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Phil Plait – Part 4

“Do you think there are any more planets in the solar system? And is there another planet in front of Mercury called Vulcan?”

Well, there might be more planets in the solar system. It’s unlikely, but it’s possible that there are planets even as big as the Earth, or bigger, way far out–way past Pluto’s orbit! Out there they’d be really cold and really dark and they’d be moving so slowly as they orbit the sun that they’d be really hard to detect. But that doesn’t mean they’re not there. There might be a planet out there and, uh, its gravity would be so weak (because its so far away) that it wouldn’t affect the planets, uh, interior to it–the planets like Jupiter and Earth and Uranus and Neptune.

So, there could be one out there, it’s just probably pretty unlikely. Now, as far as a planet closer to the Sun than Mercury: long time ago it was thought that maybe there was a planet inside Mercury’s orbit called Vulcan, they actually gave it a name, and because, uh, this was because Mercury wasn’t orbiting the Sun the way it was expected to. But it turns out that we didn’t understand everything about gravity back then.

Einstein came along and redefined what gravity was and when you use his equations, his models of how gravity works, Mercury was doing exactly what it was supposed to be doing and they realized there was no planet inside Mercury’s orbit. There might be some small asteroids in there, small chunks of rock a few hundred yards across, but they’re so close to the Sun they are really difficult to see. But there are people looking for them. We don’t know if they’re there or not, but if they are people will find them and that would be pretty interesting.

“Why can’t we live without the Sun?”

Well, we can’t because the Sun is the source of heat and light in the solar system, and so it keeps our planet warm. It keeps our atmosphere from freezing and our oceans from freezing. Plants get light from the Sun and they’re a part of the food chain. If you were to turn the Sun off, we’d freeze and the plants would all die and everything would die, so that’s why we need the Sun.

“How do stars form? What is the biggest and smallest star?”

Well, the stars form from giant gas clouds called nebulae and these things collapse under their own gravity. Sometimes two of them collide–these gas clouds collide–and it makes ‘em collapse and then there’re pockets where the gas gets really dense, and then its own gravity starts to compress it, and in the center it gets really hot, and then that is able to squeeze hydrogen atoms together to form helium in a process called fusion which generates energy–and that makes a star.

And we’ve seen this! We see this process happening in these gas clouds–pictures from Hubble; pictures from ground-based telescopes–and so we know that’s how this works, and it makes stars of all different sizes. The smallest stars are called brown dwarfs (its kind of a silly name; they’re not really brown), but they have a mass of about a twelfth of the sun which is like 80 times the mass of Jupiter. And that’s sort of the minimum mass you need to be able to get the pressure and temperature y’know high enough in the center to be able to make energy this way. Um, as far as size goes they’re about a tenth of the size of the Sun, something like that, a fifth a tenth, something like that. They’re smaller than the Sun. The biggest stars have about a hundred times the mass of the Sun, and they can be huge. They can have, y’know, five or ten or fifteen times the size of the Sun physically, but when these stars use up their fuel they can expand and they can get really huge, and-and they can be a hundred million miles across, or even more! The Sun’s less than a million miles across, so you’re talking about something that’s a hundred or two hundred times the size of the Sun, These are immense objects and we see these as well, so there’s a huge size-range of stars.

“Have the constellations changed in the last few million years?”

Yes, they have. When you go out and look at the stars night after night it doesn’t really look like they change much, but in fact they’re moving, just very slowly. And over thousands of years, and tens of hundreds of thousands of years–millions of years!–the constellations have changed as the stars change as well. So, if you could get into a time machine and go, y’know, a million years in the future you might notice that Orion and the Big Dipper and those constellations they’ll have actually changed their shape.

“If the Sun is so big, how come it doesn’t light the darkness? Why does space look black?”

Well, this is one of the most basic questions you could ask. Why is the sky dark at night? a long time ago it was thought that the universe was infinite–it went on forever–and that it was infinite in time as well–it existed forever. And if that were true, you’d expect space to be really bright because no matter where you looked in any direction you’d eventually see the surface of a star and that would all add up. It would be like sitting inside of a star, really, and so space itself would be as bright as the Sun.

But it’s not. It’s black. And that’s because the universe is not infinite either in space or time. It has an age. It started in the Big Bang about 13.7 billion years ago, so it’s not infinite in time, and it’s finite in space as well–it doesn’t go on forever. So, you’re never looking far enough to always see the surface of a star, and the universe hasn’t existed long enough for it to be infinite in space. And so that’s why it’s not as bright as a star, and it’s also dark because it’s not filled with stuff.

During the day the sky is bright because there’s air, uh, on the Earth and that takes the sunlight and spreads it out and so you see light coming from every direction. But space is empty. Its like a vacuum (not really, but close enough), and so when we look out in space we just-we just don’t see anything. And so that’s why its black. There’s just not a whole lot there.

“Do galaxies move around space? Do they ever collide? Will our galaxy collide with another one? When?”

Well, galaxies do move. Galaxies are collections of billions of stars and they have gas and dust in ‘em and they take all kinds of cool shapes. They can be like football shaped, or like balls, or they can be flat and have spiral arms, and they move around sort of like the way planets move around the Sun. Galaxies can orbit each other. As a matter of fact, we see clusters of galaxies that can have hundreds of even thousands of galaxies in ‘em, and their own gravity holds ‘em together, and its kinda like a beehive with all the bees going all around.

The galaxies all orbit each other, but that means that sometimes they collide. Galaxies actually smack into each other, and they can pass through each other actually. Stars are so small and far apart that galaxies can actually pass right through each other without actually having any stars collide. But their gravity kinda holds them together and the galaxies’ll merge and they might form sort of a ball shaped thing or-or whatever, but the Milky Way is actually going collide with the Andromeda galaxy (which is a spiral like ours), but that’s not going to happen for a billion years, so don’t wait up nights for it. It’s going to take a long time, but eventually we will collide and form a-a more massive, probably elliptical galaxy. (transcriber’s note: BA seems disturbingly enthusiastic about this prospect.)

“What is the biggest elliptical galaxy?”

Y’know, I don’t know, and I looked this up, and I was having a hard time finding what the most massive–what the biggest elliptical galaxy was. There is a galaxy called M-87. It’s-it’s relatively close by. It’s about 60 million light years away. That’s a long way! But as galaxies go that’s-that’s pretty close, and it has about a trillion times the mass of the Sun. Our Milky Way galaxy has about two hundred billion times the mass of the Sun. So, M-87 is five times more massive than our galaxy, and our galaxy’s considered to be pretty big. Now, there are probably galaxies out there bigger than M-87, but not many. It’s really one of the most massive galaxies out there. So I’m thinking roughly a trillion stars is gonna be the number of the stars in the most massive galaxy.

Apologies if this is a stupid question, I’m by no means an educated astronomer, just interested.

Cheers

Koro

I’m really confused by your no swearing policy here & not sure what words Ican and can’t say.

Please could you post a list here of all the words I can’t say so I know not to use them? ThX!