humanity’s understanding of our universe has evolved from a simple model of the sun and planets to a vast wheel of stars we now know as our galaxy.
And since the “discovery” of our Milky Way–or, more accurately, the discovery of what that hazy band of stars in the sky is–we’ve come to realize just how massive our home in the cosmos really is.
That scientific journey started with the Herschels’ mapping of what was then called the “star system.” Later astronomers began to realize just how far out from the sun the stars of our galaxy really reached. Determining distances across our galaxy was the first step to discovering its size.
Later, we began to understand its structure–mapping the extraordinarily thin disk, the chaotic central bulge, and the visible part of the halo, a sphere of stars that extends beyond the plane of the galaxy.
And since then, we’ve begun to master the next critical part of understanding our galaxy: its mass.
So, how massive is our galaxy?
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Cosmos at Your Doorstep 