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Why Is Snot Green?

And Other Extremely Important Questions (and Answers)

Roaring Brook Press

Lost in Space

The universe can be a pretty dizzying place.

It was born in an almighty explosion of energy. It's so massively, hugely, immensely enormous that it's almost impossible to imagine how big it really is. Within it, there are spinning planets, burning suns, icy comets, and vast clouds of floating dust and rock. Planets, moons, and asteroids whip around each other like cosmic dance partners. Stars are born, stars die, and stars collapse into mysterious black holes in space.

But why did it turn out that way?

Where is it all headed?

Are we all alone in it?

And come on — how big could it really be?

Want to find out? Then read on ...

I don't know about that — I can imagine some pretty big stuff ...

OK, let's give it a shot. Let's imagine the size of the universe. It's probably best to start small and work up — so let's start with something fairly big — the Earth. The Earth is about 8,000 miles wide. If you drove a tunneling car straight through the middle, you'd get to the other side in about 5½ days, going nonstop at an average highway speed of 60 mph.

That doesn't sound so far.

Right — it's not. So let's try a longer journey. Say, from here to the Moon. The Moon doesn't go around us in perfect circles — it gets closer and farther away from us at different times of the month. But, on average, it's about 240,000 miles away. It would take about 168 days to get there in a 60-mph flying space car. Even with rocket propulsion, the Apollo astronauts took about three days to get there (and it was really crowded in their spacecraft).

Similarly, the journey from Earth to the Sun is about 93 million miles, so it would take about 176 years by space car. To get right across our galaxy, the Milky Way, it would take about a million billion years (or 1,181,401,000,000,000 years to be more precise) to make the journey of 621 million billion (or 621,000,000,000,000,000) miles.

So what does that tell us?

That a space car would be cool, but at 60 mph it'd be pretty useless for getting around in space?

Errr ... yes.

That, and that the galaxy is pretty huge in itself — let alone the universe. I'm running out of space to put all the zeros after the numbers here.

All right — what if you had a space car that could go at the speed of light?

Now we're talking. The speed of light is about 670 million mph, so a car that fast could go about 6 thousand billion miles (or whole year. We call this distance a light-year, and it's much more useful for measuring the huge distances — between stars and across galaxies — that we've been talking about. For example, the Milky Way is about 100,000 light-years across, so it'd take 100,000 years for our souped-up, super-fast, light-speed car to cross it. Still way too long to manage, but easier to imagine, maybe.

Go on, then — how big is the whole universe?

Well, we can only measure the universe as far as we can see it. With the best telescopes we have, that's about 15 billion light-years (or 90 billion trillion miles — I won't even bother trying to write that out with zeros) in every direction. So at the speed of light, it'd take at least 30 billion years to cross it. That's about 16 billion years longer than the age of the universe itself.

Ah. So it's big, then?

Like I said, crazy big. And that's just the part we can see. Beyond that, we know it extends even farther because the light from the stuff we can see at the "edge" has taken 14 billion years to reach us, and the universe has expanded quite a bit since then! It might even curve back on itself, like the sea does as you sail around the globe. If that were the case, you could circle the universe and end up back where you started.

Now that would be cool.

Yes, it would. But all your friends would be billions of years older. So even if they were still around, they probably wouldn't know what cool was any more. Bummer.

But space is, well, space, isn't it? No air, no gravity, ... Well, not exactly. Gravity is actually everywhere in space.

Its pull becomes weaker the farther you move away from one particular source — like a planet — but it's still there.

And while it is true that there's no air in space, there are other things spread around it. It's only because the stuff is spread out so thin, and space is so big, that we can't detect it very easily.

So what is this "stuff"?

Mostly hydrogen and interstellar dust left over from the Big Bang.

How much of it is out there?

Well, there're billions of tons of it, but it's spread so far and wide across the universe that you won't find more than one atom per half a cubic inch of space in most places.

You've probably been told that gases spread out to fill their containers, right? Well, if there's nothing else in the container, then they do. In this case, the container — the universe — was empty and is now at least 180 billion trillion miles wide. Spread over this distance, even billions of tons of material can look like virtually nothing. It just depends on how hard you're looking for it.

OK ... so rather than say "there's nothing in space," you could say "there's almost nothing in space" instead?

Exactly. That will not only be more accurate, but it will also freak people out. Which is always fun.

Yikes. That doesn't sound good. I thought we'd just go around and around the Sun forever.

I'm afraid not. We're getting a tiny, tiny bit farther from the Sun with each lap we do around it. The Earth gets about a half inch farther away from the Sun every year.

Why's that?

It all has to do with how gravity works. A very clever scientist named Isaac Newton explained how gravity works over 300 years ago. If, like me, you can't read Latin and math gives you a headache, it basically goes like this:

• Everything attracts everything else.

• The bigger the things are, the bigger the pull.

• The closer together the things are, the bigger the pull.

• The force that causes this attraction is called gravity.

Now, the Sun is by far the biggest object in the solar system, so it pulls everything else toward it. That includes planets, comets, asteroids — everything.

Hang on a minute — so why don't the planets all just get pulled right into the Sun?

That's because the planets all formed from chunks of stuff that were already circling the Sun to begin with. When the solar system began, these chunks clumped together to form planets and settled into regular circuits (or orbits) around the Sun. Closer to the Sun, all the icy bits got vaporized, so we ended up with the small rocky planets — Mercury, Venus, Earth, and Mars. Farther away, it was cool enough for gas to hang around, so we got the gas giant planets — Jupiter, Saturn, Uranus, and Neptune.

You forgot Pluto.

No, I didn't. Most astronomers don't count it as a real planet these days. There are a whole lot of small Pluto-sized objects out there beyond Neptune, and these (it has been decided) aren't planets either.

Oh.

Anyway — as I was saying — the planets have settled into more-or-less fixed orbits around the Sun. They don't get pulled right into it because they still have some circling speed (or rather, momentum) left over from when they were just baby chunks of planet (or planetesimals, as they're called). It's like they're excitable puppies on a long leash — they're trying to whiz off into space but the Sun's gravity keeps pulling them around it instead.

So why are they gradually getting away from it, then? Because the Sun is burning up its fuel and, in doing so, it's shrinking. As it gets smaller, the strength of its pull on the planets decreases.

Doesn't that mean we're going to fly off into space and freeze?

Well — do you want the good news or the bad news?

The bad ...

Before any of this happens, the Sun will swell up into a red giant star and frazzle the Earth anyway.

Ouch. OK — the good ...

It'll take a while, so there's a good chance we'll be able to hop planets (or preferably solar systems) beforehand.

Woohoo!! Better get cracking on those spaceships.

Yep. Time's awastin' — only got about 4.5 billion years left.

You mean ... stars don't twinkle? All those nursery rhymes — they lied to me!

Well, you could see it that way. The shifting brightness and shape that we see is actually caused by churning gases in our atmosphere, which we have to look through in order to see the stars. Outside the atmosphere, the light from the stars is more constant and even, so there's no "twinkle." From down here, though, they do seem to twinkle. So they weren't really lying. Whoever they are.

Fine. If they don't twinkle, what do they do?

They burn. They burn fiercely for billions of years. Then, when they die, some can explode with enough force to sweep up 1,000 suns — leaving nothing but a vast, deadly hole in space behind them.

OK, that sounds cooler than "twinkle." Tell me more.

Are you sitting comfortably? Good. Then let's begin ...

Once upon a time there was a cool cloud of gas. It was pretty dense, but all its gas-cloud buddies thought it was cool, and everybody knew that one day it would become a star. There it was, minding its own business, doing whatever gas clouds do, until finally it collapsed. Under the pull of its own gravity, it crunched up on itself really tightly and got hotter and hotter, starting a chain reaction and turning the cloud into a huge, dangerous nuclear reactor floating in space.

This is my kind of nursery story ...

Good, now there's a great bit coming up with giants and dwarfs in it, so be quiet.

Sorry.

No problem. Where was I? Ah, yes ...

Well, by now the cloud had truly become a star. And it was enjoying itself immensely. It happily burned up its hydrogen gas — turning it into helium — for a few billion years, heating up a few nearby planets in the process. Life evolved on one or two planets, and the whole solar system bobbed along happily in its arm of the galaxy. Until one day the star had used up nearly all the hydrogen in its core and was forced to frazzle the nearby planets as it grew into a red giant.

As if that wasn't enough, its core shrank some more over the next few billion years. Then it became a giant again, then it shrank a bit again, until eventually the star had had enough and decided to go out in style. So it imploded.

The rebounding explosion — a supernova — burned brighter than the entire galaxy and left behind a huge invisible hole in space (a black hole), from which nothing that fell in could ever escape.

The end.

Wow! That was excellent. Can I see one of those now, please?

Well, these supernova explosions don't happen to all stars — they have to be massive enough to make one. In our galaxy, a new star is born and an old star dies about once a year; a supernova occurs only once every fifty years. Of course, you might spot one in another galaxy sooner if you're patient, lucky, and have a big enough telescope.

Similarly, not all supernova explosions leave behind a black hole. Plus, you can't see a black hole, unfortunately, as nothing can escape from it — not even light.

Boo. Some happy ending ...

There's no pleasing some people.

Come again?

It's true. If you think about it, the Earth is going around the Sun — a round trip of about 584 million miles — once every 365 days. This means the Earth is traveling at about 67,000 miles per hour. Just imagine — you and I, and everyone else on the planet, are doing 67,000 mph right now.

Whoa. Why does it feel like I'm sitting still, then?

Because just like when you're on a fast (but comfy) train, you don't really notice you're moving until you look out the window to see the world whipping by in the other direction.

That's what happens when we see shooting stars — we see the evidence that we're moving through space. Rather rapidly too.

Huh? How did you get all that from a little streak in the sky? Next time you're lucky enough to spot a shooting star, try and figure out which bit of the sky it came from and keep watching that spot. Chances are you'll see more of them — loads of shooting stars that seem to shoot up, down, to the left, and to the right of the same central point in the night sky. It's a bit like being directly underneath a shower head and looking up — you see droplets fanning out in every direction.

Cool — but what does that mean?

It means that the whole Earth is moving at 67,000 mph, and its path crosses chunks of rock and dust also moving through space. As each chunk slams into our atmosphere, it heats up due to friction, and most chunks burn up completely way before they hit the ground. This makes the burning streak in the sky that we call a meteor, or a shooting star. Chunks that burn for longer, we call fireballs. Chunks that make it to the ground, we call meteorites.

Got it. But if we're always moving, and always hitting rocks and stuff, why don't we see meteor showers all the time?

Well, we do hit (or get hit by) bits of dust and rock millions of times every day and night. But these are random individual chunks (or meteoroids), and it's easy to miss their meteor trails in such a big sky if you don't know where to look.

Meteor showers happen when the Earth plows through a swarm of meteoroids in space — like those left behind by passing comets as bits break off them. When this happens, you get lots of meteors at once, and you can easily spot them one after another if you're looking in the right direction.

But how would you know where to look?

Astronomers can often predict where the showers will be in the sky because we pass through some of the same swarms every year (at the same point in the Earth's circuit around the Sun). So, if you want to, you can find out where and when, and head out for an evening's meteor spotting.

Go on, give it a shot!

Really? They have rings too? How come you never see them in pictures?

Saturn's rings are easier to see — they're pretty clear to anyone looking at the planet with a half-decent telescope. In fact, Galileo spotted what he called "ears" on Saturn with his telescope way back in the seventeenth century, but he couldn't explain them. Almost fifty years later, another physicist and astronomer, Christiaan Huygens, recognized them as rings around the planet. Saturn has been famous as "the ringed planet" ever since.

Even though they couldn't see them directly, astronomers in the 1970s predicted that Neptune and Uranus would have rings; they didn't get a clear picture of them until the Voyager satellite swung by them in the 1980s. On the way there, it spotted rings around Jupiter too — which was a nice surprise. So there are pictures of rings on other planets — it's just that fewer people have seen them.

Are all those rings made of ice too?

Some are ice, some are chunks of dust or rock, and some are probably a mix of all of these.

We know that Saturn's, Neptune's, and Uranus's rings are made of millions of small chunks of dirty ice — most of them smaller than a tennis ball. Jupiter's single ring seems to be made of tiny dust particles.

How did they get there, and why doesn't Earth have them? Small planets and moons don't seem to have rings, so we think only big planets can have them. One idea is that they are formed when a moon gets too close to the planet it orbits. When this happens, tidal forces caused by the pull of the planet's gravity rip the moon apart. The pieces then spread out and encircle the planet, forming a ring that shows where the moon used to orbit. The Earth and the smaller planets aren't big enough to rip their moons apart, so they don't make rings.

So why are Saturn's rings the biggest and best, then?

Well, it's true that Saturn's rings are the most impressive. The pieces are so small, and there are so many of them, that together they look like a single big icy disk. It could be that Saturn used to have a big, icy moon that was smashed into tiny bits when it was hit by a comet or an asteroid. That'd do it.

I like that. That's definitely what happened.

It's as good a guess as any.

Oh, yeah. Of course. Er ... what?

Let's go back a bit. Isaac Newton told us how gravity works — like how and why things fall toward the ground when you drop them, and how fast you can expect them to go when you do. Agreed?

S'pose so.

OK. Well, he also told us that gravity wasn't just about things falling toward the ground, but rather all objects falling toward — or attracting — each other. This force of attraction is stronger for larger objects, and Earth is by far the largest object on ... well ... Earth. So everything on Earth is held on by the Earth's super-strong gravity. Like a big ball-shaped magnet with bits stuck all over it — not just metals, but all kinds of things, like air, water, trees, and people. That's why none of it falls off.

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Excerpted from Why Is Snot Green? by Glenn Murphy. Copyright © 2007 Glenn Murphy. Excerpted by permission of Roaring Brook Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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