The Milky Way

How Galaxies Evolve

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Ever since astronomers realized that the mysterious, fuzzy objects known as “spiral nebulae” were, in fact, separate galaxies beyond our own, we’ve sought to understand the stories of galaxies.

We’ve since discovered that most galaxies are found in galaxy clusters. There are two types: rich clusters, which are home to thousands of galaxies, and poor clusters like our own “Local Group,” which are home to only a few dozen to a few hundred galaxies.

We’ve also discovered that collisions between galaxies are quite common, and can drastically alter their shapes.

And we’ve observed something even more curious. Spiral galaxies, like our own Milky Way, are most often found in poor clusters. Rich clusters are dominated by elliptical galaxies.

It would seem that spiral galaxies thrive best in uncrowded galactic environments.

But what can that tell us about galactic evolution?

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When Galaxies Collide

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Meet the Whirlpool Galaxy, a spiral galaxy in the constellation Canes Venatici. This was actually the first galaxy to be classified as a spiral.

Specifically, the Whirlpool Galaxy is what we call a grand-design spiral: a galaxy with a very distinct pattern of two spiral arms. It was discovered by Charles Messier way back in 1773 and added to the Messier catalog as a “spiral nebula,” way back before astronomers knew what galaxies were.

But…hold on just a second.

Anybody notice there isn’t just one galaxy here?

The Whirlpool’s companion is known as NGC 5195–or, if you prefer, M51b. Together, the two galaxies are designated in the Messier catalog as M51.

We are, in fact, viewing them in the aftermath of a collision that drastically altered the shape of both galaxies. Both of these were likely once uniform disk galaxies with no spiral arms.

So, what happens when galaxies collide? And does this happen often?

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A Peek at Supermassive Black Holes

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There are few things in the universe quite as exciting as black holes.

They’re in all the movies–I even wrote a post a few years ago on what the movies get wrong about them! Though some movies, like Interstellar, do a pretty good job of capturing the relativistic effects.

There are also plenty of popular misconceptions floating about–probably thanks to those fictional portrayals!

Most of the time, when we geek out over black holes, we’re talking about stellar-mass black holes. That is, the remains of massive stars.

Wait a second…most of the time? What other kind of black hole could we be talking about?

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How Big and Bright are Galaxies?

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The brightest galaxy in our night sky is the Andromeda Galaxy, our nearest intergalactic neighbor.

Hey…notice we’re talking about intergalactic neighbors now? 😀

In most of the posts on this blog, we’ve talked about interstellar neighbors, interstellar space, etc. But now, we’ve graduated to the intergalactic frontier, and we’re not turning back–at least not for a little while!

Anyway. The Andromeda Galaxy is the brightest galaxy in our night sky. But as you may remember from my eons-old article on apparent visual magnitude…that doesn’t tell us much about how bright it actually is.

It is the closest galaxy to us, the denizens of the Milky Way, so that tells us one thing: if it appears brighter than other galaxies, it probably is.

But…let’s say we put all galaxies in the universe on an even playing field, magically all exactly the same distance from Earth.

How big and bright would they appear then?

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How Far Away are Galaxies?

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Well, I’ll give you a spoiler: they’re ridiculously far away.

Let’s consider for a moment what a light-year actually means. It sounds like a unit of time, but it’s actually the distance that light travels in one Earth year.

Think of it this way: if your name is Bob, and you can travel a certain distance in one year, that distance could be called a Bob-year.

I know it’s strange to think of light traveling at a certain speed. When you flip a light switch, the room immediately brightens. When you shine a flashlight, its beam immediately falls across the nearest surface.

But that just goes to show how insanely fast light travels. If it takes 2 million years for light to get from one object to another…imagine how far apart those objects are?

Well, that’s the case for our home galaxy, the Milky Way, and our nearest galactic neighbor, the Andromeda Galaxy.

But…wait a second. How do we know that?

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The Milky Way Demystified

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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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How Did the Milky Way Form?

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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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The Composition of the Milky Way

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This image has an empty alt attribute; its file name is andromeda-galaxy-milky-way.jpg

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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Exploring the Milky Way’s Nucleus

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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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What Are Spiral Arms?

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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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