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Transcript for Q & BA Episode 1: Galaxies

Hi. Phil Plait from here. Welcome to episode 3 of Q & BA, where I answer your questions about astronomy. Richard Sanders from Sydney, Australia [Oi!] writes in and asks, "How much of the Milky Way galaxy do we see with the naked eye?"

That's a good question. Let's go to my model of the Milky Way that I used in episode 1 because, Milky Waste not want not. [holding up CD model] The Milky Way is 100,000 light years across, so it takes light 100,000 years to go from one end of the galaxy to another. The Sun is about half way from the center of the galaxy to the edge, so that's about 25,000 light years. So 100,000 light years across; 25,000 light years to the center, and that gives you an idea of the size and scale of the Milky Way galaxy and the Sun's position in it.

Now, when you go out at night, how far away are those stars? How much of the Milky Way are we seeing? Well, you know that as something gets farther away it gets fainter, of course. The Sun is the brightest star in the sky, but that's because it's up close. If it were farther away, it would be much fainter. To get it to be just visible to the naked eye, so that if it were any fainter you wouldn't be able to see it, you'd have to move it to a distance of about 60 light years. Right at 60 light years is right on the edge of visibility.

Well, 60 light years is not very much compared to the galaxy, which is 100,000 light years across! Err! It's huge, right? Sixty light years is a little tiny piece. We only see starts that are actually really close by to us compared to the immense size of the galaxy.

Well, if you look at all the stars in the galaxy, all of them, far away, and most of them are these little, dinky, dim balls. [Image of small red dot labeled "Faint, Dinky Star" next to big yellow sun labeled, "Sun"] They're lower mass than the Sun. They're less luminous. They're fainter than the Sun and so they have to be pretty close for us to see them because at 60 light years we wouldn't see them at all!

So, even though most of the stars in the galaxy we see are faint, we only see them if they're close. So, they're all out there, but when they're far away we just don't see them and they're invisible.

So, when you look up at the sky at night, most of the starts that you're seeing are actually these dim bulbs that are close by. But stars that are more massive than the sun, are more intrinsically luminous, they're brighter [image showing relative sizes of different spectral classes of stars, OBAFGKM with the O and B class stars being bluer, and larger, labeled "Huge, bright giants". The label "Sun" points to a smallish, yellow G class star]. They can be seen from much farther away. So even though they farther away they look really bright. So it's kind of interesting. Most of the stars you see at night are faint and close, but a few of them, the brightest stars, tend to far away, but they tend to be these powerhouses of stars.

For example, Rigel, in Orion [image of the constellation Orion, with Rigel (the bright, blue star below the belt) labeled], represents Orion's left knee. It's 800 light years away. Eight hundred! That's pretty good.

Deneb, which is in the northern constellation of Cygnus, the swan, is a thousand light years away! And yet it's one of the brightest stars in the sky. But it's not the record holder.

That place belongs to a star called Mu Cephei [image of μCephei]. Mu Cephei is a moooonster. It is a red supergiant. It has about twenty times the mass of the Sun, but it is huge. If I were to represent the Sun with this Minty, a tasty Australian candy, if this were the Sun, Mu Cephei, fifteen hundred times larger. It would be 30 meters across. The size of an eight or nine story building. [Image showing dome of Keck Observatory, Hawaii with an eensy weensey door and the whole of the dome being 30m]. So Mu Cephei is huge. It's about 25 hundred to five thousand light years away. Very far away. It's hard to get the exact distances of stars like this. But that's a pretty rough estimate. That's probably pretty close.

It is so big, that if you put it in the center of our solar system, the surface of this thing would stretch out past Jupiter's orbit. [Image showing drawing of solar system out to Jupiter's orbit with the diameter of Mu Cephei shown as well, extending somewhat beyond Jupiter's orbit.] So that, that's really just about as big as a star can get. It's really a monster. And that is the farthest star you can see with the naked eye.

Now, it's not the farthest object you can see, however. In the northern hemisphere, you can see the Andromeda galaxy which is about 2 and a half million light years away. In the southern hemisphere, you can see the Magellanic Clouds. They're about, eh, 170 thousand light years away. Something like that.

People with very keen eyesight and a very dark site, can see the galaxy M 81 [image of M 81 shown.], a beautiful spiral galaxy. That's about 12 million light years away. And that's probably the record holder, the object that you can see, the most distant thing that you can just go out and see if you have really good eyesight. I can't see it but maybe you can.

But even that's not the potentially most distant object. When a star explodes, it becomes a supernova, it gets extremely bright. There's a type of exploding star called a gamma ray burst, and it's sort of a super, supernova. And they're very bright, and they can be seen from very far away. And there was one in 1999, [image of GRB 990123] that was about nine or ten billion light years away. And it was almost visible to the unaided eye. These things only last a few seconds and if you had been looking at just the right spot at just the right time, with binoculars, you could have seen this thing. But if it had been a little bit brighter, or a little bit closer, you actually could have seen an object billions of light years away with your unaided eye.

This question, "How far out of the Milky Way can we see?" it seems like a simple question. But it leads you through these steps until you find out more and more amazing about the universe. And that's why I love the simple questions, cuz you never know where they're gonna take ya.

So that's it for this episode. But! Don't forget that I need more questions. Send me more at Maybe I'll use it here on Q & BA.

So, for Q & BA and for, I'm Phil Plait.

No Minties were harmed in the creation of this video.

©2008 Phil Plait. All Rights Reserved.

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