Astrophysics for Young People in a Hurry

  Bright exploding stars like the one here, shining below the disk galaxy, helped astrophysicists determine that the universe is expanding faster than we expected.

  CONTENTS

  Prologue

  1

  The Greatest Story Ever Told

  2

  How to Communicate with Aliens

  3

  Let There Be Light

  4

  Between the Galaxies

  5

  Dark Matter

  6

  Dark Energy

  7

  My Favorite Elements

  8

  Why the World Is Round

  9

  The Invisible Universe

  10

  Around Our Solar Neighborhood

  11

  What Earth Would Look Like to an Alien

  12

  Looking Up, Thinking Big

  Glossary

  Illustration Credits

  Index

  Prologue:

  Walking Dogs to See the Stars

  I decided to become an astrophysicist when I was nine years old. I remember the night. The sky was full of stars. The Big and Little Dippers. The planets Jupiter and Saturn. A meteor streaked toward the horizon, and I saw what looked like a cloud moving across the sky. Yet it was not a cloud at all. I was looking out at our very own cosmic neighborhood, the Milky Way galaxy, a region of space crowded with one hundred billion stars. For nearly an hour I watched all this action with wonder.

  Then the lights came back on, and I found myself sitting in the American Museum of Natural History’s planetarium.

  What I’d seen was a star show, but that did not limit the impact. That night, I knew what I wanted to be when I grew up. I was going to be an astrophysicist.

  At the time I could barely pronounce the term correctly. But it is actually a rather simple concept. Astrophysics is the study of planets, stars, and other cosmic bodies and how they work and interact with one another.

  Astrophysicists study black holes, the strange monsters that swallow up all light and matter within their reach. We watch the skies for signs of supernovas, the brilliant explosions of dying stars.

  We are a curious, unusual bunch. A year, to an astrophysicist, is the time it takes for our planet to complete its annual trip around the Sun. If you attend an astrophysicist’s birthday party, you’re more likely to hear everyone sing:

  Happy orbit of the Sun to you . . .

  Science is always on our minds. As a joke, an actor friend of mine recently read me the classic bedtime story Goodnight Moon. You don’t need a scientist to tell you that cows can’t really jump over the Moon, as one does in the book. But an astrophysicist can figure out what she’d have to do to complete the feat. If the cow aims for where the Moon will be in three days, then leaps at about 25,000 miles per hour, she might have a chance.

  I didn’t know much about astrophysicists when I was nine. I merely wanted to understand what I’d seen during that planetarium show, and whether the real cosmos, the universe as a whole, was truly that fantastic. First, I began studying the sky from the rooftop of my apartment building, sneaking up with one of my friends and his handy binoculars. Later, I started a dog-walking business so I could buy my own telescope. There were large dogs, small ones, mean ones, and friendly ones. Dogs with raincoats. Dogs with hats and booties. I walked them all so I could see the stars.

  In the years since, I’ve used steadily larger telescopes, moving from that New York rooftop to South American mountaintops. Through it all, the common thread has been my desire to understand the cosmos, and to share my passion with as many people as possible.

  That includes you.

  I don’t expect that everyone who reads this book will instantly want to become an astrophysicist. But maybe it will spark your curiosity. If you have ever looked up at the night sky and wondered: What does it all mean? How does it all work? And what is my place in the universe? Then I encourage you to continue reading. Astrophysics for Young People in a Hurry will give you a basic knowledge of the major ideas and discoveries that help scientists think about the universe. If I’ve succeeded, you’ll be able to stun your parents at the dinner table, impress your teachers, and stare up at the stars on cloudless nights with a deeper sense of both understanding and wonder.

  So let’s begin. We could start with two of the grandest mysteries, dark matter and dark energy, but first we should run through what I consider to be the greatest story ever told.

  The story of life.

  In the last century, astronomers spotted eight exploding stars in this spiral galaxy—which is why it’s called the Fireworks galaxy.

  A clear view of the night sky opens your eyes and mind to the wonders of stars, interstellar dust, and our crowded Milky Way.

  1.

  The Greatest Story Ever Told

  In the beginning, nearly fourteen billion years ago, the entire universe was smaller than the period that ends this sentence.

  How much smaller? Imagine that period was a pizza. Now slice the pizza into a trillion pieces. Everything, including the particles that make up your body, the trees or buildings outside your window, your friend’s socks, petunias, your school, our planet’s towering mountains and deep oceans, the solar system, the distant galaxies—all of the space and energy and matter in the cosmos was crammed into that point.

  And it was hot.

  Conditions were so hot, with so much packed into such a small space, the universe could only do one thing.

  Expand.

  Rapidly.

  Today, we call this event the big bang, and in a tiny fraction of a second (specifically, one-ten-million-trillion-trillion-trillionth of a second), the universe grew tremendously.

  What do we know about this first instant in the life of our cosmos? Very little, unfortunately. Today, we have found that four basic forces control everything from the orbits of planets to the little particles that make up our bodies. But in that instant after the big bang, all these forces were rolled into one.

  As the universe expanded, it cooled.

  By the end of this blip of time, which is known among scientists as the Planck era, named for the German physicist Max Planck, one force wriggled free of the others. This force, gravity, holds together the stars and planets that form galaxies, keeps Earth in orbit around the Sun, and prevents ten-year-olds from dunking basketballs. Among other things. For a simple demonstration of gravity’s constant pull, close this book, lift it a few inches off the nearest table, and then let go. That is gravity at work.

  (If your book did not fall, please find your nearest astrophysicist and declare a cosmic emergency.)

  In the first few instants of the early universe, however, there were no planets or books or ten-year-old basketball players for gravity to act upon. Gravity does its best work with large objects, and everything in the early universe was still unimaginably small.

  But this was only the beginning.

  The cosmos continued growing.

  Next, the other three main forces of nature separated from each other.* The main job of these forces is to control the tiny particles, or chunks of matter, that fill the cosmos.

  Could You Dunk on Mars?

  Let’s assume you could actually get to Mars, which is not an easy task, and that you had a spacesuit that allows enough freedom of movement to let you jump. The strength of gravity on a given planet or moon depends on its mass. Since Mars is less massive than Earth, gravity is a little more than 1/3 as strong. So, there’s a chance you could jump high enough. But I hope, if you do manage to make it to Mars one day, that you won’t waste your time playing basketball.
There will be many more interesting things to see and do.

  Once the four forces had all split apart, we had what we needed to build a universe.

  A trillionth of a second has passed since the beginning.

  The universe was still unimaginably tiny, hot, and starting to become crowded with particles. At this point, the particles came in two types, called quarks—which rhymes with marks—and leptons. Quarks are quirky beasts. You’ll never catch a quark all by itself; it will always be clutching others nearby. I’m sure you have at least one friend or classmate who behaves similarly. Quarks are like those kids who never want to do anything alone, not even walk to the restroom.

  The force that keeps two or more quarks together actually grows stronger the more you separate them—as if they were attached by some sort of miniature invisible rubber band. Separate them enough, the rubber band snaps and the stored energy creates a new quark at each end, giving each one of the separated pair a new friend. Imagine if the same thing happened to those inseparable kids at your school, and they all sprouted doubles. Your teachers would undoubtedly be stumped.

  The Many Names of Matter

  I was warned that it would be unwise to introduce so many names and terms to young readers. So I’ll resist the temptation to detail all the different types of quarks in the universe—up, down, strange, charmed. But I do think you should know of quarks and leptons. The entire visible universe is built from them. Including you. Plus, I’ve noticed that kids have absolutely no trouble memorizing the complex names of various dinosaurs. Sure, some dinosaurs are ferocious and terrifying, which makes them worth memorizing. But again, we’re talking about the stuff that makes up the universe! Particles are fascinating, too, even if they’re less ferocious. Without them, we wouldn’t have had those dinosaurs in the first place.

  The leptons, on the other hand, are loners. The force that joins quarks together has no effect on leptons, so they don’t clump together in groups. The best-known lepton is the electron.

  In addition to these particles, the cosmos was seething with energy, and this energy was contained in little wavelike packets or bunches of light energy called photons.

  This is where things get weird.

  The universe was so hot that these photons routinely converted into matter-antimatter particle pairs. And those pairs would collide, transforming into photons once again. But for mysterious reasons, one in a billion of these conversions made just a matter particle, without its antimatter friend. If not for these lone survivors, the universe would have no matter in it at all. And that’s a good thing, too. Because we’re all made of matter.

  We do exist, and we know that as time passed, the cosmos continued to expand and cool. As it grew larger than the size of our solar system, the temperature dropped rapidly. The universe was still incredibly hot, but the temperature had fallen below a trillion degrees Kelvin.

  Antimatter

  All the major particles in the universe, including the quarks and leptons we just met, have antimatter twins that are their opposites in every way. Take the electron, the most popular member of the lepton family of particles. The electron has a negative charge, but its antimatter opposite, the positron, has a positive charge. We don’t see antimatter around much, though, because once a particle of antimatter is created, it immediately seeks out its matter twin, and these meetings never go well. The twins destroy each other, converting into a burst of energy. (See the story about physicist George Gamow’s Mr. Tompkins in chapter 3.) Today, scientists create antimatter particles in giant experiments that smash together atoms. We observe them following high-energy collisions in space. But antimatter is probably easiest to find in science fiction plots. It fuels the engines in the famed Enterprise of the Star Trek television show and movies, and appears repeatedly in comics.

  How We Measure Temperature

  Maybe you’ve learned this already, but there are several different ways to describe the temperature of a system. Here in the United States, we speak of degrees Fahrenheit. In Europe and much of the rest of the world, the standard is degrees Celsius. Astrophysicists use Kelvin, a standard in which zero is really zero. You can’t get any colder. So, a trillion degrees Kelvin is much hotter than a trillion degrees Fahrenheit or Celsius. I have nothing against the other standards. In my daily life, I’m fine with Fahrenheit. But when I’m thinking about the universe, it’s all Kelvin.

  A millionth of a second has passed since the beginning.

  The universe had grown from a tiny fraction of the period at the end of this sentence to the size of our solar system. That’s almost three hundred billion kilometers, or more than one hundred and eighty billion miles, across.

  A trillion degrees Kelvin is much, much hotter than the surface of the Sun. But compared with that very first instant following the big bang, this was cool. This lukewarm universe was no longer hot enough or crowded enough to cook quarks, and so they all grabbed dance partners, creating heavier particles. These combinations of quarks soon resulted in the appearance of more familiar forms of matter like protons and neutrons.

  A Simple Recipe for Matter in the Universe

  1.Start with quarks and leptons.

  2.Add quarks together to form protons and neutrons.

  3.Combine the protons, neutrons, and electrons (a negatively charged type of lepton) to build your first atoms.

  4.Mix these atoms together to make molecules.

  5.Accumulate molecules in different forms and combinations to make planets, and petunias, and people.

  By now, one second of time has passed since the beginning.

  The universe has grown to a few light-years across, about the distance from the Sun to its closest neighboring stars. The temperature has dropped to a billion degrees. This is still plenty hot—enough to cook the little electrons and their opposites, positrons. The two different particles pop into existence, annihilate each other, and disappear. But what was true for other particles becomes true for electrons: only one in a billion survives.

  The rest destroy each other.

  The temperature of the cosmos drops below a hundred million degrees, but it’s still hotter than the surface of the Sun.

  Larger particles begin to fuse with each other. The basic ingredients for the atoms that make up our visible world today—including the stars and planets, the trees or buildings outside your window, your friend’s socks, my moustache—are finally coming together. Protons fuse with other protons as well as with neutrons, forming the center of the atom, called the nucleus.

  The Four Fundamental Forces

  Here are the four fundamental forces that control our universe:

  1.Gravity, which you know.

  2.The strong force holds particles together in the center of atoms.

  3.The weak force causes atoms to break down and release energy. Also, it’s not actually weak. It’s way stronger than gravity. But it’s not as powerful as the strong force.

  4.The electromagnetic force binds negatively charged electrons to the positively charged protons in the center of atoms. It also binds the collections of atoms known as molecules.

  But let’s keep this simple: Gravity binds the big stuff, and the three other forces work on the little things.

  Two minutes have now passed since the beginning.

  Normally, the electrons whipping around the universe would be attracted to the protons and nuclei. Electrons have a negative charge. The protons and nuclei have positive charges, and opposites attract. Why do they have positive and negative charges? And why, you ask, do opposites attract?

  They just do.

  I wish I had a better answer for you, but the universe is under no obligation to make sense to us. What I can say is that many, many years of scientific research have backed up both of these ideas.

  Now, given their attraction, you’d think the protons and electrons would latch onto each other. For thousands of years, though, the universe was still too hot for them to settle down. The electrons roamed free, batting phot
ons back and forth, something free electrons like to do.

  What is charge?

  Each of us humans has various qualities or characteristics. Maybe we’re kind or charitable or unfriendly. These properties help define us. Charge is one of the basic properties of matter. Some particles, like protons, have positive charge. Others have negative charge. And still others, such as neutrons, have no charge at all. When two particles have the same charge, they are pushed apart. If they have opposite charge, such as protons and electrons, they are drawn closer together.

  This came to an end when the temperature of the universe fell below 3,000 degrees Kelvin (about half the temperature of the Sun’s surface), and all the free electrons combined with those positively charged protons. When they joined, all those photons could now cross the universe, untouched—light that scientists can still detect today. We will talk about it more in chapter 3.

  Three hundred and eighty thousand years have passed since the beginning.

  The universe continued to expand like a balloon that never pops. As it grew, it cooled, and gravity started to do its work. For the first few hundred thousand years, particles were racing everywhere, like kindergartners set loose on a playground. Then gravity began pulling these pieces together into the cosmic cities called galaxies.

  Nearly a hundred billion galaxies formed.

  Each galaxy contained hundreds of billions of stars.

  These stars acted like pressure cookers, forcing the tiny particles to bond together into larger and larger elements. The biggest stars would build up so much heat and pressure that they manufactured heavy elements like iron.

  This view from a telescope shows hundreds of thousands of stars near the center of our Milky Way galaxy.