Transcript for Q & BA Episode 1: Galaxies

Hi. Phil Plait from BadAstronomy.com here. Welcome to the very first question of Q & BA where you ask me questions about Astronomy. I got a lot of questions this week but galaxies really topped the list of topics. A lot of people asked me about galaxies, including:

MaryCCC, Jack Yomens, Mario Ramero, Dan Holland, Michelle Turner, and Joshua C.

They asked me what a galaxy is, can I see one using my own telescope, and, I know we live in a galaxy, but how do we know what kind it is?

Well, let's go over the basics. A galaxy is a collection of gas and dust and stars, held together by their mutual gravity. Now, small galaxies might have only a few million stars. Big ones, like our own Milky Way, have over a hundred billion stars. Really big ones can have trillions of stars.

There are four basic kinds of galaxies. The first kind is elliptical. Now these are elliptical shaped! Duh, right? M87 is a pretty typical example [image of M87]. It's a giant sphere of stars. Some of them are more stretched out, more elliptical than that. Those are called prolate elliptical galaxies.

Another kind are the irregular galaxies [image of NGC 4449]. These are shapeless smears and smudges. They really don't have much of a shape at all.

Peculiar galaxies do have a shape. They're weird. They're peculiar. They can be hoop or ring shape [image of Hoag's Object]. Sometimes when galaxies collide, you get all sorts of weird patterns out of them [image of the Antennae Galaxies]. They still have a shape, but they're weird.

But when most people think of galaxies, they think of those magnificent pinwheel, spiral-arm galaxies, right? Well, I have a model of one here to show you about the different components of it.

I took a picture of a spiral galaxy drawing, and glued it onto a compact disk, a CD. Now to get the proportions right, I actually stacked several of them up (looks to be 4 CDs thick). The spiral arms are actually part of a disk of stars and gas and dust. Now, in a big spiral, like the Milky Way, that disk might be 100,000 light years across, but only a few thousand light years thick. See [shows thickness of CD stack]? That proportion's about right; the width here to the thickness.

In the center is a bulge, a hub, of old red stars. Now, I have modeled that using a red rubber ball which I have cut in half and literally taped on. I know. Very sophisticated.

There are actually two different types of spiral galaxies. There's the kind I just described, but there's also the barred spiral galaxy [image of NGC 1300]. It's called that because they have a bar of stars across the center, a rectangular bar and the spiral arms emanate from the ends of those. They're very pretty.

Now, can you see galaxies like this from your own backyard? And the answer is "Yes! You can." In fact, even with a pair of binoculars you can see dozens or hundreds of galaxies. With a telescope you can see thousands. You're probably not going to see much detail because these pictures I was showing you, I mean, those were from telescopes like Hubble, which is this huge telescope with very fancy cameras on it, and with your own eye, you're not going to see a whole lot. But you can see some stuff and it can be pretty interesting.

In the northern hemisphere the Andromeda Galaxy [image of M31], another big spiral galaxy like the Milky Way, can be seen even without a telescope. You can just look up and see it, if you have a dark site. With binoculars, you can see it pretty clearly and with a telescope, it looks pretty cool.

In the southern hemisphere, you have the Magellanic Clouds, the Large and Small Magellanic Clouds. Those are also naked eye galaxies. Those are irregular galaxies. Also pretty cool through a telescope, or even though binoculars.

Now, we live in the Milky Way galaxy and we know what kind of galaxy it is. Now how do we know that? Well, let's think about it for a second. If we lived in an elliptical galaxy, we were somewhere, you know, in this big ball of, of stars, everywhere we look in the sky, we'd see stars scattered all around us. We don't actually see that. When you go out at night, you see the Milky Way, this diffuse band of light going across the sky [wide angle image of Milky Way]. That's actually made up of billions of stars. Their combined light is making that glow. And if you look towards the constellation of Sagittarius, it actually puffs out. It bulges there. That sound familiar?

[Holding up the CD model] Ha ha ha! Sounds like a spiral, doesn't it? Now, if you're outside a spiral galaxy, looking down on it, you can see those spiral arms, and it's pretty easy to tell it's a spiral. But if you're inside of it, it's not so easy, right [holding up CD model showing edge on]? Because you're inside of it, and you're seeing it edge on, like this. So what you would see, is that band of stars [pointing to edges of disk] like that, and when you look towards the center, you'd see the bulge.

Now, we know the sun is half way out of our galaxy, from the center to the edge that is. So when we look towards the center, that's the direction towards Sagittarius, that's when we see that big ball in the sky, so we know that we live in a spiral galaxy. In fact, very careful measurements of the stars in the very center of our galaxy has revealed that we actually live in a barred spiral [image of artist's conception of barred Milky Way], and the bar is quite big. It's about 25 or 27 light years across. So from a distance, outside of our galaxy, it'd be really magnificent to see it.

This brings me to an interesting point. Those pictures I showed you, those were from Hubble and from other telescopes; Big telescopes. And it turns out, no matter where we point these things, we see galaxies. They are everywhere. A hundred years ago, astronomers were arguing over what galaxies were. Now we understand what they are pretty well. We know that there are hundreds of billions of them in the universe and that our galaxy is just one among them and we have about a hundred or two hundred billion stars in our galaxy. And our Sun is just one among many of them.

We used to think that we were the center of the universe; that we were pretty special. But now that we know we're just one planet around one star in one galaxy in a really big universe. That's one of the reasons I love science. It really tells us more about ourselves and our place in the universe and I think that's one of the reasons that astronomy is so, so cool.

From BadAstronomy.com, this is Phil Plait.