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  To curious minds everywhere

  PART 1 The Great Beyond

  Answers to the Big Questions

  1 How Big Is Our Galaxy?

  Our Earth is floating through a beautiful whirlpool of stars almost too large to imagine. We call our galaxy the Milky Way because it looks like someone spilled milk across the night sky. You can see it with your own eyes, but it’s a little tricky. You have to find a dark place where there are no streetlights, away from towns and cities, on a clear night when the moon is not up. If you can find a spot like that—in the country, beside a lake, on a farm—and look up on a summer night, you’ll see a ghostly glow arcing across the entire sky, a bridge of stars that reaches from horizon to horizon.

  That’s our home galaxy. But there’s much more to it than meets the eye. Looking up from the ground at night, we don’t see the whole galaxy because we’re inside it, the same way you can’t see your entire town or city from your front door.

  If you could soar above the Milky Way, what a spectacular sight you would witness—a luminous swirl of stars with four graceful, curving arms wrapping around one another in a pinwheel shape. Our sun is just one of hundreds of billions of stars on that merry-go-round. We live on the inside edge of one of the galaxy’s curving arms, about two-thirds of the way out from the center, and the nearest star to our sun is more than four years away at the speed of light.

  If you wanted to travel around the entire galaxy the way fictional starships in movies do, you’d need a fast ship—one that could move at the speed of light, at the very least. And that is crazy fast. At light speed, you’d cover three hundred thousand kilometers, or seven times around the Earth, in one second. You could visit the moon and come home again in a second and a half, or travel to Mars in twenty minutes.

  But even traveling at that incredible speed, it would still take you thirty thousand years to travel from Earth to the center of the galaxy. And if you tried to cross from one side to the other, it would take one hundred thousand years.

  SPACE PLACES

  Take a walk through the Milky Way Galaxy in the Galaxy Garden in Hawaii. This big, circular garden was designed in the exact shape of the Milky Way, with different plants representing the stars and nebulae. A funnel-shaped fountain in the middle of the garden represents the black hole in the center of the galaxy, and all of the plants are arranged in curving arms that resemble the spiral shape of our galaxy.

  If you want to find the Earth, you will have to look hard, because on the scale of this garden, it is too small to see with the eye. But there is a spot, a little more than halfway out from the center fountain, where a little dot on the leaf of one plant represents our sun.

  If you could make the journey to the center of the galaxy, you’d see it for what it truly is—a city of stars. About 300,000,000,000, or three hundred billion stars.

  That number is so huge, it’s hard to wrap your head around. But you can get a sense of how many that is by imagining every star as a grain of sand. If you pick up a handful of sand, you’re holding a few thousand grains. That’s about the number of stars you can see with your eyes on a clear night. But that’s only a tiny fraction of what’s really out there.

  An average dump truck holds approximately as many grains of sand as there are stars in the Milky Way. Imagine that dump truck full of sand spread out roughly thirty meters across, or the area of a baseball diamond. That’s the size of our galaxy. But it’s only one of about a hundred thousand million other galaxies that make up the rest of the known universe.

  So how many stars are there in the universe? At least this many: 200,000,000,000,000,000,000,000. That number is so big, it doesn’t even have a name.

  If all the stars in the universe were turned into grains of sand, they would cover all the beaches in the world.

  After you’ve pondered that, pick up just one little grain of sand on the end of your finger and look at it. That’s one star—our sun. At that scale, the Earth is not even visible to your eye.

  The universe is incredibly vast and filled with an unimaginable number of stars. It can make you feel small and insignificant when you think about it all. But remember this: we may be small, but at least we know our place in the universe!

  Of course, for now, we don’t have any spaceships that can take us across the galaxy, so we have to journey there in our imaginations. To do so, lie on the ground. Don’t think about the sky as “up.” Think of it as a huge space that reaches out in all directions. Remember, there are just as many stars “below” you as there are above, because you’re lying on a big ball. And you are not on top of the ball—that’s the North Pole. You’re somewhere on the side. (The next time you see a globe of the Earth, find your city and see what side you live on.) If you think of yourself like a fly on a wall, with the ground behind you instead of below you, and the stars in front of you instead of above, you’ll get a truer sense of your place in space.

  THE FUTURE IS NOW

  It doesn’t happen very often, but every once in a while, two galaxies run into each other. Don’t worry, no one gets hurt. Galaxies are mostly made of empty space, with the stars so far apart that two galaxies can pass right through each other without anything actually touching. But when they get too close, gravity can pull them out of shape, rearranging those long spiral arms.

  Eventually, our Milky Way is going to collide with the next-biggest galaxy close to us, Andromeda. Luckily, that won’t happen for another four billion years! When it does, though, there will be two Milky Ways in the sky!

  YOU TRY IT! Model Galaxy

  WHAT YOU NEED

  A large bowl

  Water

  Food coloring or milk

  WHAT TO DO

  Fill the bowl with water.

  Using your finger, swirl the water around in a circle until it dips down a little in the center.

  Let the water spin on its own for five seconds, then carefully pour just one or two drops of food coloring or milk in the center.

  Watch the shape the food coloring or milk takes.

  You have just made a model of the Milky Way!

  The food coloring or milk should take on a spiral shape, with long arms wrapped around the center of the bowl. That is the shape of our Milky Way Galaxy. The sun and billions of other stars all swarm around a central bulge. Galaxies come in many different shapes, but the spirals, like ours, are the most beautiful. Don’t you agree?

  2 Is There Life in Space?

  With so many stars and planets in the universe, it seems silly to think that we’re living on the only planet with life. So if there are alien life-forms out there, where are they? No other life has been found on worlds other than Earth, but that doesn’t mean it’s not out there.

  Consider how many different forms of life there are on our planet. Plants and animals have found ways to live in extreme environments—not only in lush jungles and forests but also in hot, dry deserts, in frigid ice, and even in boiling water at the bottom of the ocean. Living things that survive in extreme environments are called “extremophiles.” And if extremophiles can exist in the most challenging landscapes on Earth, what about in weird environments on other worlds? Could there be life in the incredibly cold, dry deserts of Mars? Might there be creatures floating around in the orange clouds of Jupiter? Or str
ange life-forms hiding under the ice of distant moons?

  We don’t know the answers to these questions yet, but one thing we do know is that when we find life on other worlds, it probably won’t be the little people with big heads and huge eyes we see in the movies. Those movie aliens are often large creatures, too, but when we do find life on other worlds, there is a good chance it will be very small. Higher forms of life, like animals, fish, and birds, take billions of years to evolve. We will likely find microscopic cells floating in water, simple plants, or pond scum. That’s what life on Earth was like for most of its history, and even today, there are far more insects and microscopic creatures than there are people or elephants.

  Another thing to consider is that all life you see around you—whether plant, animal, bird, or insect—is made of the same basic ingredient as you are: carbon. Some scientists have suggested that alien life could be based on another element, such as silicon, which makes up rocks and sand. Can you imagine living rocks? We might not even recognize them as alive. Alien life may be so strange-looking to us we might have a hard time relating to it.

  So where do we start our search for life on other worlds?

  We’ve begun our search of Mars, using robots. Mars is cold and dry today, but scientists believe there used to be lakes, rivers, and even an ocean on its surface about three billion years ago, roughly the same time that life was just starting to appear on planet Earth. Once upon a time, you could have gone swimming and sailing on Mars! Was there life in those Martian oceans? That’s one of the questions our robots are trying to answer. They are searching for signs of life, but so far, they haven’t turned up anything.

  Today, Mars remains covered with markings that look like rivers and lakes. The only problem is that we don’t see any water in them. It seems they dried up when Mars went into an ice age. Today, it’s hard to find a place on Mars that isn’t below freezing, which decreases the chances of finding alien life-forms.

  But maybe we’re looking in the wrong places. What if there’s life in Mars rather than on it? There may be caves where it’s warmer and wetter, caverns where extremophiles could survive. The problem is, you have to know where the entrances to Martian caves are. So far, our robotic rovers haven’t had much luck in finding any. And even if we do find caves on Mars, there’s another problem. The air on Mars is different from the air on Earth. Martian air doesn’t have oxygen in it, so we won’t recognize any life we find there.

  One thing we see on Mars is lots of ice. Ice is everywhere in space! And we already know that even in the coldest places on Earth, microscopic life can survive. If life can exist underneath ice on Earth, perhaps we could find life under the ice on Mars. All we need is some way of digging down into that frozen water to find it.

  Some scientists think there’s a better chance of finding life on Europa, one of Jupiter’s moons, than on Mars. Europa is completely covered in ice, and under that ice is an ocean containing more salt water than all of the oceans on Earth. There could be hot-water vents at the bottom similar to those on our planet, with their own kind of microscopic extremophile life.

  Another way we might find out if there’s life in space is through space probes sent to comets. When the satellite Stardust flew to a comet in 2004, it scooped up some material from the comet’s tail, then returned to Earth. The comet dust it retrieved contains the same basic chemicals—called organics—that life is made of, which begs the question: Is it possible that life on Earth came from comets?

  This concept has a name: “panspermia,” meaning “life that spreads around the universe via comets.” Microscopic organisms (or even the chemicals that make up life) buried deep inside an icy comet could be protected from the harsh environment of space for millions of years. Then, when a comet strikes a planet such as Earth, which is warm and has lots of water, those life-giving ingredients may become the seeds for a whole generation of creatures. We know the Earth has been hit by comets many times in the distant past. What does this mean? Maybe we’re all descendants of alien matter from space!

  If you’re old enough, you may recall ads for sea monkeys in the backs of comic books. News flash: there’s no such thing as a sea monkey. These creatures are actually tiny shrimp, and they have an amazing ability to survive when their ponds dry up. They can dry out so much that they look like dirt and appear dead. They can stay in this state of suspended animation for many, many years. To bring them fully back to life, all you need to do is put them in water.

  It’s astounding how something that seemed dead can come back to life. This kind of hibernation could allow life to travel through space hidden inside a comet. How many seeds of life are floating around out there right now, just waiting to run into a warm, wet planet like ours? When we search for life on other worlds, this is the type of creature we’re likely to run into first. They’re life-forms that, in many ways, are more resilient than us.

  ON THE DRAWING BOARD

  Is there anyone out there we can talk to? With all the billions of stars in billions of galaxies in the universe, it seems unlikely we are the only intelligent life. One way to search for other communicative civilizations is to listen for radio signals. We’ve been keeping an ear out for aliens for some time now. A number of radio telescopes use a process called SETI—Search for Extraterrestrial Intelligence—to listen for alien radio signals. So far, nothing has turned up.

  If you want to join the search for aliens, you can help through a project called SETI@home. There are so many signals coming from space that scientists can’t analyze them all. Through SETI@home, you can link your computer to a worldwide network that looks for alien signals in the data gathered by telescopes. Maybe you will be the first to locate a signal from an alien civilization!

  Other planets may have water, but they are frozen planets. Venus, on the other hand, is a hot planet where water, if there is any, would exist only as a gas. The Earth is right in the middle. It’s sort of like Goldilocks: not too hot, not too cold, but just right. Astronomers call this midrange “the habitable zone” or the “Goldilocks zone.” In our solar system, this zone extends inward just a little toward Venus and outward to Mars. Astronomers and scientists continue to look for planets that might be habitable like ours somewhere in the galaxy. We have found a few, but no signs of life.…yet.

  YOU TRY IT! Alien Linguistics

  What would you say to an alien if you had the chance?

  WHAT YOU NEED

  Blank paper

  Markers, pens, or colored pencils

  A few friends

  WHAT TO DO

  Divide yourselves into pairs. Each pair should have one piece of paper and a couple of writing utensils.

  Separate yourselves so you can’t see what the others are doing.

  Come up with a message that you would like to send to an alien civilization. It could be a greeting that says “Hello.” Or perhaps you are in trouble and need to ask for help. Or maybe you want to invite them to come visit. Be creative, and don’t tell the other group what you’re trying to say.

  Write your message on the paper for the other group. One rule: you cannot use English words, numbers, or any symbols that we use on Earth because the aliens will not know our language. It has to be some kind of drawing that gets your message across.

  When you have completed your message, gather everyone back together and see if the other pairs can understand your message. Can you understand what they were trying to say to you? It is harder than you think!

  3 Where Do Stars Come From?

  Stars come from clouds. Just as clouds in the sky make raindrops, enormous clouds in space give birth to stars.

  Space clouds are called nebulae. They look like beautiful space flowers drifting among the stars. Some are so large, it would take longer than your entire life to cross from one side to the other, even if you were moving at the speed of light. Nebulae are mostly made of hydrogen gas, which is very light, and they are found everywhere in the universe. It makes sense, then, that most star
s, including our sun, are made almost entirely of hydrogen, too.

  But while these nebulae are beautiful to look at through a telescope, if you were to pass through one on a spaceship, they sure wouldn’t smell like flowers. They’re full of dust and other carbon-based gases that are similar to what comes out of a car’s tailpipe. So nebulae would probably smell like a combination of vehicle exhaust, smoke, and burned hamburger. The plus side? That space pollution is the fertilizer for new solar systems.

  Clouds in space usually have some parts that are thicker than others. And in space, whenever anything—whether it’s gas or rocks—gathers together in clumps, these clumps have more gravity than the rest of the material around them. The clumps within a space cloud attract more gas and dust, and as they do, their gravity grows, squeezing everything toward the center until a ball is formed. If that ball gets big enough, the center becomes extremely hot. Nuclear fires ignite… and a star is born.

  Meanwhile, other smaller clumps of matter—the ones that don’t fall into the center and become the star itself—circle around the new star, forming little balls that eventually grow to become planets. This is how the Earth and all the planets in our solar system were formed four and a half billion years ago.

  You might be wondering, “Where did those big clouds in space come from in the first place?”

  It turns out that many nebulae come from old stars that died a long time ago. Stars are born, but they also die. And when they do, their remains become new stars.

  Stars can live for billions of years. (Ours, the sun, is 4.5 billion years old, but only halfway through its life.) Eventually, every star runs out of gas. The nuclear fires inside stop pushing outward, and gravity crushes the star down, squeezing it into a smaller space. The squeezing makes the center of the star even hotter, and for a short while, a different kind of burning at the center of the star begins pushing back out again. This puffs the star out into a big, red giant.