Video Transcript: Classifying Galaxies

Have you ever gone to a museum of natural history?  These are museums where they have all kinds of examples of maybe different rocks and minerals, and different examples of insects or animals and you get to explore all the variety that there is in the natural world.  One of my favorite places to visit is the Harvard Museum of Natural History, which is in Boston.  I lived in Boston for a while and I would visit this museum several times.  And looking at the butterflies is one example of how many different kinds of butterflies there are in the living world.  Or beetles; there’s so many, thousands and thousands of different species of beetles, each with their own unique colorings, and, and sizes, and all sorts of different things.  I can't help but think about that when I think about galaxies, and classifying galaxies, because there are so many different kinds of galaxies, and not so many different… the variety just put that way, the variety of galaxies is astounding, every galaxy looks a little different, it seems like.  

And so when astronomers look at all these different galaxies, they try to make sense out of them.  They try to kind of put them in groups and say which galaxies are similar to each other, in the hopes that maybe what we can do is figure out how those galaxies are related to each other.  And I'm trying to think of a good comparison.  I think the living world is a is a good one, where you say these kinds of trees have leaves that grow and fall every year.  So even though these trees are very different from one another, they're the same kind of tree, whereas evergreen trees, they keep their needles all year round.  So they're many different species of tree, but they fall into these categories.  In the same way, we want to try to do the same thing with galaxies. 

The first picture in the gallery, you can see a sense of this beautiful diversity of galaxies that there are.  These all happen to be examples of spiral galaxies.  When we get to other kinds of galaxies, there's even more.  And one of my favorite categories of galaxies actually like the irregular galaxies, you'll see rings of stars, you'll see all kinds of crazy jumbled messes, but every galaxy has its own little unique look to it. 

Now, what I'd like to do is show you this example.  Oh, in fact, there's a small version of right here.  But the second picture in the Gallery is one of my favorite galaxies, and that is M 51, the Whirlpool galaxy.  And what this shows is that galaxies can actually interact with each other.  So you have this really large spiral galaxy, then you see a smaller Galaxy off to the side that's interacting gravitationally with it.  And they're sort of in the process of colliding with each other.  Again, we can't actually see them colliding.  What we can do is we can measure their speeds and we can tell how close they are, how much they weigh, things like that.  Until they're in the process over the course of millions of years, like the slow motion collision with each other.  And when two galaxies collide, this is a crazy thing.  When they collide, in a sense, they don't really hit each other, because they're made up of these tiny little stars.  So it's almost like two swarms of bees.  If these two swarms of bees are flying at each other and they hit each other, the swarm will get kind of messed up, but the chances that individual bees are actually going to touch each other is very low.  And the same is true in a galaxy; the chances that stars are actually going to hit each other is basically zero, but the galaxies are going to be disrupted and all the gravity is going to get messed up and probably the spiral arms are going to get all discombobulated because of this interaction between the two. 

So we're seeing that process just starting here.  In other galaxies, we can see cases where they're currently colliding or just past collision.  So we can see that galaxies interact with each other over time throughout the universe. 

Now, we've talked a lot about spirals, but there's another kind of galaxy that's actually more common in the universe.  And you can see that in picture three, this is called an elliptical galaxy.  And an elliptical galaxy is essentially a big ball of stars; like a huge ball of stars.  So we're getting we're talking about 100 billion stars in one of these galaxies. 

Now, not all galaxies are that big. They can range in size.  Some are very small, some are really big, but on the big side of things, they can be that large, 100 billion stars, and you'll see that an elliptical galaxy looks a lot like the halo of a spiral galaxy.  It's kind of this diffuse glow.  It's bright in the middle, it gets fainter as you get further and further out, it looks like it's just kind of like a glowing diffuse ball.  Really, it's all of these stars, we can't see the individual stars necessarily, we can just see the collective light from all of these stars inside the galaxy. 

Now, not all ellipticals are actually spheres; some of them may be stretched and be more like an egg shape.  So they come in many different shapes and forms.  Now the question can be raised, how do spiral galaxies and elliptical galaxies relate to each other?  I mean, what's going on here?  Are they connected somehow?  Do galaxies start as spirals and then eventually become ellipticals?  Do they start as ellipticals and eventually become spirals?  Well, we don't really know, we don't really know.  Some of the ideas are that spiral galaxies to begin that way, as spirals, they collide with each other, and then they become ellipticals.  But we see several spiral galaxies that appear to be very old, just as old as ellipticals, so there's no sense that one is older than the other, or that spirals always becoming elliptical.  So that's something that astronomers continue to work out, how are these things related to each other?  And it's very difficult when we can't watch them interacting with each other; we can't see how they're changing as much over time as we'd like to. 

One way we can do that is by looking deep, deep in the sky to the faintest galaxies, because as we look further back to those faint galaxies, we're looking back in time.  We'll talk about that more later.  I won't get ahead of myself, I get excited about this stuff. I want to get ahead.  Okay.  So, in the relationship between the two, the most likely case is that spirals kind of can make their way and become ellipticals, but not all spiral galaxies do that. That's kind of our leading understanding. 

Alright, so as I mentioned before, to that, there are other kinds of spirals, you know, there's these barred spiral galaxies, which can look like this.  It’s really amazing, this straight line bar across the center of the galaxy and then the spiral arms that come off of it.  So astronomers are trying to understand these different shapes, do a lot of simulations, where they're running computer programs with hundreds of billions of stars in it, doing the gravity of all these things, figuring out where with the spirals form, how different shapes could arise, and why they might look the way they do. 

Astronomers have tried to understand all these different kinds of galaxies, and try to put them in different categories. This is something that Edwin Hubble took on this challenge; went through cataloging a bunch of different galaxies, tried to put them in different categories, and that's how we ended up with what's called the Hubble Tuning Fork Diagram.  This is the last picture in the gallery.  And it's the current classification scheme that we use for organizing galaxies.  And there's basically three categories, maybe four categories that we put galaxies into.

There's the ellipticals, that's the start of the tuning fork; ellipticals.  So that's the E; we give them a designation of a capital E, and then we give them a number.  And that number represents how circular it is versus how elongated it is.  So if it's like a perfect circle, like a sphere, we call it an E0 (zero).  And as it gets longer and longer, more like an egg, maybe even all the way to be like a cigar, we call it E5, E6, E7, all the way up really to E9. 

And then we have different categories of spiral and barred spiral galaxies.  So a spiral galaxy, as you imagine just has spiral arms, where those spiral arms connect all the way to the center of the galaxy.  A barred spiral is where there's a clear bar in the middle and the spirals come out of that bar.  And again, just like with the ellipticals, we give the designation of 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, depending on whether it's like face on, we can see a nice circle, or whether it's kind of flat, or flattened out.  

Now, this is really easy to classify galaxies like that when we can see a big beautiful picture, but the majority of galaxy pictures we have the galaxies are tiny, you can hardly see them in the picture because they're so far away, and so it can be difficult to tell whether it's a spiral or a barred spiral and astronomers struggle sometimes to classify these galaxies.  

And the fourth category is sort of a catch all; we call it irregular galaxies.  Those are galaxies that don't appear to show any of the same patterns as the others.  They have dust bands, but they're not really spirals.  So they're not ellipticals and not barred spirals, they just don't fit.  And so we call those irregular galaxies.  Roughly like 3% of galaxies would fall into that category. 

Now, one of the dangers here, oh, right, right, right, one thing I forgot about the spirals is that we also give them this little a, b and c.  The a, b, and c is a is meant to distinguish how tightly wound the spirals are.  In some galaxies, you have this really tight wind, where you can see the spirals going all the way around several times in this galaxy, and that's what we denote with like a lowercase c, that would be on the far side.  Lots and lots of spiral arms.  Whereas on the a side, there's less spirals and they're more spread out.  So they're not as tightly wound about.  

So what good does this do to label galaxies this way?  Well, the hope is that by getting galaxies in different categories, you can kind of try to find patterns and trends, how these things are related to each other.  And Hubble's diagram is kind of misleading, because it sort of suggests that maybe a galaxy would make its way along this tuning fork and that it changes from one to the other all the way down.  But that's not really the case.  And we don't really know how these things are related to each other.  How is an Sa galaxy related to an Sc galaxy?  Can it change and become that?  We just don't know how these things work yet.  At least I don't and I've never read about it.  Maybe someone out there does, but I certainly don't know.  

Okay, so that's the Hubble classification.  And you get a sense of how astronomers have tried to address these problems looking for patterns.  Sometimes it's successful.  Sometimes it's not.  Sometimes we're partway through and we haven't figured it out yet.  But that's the classification system for galaxies. 

Okay. We'll see you next time.