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Book Summary: Welcome to the Universe – An Astrophysical Tour

Welcome to the Universe (2016) is a mind-blowing and breathtaking introduction to astrophysics, based on the popular course the three authors cotaught at Princeton University. It takes everyone – even the nonscience-minded – on a trip through the known universe, stopping to examine stars, galaxies, black holes, and more, all while presenting fascinating theories regarding time travel, the big bang, and the prospect of life in other galaxies.

Book Summary: Welcome to the Universe - An Astrophysical Tour

Content Summary

Introduction: A riveting, whirlwind tour through the universe.
Getting situated.
The sun.
Our solar system.
Light and distance.
Nebulas, galaxies, and dark matter.
Black holes.
The shape of the universe.
Time travel and wormholes.
Life outside our solar system.
About the author
Table of Contents
Video and Podcast
Read an Excerpt/PDF Preview


Physics, Astronomy, Space, Popular Science, History, Education, Science Nature, Cosmology

Introduction: A riveting, whirlwind tour through the universe.

The universe is 13.8 billion years old. Imagine this stretch of time as being equivalent to the length of a football field. On it, every step you take is equivalent to about 50 million years. Where do you think the entire span of human history fits in?

Right at the end . . . about the width of a single human hair.

We humans tend to feel like we’re important. In reality, we occupy a humble corner of an unassuming galaxy in a completely average stretch of space and time.

Lucky for you, this summary takes you on a trip far beyond the rock we call home. As we fly through the astrophysical classic, Welcome to the Universe, written by preeminent astronomers Neil deGrass Tyson, Michael A. Strauss, and J. Richard Gott, we’ll be visiting stars, galaxies, black holes, and even do a bit of time traveling – so, if you’re ready, strap in, and get ready for lift-off!

In this summary, you’ll learn

  • why Pluto isn’t a planet;
  • what lies at the center of every galaxy; and
  • how you can travel back in time to shake your own hand.

Getting situated.

Okay, first stop. Lower Earth orbit.

Before heading out any further, we need to get you situated with our starting point – Earth. Our little blue marble of life. For many of you, this will seem like your very first trip to space. But in reality, you’ve been rocketing through space all your lives. Because, realize it or not, we’re all living on a rock moving roughly 100,000 kilometers per hour through our solar system. And in that sense, Earth already is a kind of spaceship.

Now, looking at Earth from space, I’m going to ask that everyone tilt their heads just slightly to the right. That’s it. It’s only now that you’re actually looking at our planet head-on. This is because as Earth orbits the sun, it’s perpetually tilted at an angle of 23.5°. As we orbit the sun once a year, Earth keeps a constant tilt, maintaining the exact same orientation for the entire journey.

Alright, now look toward the edges of Earth. See that creeping shadow? Well as you might have guessed, that’s the night beginning for those parts of Earth. But, what you might not realize, unless you’re looking at Earth head-on, is that it’s always exactly 50 percent covered in sunlight and exactly 50 percent in darkness. Forget daylight saving time, long winters, summer solstices, that’s just where you happen to be located on this tilt. People in Antarctica can see daylight for 24 hours in December . . . but Earth as a whole? Without exception, it’s always 50/50.

And it’s this very tilt that dictates everything we know about the sky. From the stars we see, to the path of the sun. Many people believe that high noon means the sun is directly overhead. But, in fact, if you’re looking from the US, you’ll never, at no point of day or year, experience the sun directly over your head. It just doesn’t happen. Because in the US, you’re always witnessing the sun at an angle. This also means you’ll never see all the same star constellations that the Southern Hemisphere does, and vice versa.

So, if you can all untilt your necks . . . now we’re situated and looking straight ahead. It’s time to start our tour through space at our solar system’s crown jewel – the sun.

The sun.

There it is, the sun!

From Earth, it might appear as yellow, but in reality, it’s radiating roughly equal amounts of all the visible colors, which instead gives us white light, not yellow. But burning away somewhere around 6,000 degrees K – it’s quite a bit hotter than your normal white flame.

A cooler star than the sun – say with a temperature of only 1,000 K – looks red, because it emits more low-energy red light than high-energy blue light. That’s why, when you look at the other end of the heat spectrum, say at a really hot star of 30,000 K – it appears as a rich blue color.

But, whether hot or cool, every star contains a kind of thermonuclear hydrogen furnace at its core. And the hotter they burn, the faster they run through their hydrogen supply. That’s why the bluest, hottest stars die quicker than others – only lasting for about 10 million years. While a cooler star, like the sun, will have a slow burn – living for a total of 10 billion years. The cooler the temperature, the longer the lifespan.

Now if we were able to crack open the sun and look directly into the middle – we’d see a glowing core smack-dab in the center. That’s the thermonuclear furnace we were talking about. It not only keeps the center of a star hot, but also performs another amazing function: generating elements. Because it’s so hot in there, the normal rules of electromagnetism cease to apply, and the hydrogen protons attract instead of repelling each other. And when protons attract, they collide. Thus forming new forms of matter.

At the center of our sun, four million tons of matter are converted into energy every second. And it’s almost entirely Hydrogen fusing to become Helium. This process continues for about 90 percent of a star’s lifetime – creating energy from hydrogen and keeping the core hot and stable. But eventually, and we’re talking like five billion years from now, the hydrogen fuel at the sun’s core will begin to run out, shifting to a largely Helium core. And this is where things will get out of hand, pretty quickly. With no hydrogen, the core will grow increasingly unstable and begin to collapse – becoming hotter and hotter as it burns through any leftover hydrogen in the outer layers. As a result, the sun’s envelope will expand, and it will become what we call a red giant. It looks exactly how it sounds, red . . . and giant.

As the red giant continues to grow, the core gets hotter and helium begins fusing to become carbon. Carbon then becomes oxygen. Until eventually, the core builds up what looks like something resembling an onion, layered with all these different elements.

But as these layers continue to grow and expand, the whole star will inevitably burn out, collapse, and throw off its gaseous envelope, releasing all of its stellar guts into the galaxy. This is where the sun will end – resting into its final stage as a humble white dwarf.

For bigger stars, this can result in a reaction so massive it explodes into a supernova, in turn collapsing into a neutron star or even a black hole!

Earth, of course, will become a charred ember long before any of this happens. In a mere one billion years, our sun will already have grown large enough to cause Earth’s oceans to boil and evaporate into the atmosphere, and life as we know it will cease to exist . . .

And on this happy note, it’s time to take a quick tour of the rest of our solar system while we still can!

Our solar system.

The enriched gases that explode from a star after its life has ended can eventually form into balls of solid matter containing elements like oxygen, silicon, and iron. In other words: terrestrial planets. In our solar system, we have nine – shoot – I mean, eight planets . . . I’ll get to that miscount momentarily. But first, look outside your windows and you’ll find yourselves passing the first family of planets that our wonderful solar system has to offer.

Those would be Mercury, Venus, Earth, and Mars. All of them are small and rocky objects that orbit the sun. They certainly have many differences, but they’re still closer to each other than they are to anything else in the solar system.

Now take a look to your right, and you will see the next family in the solar system – collectively known as the gas giants. These are Jupiter, Saturn, Uranus, and Neptune. Again, these planets are certainly very different from one another, but they all share the traits of being big and low-density.

Then there’s everyone’s favorite problem child: Pluto. Is it a planet or not? You may have already taken a side when it comes to this controversy.

No offense to Pluto, but it just doesn’t fit in with either of the two other families of planets. First of all, its orbit is all wrong – it crosses with Neptune’s, which isn’t at all how a planet should behave. Plus, its orbit is at an angle relative to the plane of all the other planets. That’s another strike against it.

Since 1992, scientists have found over a thousand objects like Pluto – icy bodies beyond Neptune, many with orbits similar to Pluto’s. Together, these small, icy bodies make up what we call the Kuiper Belt. These are Pluto’s true brothers and sisters. Pluto just happens to be the biggest and brightest one of them all.

I’ve noticed one or two of you have taken out the stellar blankets provided for you. You’ve likely noticed that the universe is no stranger to very cool temperatures. As a whole, the universe has a temperature left over from the big bang, when all the matter in the universe was condensed and then rapidly expanded. Right now, it’s at 2.7 K⁠ – and dropping. The data shows that the universe is going to keep expanding, steadily approaching 0 K, or absolute zero. One by one, the stars will use up all their fuel, dying out and disappearing from the sky until every last light goes out.

Light and distance.

As we start to head outside of our own solar system, you’ll notice that everywhere you turn the sky is full of bright, shining stars.

It’s very likely that you have quite a few misconceptions about them, though. For example, you’ve probably heard it repeated that the North Star – Polaris – is the brightest in the sky. That’s actually incorrect. In fact, Polaris isn’t even in the top ten . . . or 20 . . . or 30 brightest stars. It comes in at a lowly 45th place. The brightest is actually Sirius, otherwise known as the Dog Star.

And as we start to speed up, this might be a good time to wrap our heads around distances. Take our sun for example. Even though the sun is close to Earth, it’s not exactly close. The sun is 150 million kilometers away. But we don’t typically measure the distance in kilometers or miles. Instead, we use the amount of time it takes light to travel the distance. The speed of light is 300,000 kilometers per second. The distance from the sun to Earth is about eight light-minutes – so, in other words, it takes light eight minutes to travel that distance.

For comparison, the stars in the system known as Alpha Centauri – our “nearest” neighbors – are about four light-years away. This means that the light we see from those stars was actually emitted four years ago. Any time we gaze out into the universe, we’re looking back in time!

But on that note, what are we even looking at when we look at the stars? What is light? Well, it consists of photons, which are simultaneously particles and waves. The amount of energy they can carry differs, which creates different “flavors” of photons.

One of the photon flavors is visible light, to which the human eye is sensitive – white, red, orange, yellow, green, blue, and violet light. Then there are the flavors that our eyes are not sensitive to. These include infrared light, microwaves, and radio waves, which are all “below” red on the light spectrum. And then there’s ultraviolet light, X-rays, and gamma rays “above” violet on the spectrum.

As you move along the spectrum up toward gamma rays, the energy contained in each photon increases. There’s a reason everyone tells you to wear sunscreen to protect yourself from UV rays and why you need to wear a lead blanket while getting an X-ray!

We’ve still only scratched the surface when it comes to stars. To see even more, we’ll have to go much further than we are now. Hold on tight.

Nebulas, galaxies, and dark matter.

As we begin to dart through the Milky Way galaxy, you’ll notice stars aren’t lone wolves. Instead, they form in clusters.

Some clusters contain just a few hundred stars. In that case, we call them open clusters. Others have hundreds of thousands of stars, in which case we call them globular clusters. In both cases, the stars in any given cluster share a birthday – they all formed from a gas cloud at the same time.

The Pleiades, which you can see ahead, is an open cluster probably less than 100 million years old. It’s full of young, bright, blue stars – which, if you remember, means they’re incredibly hot. But there are also some cooler, red stars nearby. There’s no reason to believe this isn’t just a coincidence. Some stars start out hot, others cool – they’re born that way!

Next up, you’ll find that we’re now passing the Orion Nebula. This clump of gas and dust particles lies within our own galaxy, and it’s a veritable stellar nursery. About 700 stars are in the process of being born here right now!

Nebulas like this one are enriched with heavy elements previously formed in the inner cores of dying stars. Gravity always works to pull the gas and dust together and create something out of it. So, just like we learned how leftover elements can sometimes become new planetary systems, they can also become new stars! But when this happens, as the material is pulled inward, it heats up. Eventually, it gets so hot and dense that thermonuclear reactions begin to take place – and boom, you have a newborn star.

The Milky Way contains roughly one to three hundred billion stars. They’re arranged in a flattened disk with a total diameter of about 100,000 light-years. At the center, there’s a thicker, lumpier distribution of stars about 20,000 light-years long. This is called the bulge. Star formation occurs almost exclusively in the disk-like spiral arms that radiate out from the bulge.

And there’s something else going on in the arms, too. The mass of the Milky Way, which we’ve been able to calculate, isn’t matched by the number of stars we can observe. This has led scientists to postulate the existence of dark matter which contributes to the mass of the universe. In fact, we now think that the vast majority of the Milky Way’s mass exists in the form of dark matter, even though we haven’t yet been able to observe it directly or identify the elementary particles from which it’s made.

Dark matter is interesting enough but equally fascinating is what lies at the center of the Milky Way. The stars at the very center are orbiting something – an invisible, extremely massive something, 4 million times the mass of the sun. Can you guess what it is? We’ll talk about it – and its brethren – next.

Black holes.

Objects so massive that not even light can escape them – these are what lie at the center of galaxies. We’re talking, of course, about black holes.

At the very center of every large galaxy with a significant bulge, there lies a supermassive black hole. That includes the Milky Way. But actually, in comparison, ours is kind of on the wimpy side. Our black hole has a mass of just 4 million suns, while others are worth several billion suns.

Black holes are truly fascinating objects. Unfortunately, we can’t exactly travel to one and see what happens inside. Why? Well, think about what happens when you throw a ball up in the air on Earth. Normally, it goes up and then comes down. But if you threw the ball up fast enough, it would escape Earth’s gravitational field, never to return. Here, “fast enough” would mean a speed of 25,000 miles per hour – that’s Earth’s escape velocity or the speed at which something must travel to escape its gravitational field.

Black holes are so compact and so dense that they have an escape velocity greater than the speed of light. Because of that, not even light can travel fast enough to escape them. Now, you may be wondering: what would happen to you if you were to try vacationing inside a black hole?

Well, as long as you stayed outside a certain radius of the black hole, everything would be just fine. Even inside the black hole, you wouldn’t die immediately. In fact, things would only start to get ugly once you crossed something called the Schwarzschild radius, otherwise known as the event horizon. This is the point at which the escape velocity of the black hole exceeds the speed of light. Once you crossed that radius, it would be game over. Except that the event horizon of a black hole is completely invisible, so you’d never even know when you crossed it. You could even be crossing the Schwarzschild radius of some massive black hole right now, none the wiser!

After crossing the Schwarzschild radius, as you traveled further and further into the black hole, your body would begin to get stretched. If you were falling feet first, your feet would be pulled downward by the mass at the center of the black hole. Meanwhile, your left and right shoulders would be drawn inward toward the center. You’d slowly be crushed from both sides while being stretched – like getting turned into a piece of spaghetti. And what do you know? The real, technical term for this process is spaghettification. And no, I’m not kidding!

Fortunately, death by spaghettification is pretty quick, taking just 0.09 seconds in a 3-billion-solar-mass black hole. But we can never actually observe this process happening directly. No outside observer can witness anything that occurs beyond the event horizon of a black hole, in the same way that you can’t see past the horizon from anywhere you stand on Earth.

The shape of the universe.

If you recall, we dipped our toes into the topic of the big bang earlier on our tour. We briefly discussed the explosion of matter that resulted in the universe we know today – a universe that will die when all its energy has been used up.

The big bang model makes several predictions. As scientists, we compare those predictions with what we observe. The results we find show us how accurate the predictions are. And so far, the big bang model has passed every test we’ve thrown at it.

One such prediction is that the universe should be expanding – and that’s exactly what it’s doing.

In a way, the universe is like a loaf of raisin bread. The galaxies are like the raisins, while the dough is the space between them. The dough starts at the big bang, in a highly compressed state, with all the raisins close together.

As the dough expands in the oven, the raisins move farther and farther away from one another. From each individual raisin’s perspective, all the other raisins are receding away from it. Similarly, from our vantage point in the Milky Way, all other galaxies appear to be moving away from us – even though in reality, we’re also moving. Furthermore, more distant raisins, or galaxies, appear to be receding twice as fast as the closer raisins, because there’s twice as much expanding dough in between.

This analogy isn’t perfect, because a loaf of raisin bread isn’t infinite and has edges, unlike the universe. But, just as in the raisin bread, the raisins themselves – meaning the galaxies – aren’t expanding. Only the space in between them is.

But if the universe isn’t shaped like a loaf of bread. What is its shape?

The key to this question lies in understanding how many dimensions the universe has. The answer is four. That’s because you need four coordinates in order to pinpoint any event – three dimensions of space and one dimension of time. Given these four dimensions, you might represent the universe as a diagram in the shape of an American football, just as physicist Alexander Friedmann did back in 1922.

In Friedmann’s diagram, time starts at the point at the very bottom of the football, with the big bang, and ends at the very top. At the big bang, the galaxies all fly away from each other until the point of maximum expansion in the middle of the football. We haven’t yet reached this point in time, because, as we’ve confirmed, the universe is still expanding.

After the point of maximum expansion in Friedmann’s model, galaxies begin moving back toward each other. The distances eventually shrink until they all crash back together again at what is known as the big crunch.

This model of the universe is just one potential way of looking at things. Nothing states that it must take the shape of an American football. The universe could also, for instance, look something like a Victorian woman’s corset. But whatever shape you choose the universe to have, it’s now time to turn to one of my favorite topics: time travel.

Time travel and wormholes.

Time travel as depicted in most science fiction has one major flaw. In order to return to the past via a time machine, according to Einstein, a time traveler must be able to exceed the speed of light. But we know that traveling faster than light is physically impossible.

However, nothing says you can’t take a shortcut to beat light to the finish line. There are two ways of doing that: going through a wormhole or around a cosmic string.

Wormholes are short tunnels connecting two distant points in curved spacetime. There are a few different types of wormholes that can exist. One lies within black holes. These can theoretically connect two different universes, like two funnels taped together. The wormhole is like the center point connecting the two narrow ends of the funnels.

You can’t pass through these types of wormholes because you’d be required to travel at the speed of light. Other wormholes, however, are traversable. We haven’t yet discovered any of these, but they are theoretically possible.

Here’s how it could work. One end of the tunnel might be somewhere near Earth, while the other end could be four light-years away, at the star system Alpha Centauri. But the wormhole tunnel itself is only ten feet long.

You can think about the wormhole as a dinner table with a hole drilled through it. Say there are some ants at the top, trying to get to the underside. They could go the long way, running along the top, over the edge, and onto the bottom. Or they could travel by wormhole, popping into the hole in the table and immediately finding themselves on the bottom. You aren’t breaking any of the laws of physics here – you’re just taking a shortcut.

The time travel aspect comes in when you pull the wormhole gravitationally – perhaps with a giant spaceship. Say that, as in our previous example, one mouth of the wormhole is near Earth and the other is near Alpha Centauri. If, on January 1st, 3000, you use the spaceship to gravitationally pull one wormhole mouth on a 5-year round-trip journey at 99.5 percent of the speed of light, people on Earth would see it return just over five years later. Inside the wormhole tunnel itself, however, time would be moving ten times slower since it’s traveling at almost the speed of light – so instead of five years, only six months would have passed inside the wormhole, and therefore also on the Alpha Centauri side. So, if you jumped into the wormhole’s mouth upon its return to Earth on January 10, 3005, you would reach Alpha Centauri on July 1, 3000, four and a half years earlier. Then, because Alpha Centauri is only four light-years away through ordinary space, if you get in a spaceship and travel back to Earth at 99.5 percent of the speed of light, you’ll return in a little over four years. And as a result, you’d likely arrive back on Earth on July 8, 3004 – six months before you jumped through the wormhole to begin with. Just in time to shake your own hand and wish yourself luck on your own journey!

Now, what about time-traveling by cosmic string? A cosmic string is an extremely thin thread – thinner than an atomic nucleus – that is highly energy-dense. These strings have no ends and are made of energy left over from the early universe. They can either be infinite in extent or occur in closed loops – like strands of spaghetti and SpaghettiOs.

But these cosmic strings are no small fries. They’re expected to be extremely massive – about a million-billion tons per centimeter.

Theoretically, a person could also use the warping of spacetime caused by strings in order to time travel. But this could only happen if two strings got close enough for you to circle them. If the strings were far apart, it would take too long to circle them – too long to get back to your own past. In practice, you’re very unlikely to get so lucky to come across two cosmic strings passing each other in such a way to make a time machine.

Fleshing out all these complicated theories about time travel requires much more information about quantum mechanics. Most importantly, we must marry general relativity and quantum mechanics such that we can understand whether it really is possible to construct a time machine and visit the past. Perhaps some laws of physics we discover one day will teach us it’s impossible. But for now, the door – or perhaps the wormhole – is still open.

Life outside our solar system.

We’ve been exploring the universe for a while now. How’s everyone feeling?

By this point, you have a solid grasp of the physical material that makes up the universe, as well as some more theoretical concepts. But we’ve saved the best for last: the question of whether there is intelligent life in the universe aside from humanity.

Life as we know it requires a few things in order to exist. Number one on the list is liquid water. If your planet is too close to a star like the sun, the water evaporates. Too far away and it freezes.

Of course, the situation is far more complicated than that. Different stars have different luminosities, which means they have habitable zones of different sizes. And, based on everything we know, you need a planet orbiting a star in order to have life. You also need enough time – time for the star to be created, then the planet, and then billions of years for life to have a chance to evolve. That means the star has to be long-lived. The most massive stars only live for about 10 million years, which makes life near one of those pretty hopeless.

That’s already quite a few requirements. But there are a lot more! That’s especially true if we’re talking about life with which we might be able to converse – intelligent life. Even that, though, isn’t enough. You also need that intelligent life form to be able to send signals across incredibly long distances in space, and for us to have caught the life form at the right moment in its history. If an intelligent life form is even just 1,000 light-years away, it had to have been transmitting signals across space 1,000 years ago in order for its signals to be reaching us now.

We could go on. But perhaps it’s better, for the last stop on this journey, to take you to one of our most promising candidates where there could be life.

That candidate is called Kepler 62e. Its radius is 1.61 times as large as Earth’s, and it only receives 20 percent more radiation per square meter from its star – Kepler 62 – than we do from the sun. That makes it pretty likely to lie in a habitable zone. Kepler 62e could be either rocky or icy with an ocean – we aren’t really sure.

That’s a concrete example – now, let’s think about the question numerically. Luckily, a certain astrophysicist called Frank Drake has already helped us out here. He came up with something we now know as the Drake equation, which helps us estimate the number of potentially life-supporting planets in our galaxy.

The Drake equation looks into the fraction of suitable stars with a planet in the habitable zone. So, consider a spherical region of space, with a radius of 40 light-years. Within that sphere, you’ll have something like 1,000 stars. Using the Drake equation under our specific perimeters, you’ll likely find an average of six habitable planets within this radius alone. And remember – 40 light-years is tiny in comparison to the expanse of the entire galaxy, not to mention the entire universe!

The next part of the equation involves calculating the number of planets that have developed a technology capable of communicating across interstellar distances – and which are communicating during the epoch at which we’re observing them now. The chance of catching a planet during that specific phase at a random time is equivalent to the average longevity of radio-transmitting civilizations divided by the age of the galaxy. And that’s where things get complicated because we only have one example of an intelligent civilization: our own. One estimate suggests that the average lifespan of a radio-transmitting civilization is likely to be 12,000 years, though of course there are other possibilities.

Plug the numbers into the Drake equation, and voilà, you’re left with an estimated number of communicating civilizations! There are many different interpretations out there on exactly what measurements are best to plug into the Drake equation, but for fun, the authors of this book threw in their best estimates and found that there could be up to a hundred civilizations in the Milky Way galaxy capable of communicating with radio waves currently. However, as of right now, we haven’t found any of these . . .⁠ and it doesn’t seem like they’ve found us. All that remains is to keep on searching!


The key message in this summary is:

The universe is so much bigger, hotter, denser, and weirder than we typically realize. It can be easy to convince ourselves that Earth is somehow special, but in reality, we and our planet occupy a corner of space that’s not much different from any other. Stars, planets, galaxies, black holes, wormholes – these are just a few of the things that exist, or may exist, in the vast expanse of the universe. And as we grow along with our universe, we continue to discover new things about space, time, and our very existence every day.

About the author

Neil DeGrasse Tyson is an astrophysicist with the American Museum of Natural History, director of the world-famous Hayden Planetarium, and the award-winning author of Death by Black Hole, Origins, and The Pluto Files. He lives in New York City.

J. RICHARD GOTT III is a professor of astrophysical sciences at Princeton University. For fourteen years he served as the chairman of the judges of the National Westinghouse and Intel Science Talent Search, the premier science competition for high school students. The recipient of the President’s Award for Distinguished Teaching, Gott has written on time travel for Time and on other topics for Scientific American, New Scientist, and American Scientist.

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Table of Contents

Preface 11
Part I Stars, Planets, and Life 15
1 The Size and Scale of the Universe 17
Neil deGrasse Tyson
2 From the Day and Night Sk y to Planetary Orbits 26
Neil deGrasse Tyson
3 Newton’s Laws 42
Michael A. Strauss
4 How Stars Radiate Energy (I) 54
Neil deGrasse Tyson
5 How Stars Radiate Energy (II ) 71
Neil deGrasse Tyson
6 Stellar Spectra 81
Neil deGrasse Tyson
7 The Lives and Deaths of Stars (I) 93
Neil deGrasse Tyson
8 The Lives and Deaths of Stars (II ) 111
Michael A. Strauss
9 Why Pluto Is Not a Planet 126
Neil deGrasse Tyson
10 The Search for Life in the Galaxy 146
Neil deGrasse Tyson
Part II Galaxies 171
11 The Interstellar Medium 173
Michael A. Strauss
12 Our Milk y Way 183
Michael A. Strauss
13 The Universe of Galaxies 197
Michael A. Strauss
14 The Expansion of the Universe 207
Michael A. Strauss
15 The Early Universe 222
Michael A. Strauss
16 Quasars and Supermassive Black Holes 241
Michael A. Strauss
Part III Einstein and the Universe 255
17 Einstein’s Road to Relativity 257
J. Richard Gott
18 Implications of Special Relativity 270
J. Richard Gott
19 Einstein’s General Theory of Relativity 289
J. Richard Gott
20 Black Holes 300
J. Richard Gott
21 Cosmic Strings, Wormholes, and Time Travel 321
J. Richard Gott
22 The Shape of the Universe and the Big Bang 347
J. Richard Gott
23 Inflation and Recent Developments in Cosmology 374
J. Richard Gott
24 Our Future in the Universe 400
J. Richard Gott
Acknowledgments 425
Appendix 1 Derivation of E = mc 2 427
Appendix 2 Bekenstein, Entropy of Black Holes, and Information 431
Notes 433
Suggested Reading 439
Index 441


The New York Times bestselling tour of the cosmos from three of today’s leading astrophysicists

Welcome to the Universe is a personal guided tour of the cosmos by three of today’s leading astrophysicists. Inspired by the enormously popular introductory astronomy course that Neil deGrasse Tyson, Michael A. Strauss, and J. Richard Gott taught together at Princeton, this book covers it all—from planets, stars, and galaxies to black holes, wormholes, and time travel.

Describing the latest discoveries in astrophysics, the informative and entertaining narrative propels you from our home solar system to the outermost frontiers of space. How do stars live and die? Why did Pluto lose its planetary status? What are the prospects of intelligent life elsewhere in the universe? How did the universe begin? Why is it expanding and why is its expansion accelerating? Is our universe alone or part of an infinite multiverse? Answering these and many other questions, the authors open your eyes to the wonders of the cosmos, sharing their knowledge of how the universe works.

Breathtaking in scope and stunningly illustrated throughout, Welcome to the Universe is for those who hunger for insights into our evolving universe that only world-class astrophysicists can provide.


“Welcome to the Universe in 3D provides a fascinating journey through the observable universe, from Earth to the big bang. This unique book gives readers a sense of the vast spatial and temporal scales that set the stage for our existence.”—Abraham Loeb, author of How Did the First Stars and Galaxies Form?

“A New York Times Bestseller”

“One of Men’s Journal’s 40 Best Books of 2016”

“One of Symmetry Magazine’s Physics Books of 2016”

“One of Ars Technica’s 12 engrossing nonfiction books from 2016”

“Honorable Mention for the 2017 PROSE Award in Cosmology and Astronomy, Association of American Publishers”

“One of’s 10 Best Popular Science Books of 2016: Maths, Physics, Chemistry”

“Longlisted for the 2018 AAAS/Subaru SB&F Prizes for Excellence in Science Books, Young Adult Science Books”

“Riveting questions fielded by three top astrophysicists in engaging style, with great illustrations and just a handful of equations. They may just have produced the best book about the universe in the universe.” ― New Scientist

“Reading through is akin to receiving a private museum tour from an expert scientist. . . . The authors present challenging content in accessible prose as they lead readers from our solar system to the edge of the visible universe, getting into the how and the what of just about everything there is to know about the cosmos. . . . As Tyson, Strauss, and Gott explain the cutting-edge physics of multiverses, superstring theory, M-theory, and the benefits of colonizing space, even seasoned science readers will learn something new.” ― Publishers Weekly

“As citizens of the cosmos, we are duty bound to explore it. So opine astrophysicists Neil deGrasse Tyson, Michael Struass, and Richard Gott, guides on this bracing expedition through dusty galactic hinterlands and the vast theoretical vistas of Albert Einstein’s work.” ― Nature

“All three [authors] write in informal, conversational tones, and the text is sprinkled with genuinely funny non sequiturs, such as a brief rumination on dwarfs versus dwarves and commentary on English-speaking aliens in Star Trek. . . . What the book does very well is to present not just what we know about the universe but how we know it.” ― Science

“An accessible and comprehensive overview of our universe by three eminent astrophysicists. . . . An entertaining introduction to astronomy.” ― Kirkus Reviews

“Three of the leading voices in astrophysics take us on a well-illustrated tour that includes Pluto, questions of intelligent life, and whether the universe is infinite.” ― Philadelphia Inquirer

“The text is written in an informal and approachable style, referencing many popular-culture icons. . . . This book will open up some of the newest and most sophisticated concepts in astrophysics to a general audience, helping all of us better understand the universe we live in.” ― Booklist

“This book is anything but another ho-hum book on astrophysics. . . . Unlike many popular scientific books that are very esoteric, this one is more like a conversation between expert and interested lay person. . . . [Welcome to the Universe] will be a great read for any non-scientist but also science curious persons. It is certainly a good book for the teacher of science at any level as well as the high school and college student. Any reader will be able to see how some complex scientific thoughts fit together.” ― NSTA Recommends

“Their laudable goal is communicating vast, cosmic ideas in ways that are accessible without being simplistic.” ― Washington Post

“If you have a passing interest in astrophysics and would like to deepen it, this book is for you. . . . An authoritative book written with humour and charm.”—Marcus Chown, Times Higher Education

“This is an important book. Part fascinating story, part reference book, and part astrophysical textbook, the work presents an information-rich summary of the current state of human knowledge of the cosmos. . . . Reading this book, which packages many entertaining treatments of concepts in astronomy and astrophysics, will make you a whole lot smarter about how the universe works. It is highly recommended.”—David Eicher,,

“The authors remind us that even though people are not the center of the universe, we are an intelligent species able to measure, theorize, comprehend, and explore the limits of knowledge. An excellent introduction that will equip readers to follow current astronomical discoveries.” ― Library Journal

“Well written with clear, helpful graphics and glossy pictures accompanying the text. This book would be ideal for those who want a slightly more technical read.”—Dr. Chris North, BBC Sky at Night Magazine

“Astrophysicists Neil deGrasse Tyson, Michael Strauss and J. Richard Gott team up for a readable survey of the universe, from our solar system’s worlds to cosmic inflation and the multiverse. They don’t stint on the details, and yes, there’s some math involved, but it’s well worth the journey.”—Alan Boyle, GeekWire

“Don’t know the difference between a pulsar and a quasar? Pick up this endlessly fascinating book by three astrophysicists that provides a clear, readable introduction to the inner workings of our universe.” ― Men’s Journal

“Looking like a cross between a textbook and a coffee-table book, Welcome to the Universe is an extremely readable compilation of introductory astronomy lectures for non-science students. . . . Their talks present physics with clarity and a little levity–with references to pop culture items such as Toy Story and Bill and Ted’s Excellent Adventure. Gott even tackles time travel. What’s not to like?” ― Symmetry Magazine

“Welcome to the Universe is going to turn your head around, because, frankly, what you think you know about the universe is probably wrong. . . . Welcome to the Universe deserves numerous curtain calls for allowing the cosmos to embrace our existential thinking like a great Whitmanesque hug.”—Peter Lewis, Philadelphia Inquirer

“Learn about everything from the birth of the Universe and quasars to dark energy and exoplanets from three of the coolest guys you’ll ever meet.”—Annalee Newitz, Ars Technica

“Welcome to the Universe is more than a breathtaking guide to the cosmos. It is a unique bridge between popular science and textbooks, admirably achieving Tyson’s goal to ‘empower you to understand the operations of nature.'” ― Cosmos Magazine

“A unique intergalactic voyage from our solar system to the outermost frontiers of the universe.”—Lisa Kaaki, Arab News

“This entertaining and enlightening book is an overview of the latest discoveries in astrophysics. . . . The writing is witty yet informative, and the book is beautifully illustrated. [Welcome to the Universe] will appeal to all those who wish to learn more about the universe from three internationally prominent astrophysicists.”—,

“In an informative and entertaining way, the book takes us from the latest discoveries to the edge of outer space, from planets, stars, galaxies, to black holes, wormholes, and time travel.”—Wan Lixin, Shanghai Daily

“The book’s breadth is impressive. It starts with the basics (the size and scale of the universe) and finishes with a discussion of Einstein, general relativity, and the universe’s fate. . . . The book’s strength is the authors’ ability to write conversationally.” ― Air & Space Magazine

“This is a great book, managing to be both hugely informative and entertaining–undoubtedly the best and most comprehensive of its kind that I’ve come across.”—FictionFan

“The authors maintain the individual charms of their distinct voices chapter by chapter so the reader has the visceral sense of science shared, passed from one mind to another, almost as though through an oral history―ultimately, a warm welcome to the universe.”―Janna Levin, author of Black Hole Blues and Other Songs from Outer Space

“Readers will enjoy the big ideas in this lively and enjoyable book.”―Robert P. Kirshner, author of The Extravagant Universe

“All three of these authors are experts in the field, and they are also engaging writers. This is a very good book. There is nothing on the market that quite matches it.”―Sean Carroll, author of The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World

“As an astronomer, I admire the clever and artful way so much frontier cosmology is covered in this book. I enjoyed reading it immensely.”―Chris Impey, coauthor of Dreams of Other Worlds: The Amazing Story of Unmanned Space Exploration

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We begin with the stars, then ascend up and away out to the galaxy, the universe, and beyond. What did Buzz Lightyear say in Toy Story? “To Infinity and Beyond!”

It’s a big universe. I want to introduce you to the size and scale of the cosmos, which is bigger than you think. It’s hotter than you think. It is denser than you think. It’s more rarified than you think. Everything you think about the universe is less exotic than it actually is. Let’s get some machinery together before we begin. I want to take you on a tour of numbers small and large, just so we can loosen up our vocabulary, loosen up our sense of the sizes of things in the universe. Let me just start out with the number 1. You’ve seen this number before. There are no zeros in it. If we wrote this in exponential notation, it is ten to the zero power, 10. The number 1 has no zeros to the right of that 1, as indicated by the zero exponent. Of course, 10 can be written as 10 to the first power, 101. Let’s go to a thousand — 103. What’s the metric prefix for a thousand? Kilo- kilogram — a thousand grams; kilometer — a thousand meters. Let’s go up another 3 zeros, to a million, 106, whose prefix is mega-. Maybe this is the highest they had learned how to count at the time they invented the megaphone; perhaps if they had known about a billion, by appending three more zeroes, giving 109, they would have called them “gigaphones.” If you study file sizes on your computer, then you’re familiar with these two words, “megabytes” and “gigabytes.” A gigabyte is a billion bytes. I’m not convinced you know how big a billion actually is. Let’s look around the world and ask what kinds of things come in billions.

First, there are 7 billion people in the world.

Bill Gates? What’s he up to? Last I checked, he’s up to about 80 billion dollars. He’s the patron saint of geeks; for the first time, geeks actually control the world. For most of human history that was not the case. Times have changed. Where have you seen 100 billion? Well, not quite 100 billion. McDonald’s. “Over 99 Billion Served.” That’s the biggest number you ever see in the street. I remember when they started counting. My childhood McDonald’s proudly displayed “Over 8 Billion Served.” The McDonald’s sign never displayed 100 billion, because they allocated only two numerical slots for their burger count, and so, they just stopped at 99 billion. Then they pulled a Carl Sagan on us all and now say, “billions and billions served.”

Take 100 billion hamburgers, and lay them end to end. Start at New York City, and go west. Will you get to Chicago? Of course. Will you get to California? Yes, of course. Find some way to float them. This calculation works for the diameter of the bun (4 inches), because the burger itself is somewhat smaller than the bun. So for this calculation, it’s all about the bun. Now float them across the ocean, along a great circle route, and you will cross the Pacific, pass Australia, Africa, and come back across the Atlantic Ocean, finally arriving back in New York City with your 100 billion hamburgers. That’s a lot of hamburgers. But in fact you have some left over after you have circled the circumference of Earth. Do you know what you do with what you have left over? You make the trip all over again, 215 more times! Now you still have some left over. You’re bored going around Earth, so what do you do? You stack them. So after you’ve gone around Earth 216 times, then you stack them. How high do you go? You’ll go to the Moon, and back, with stacked hamburgers (each 2 inches tall) after you’ve already been around the world 216 times, and only then will you have used your 100 billion hamburgers. That’s why cows are scared of McDonald’s. By comparison, the Milky Way galaxy has about 300 billion stars. So McDonald’s is gearing up for the cosmos.

When you are 31 years, 7 months, 9 hours, 4 minutes, and 20 seconds old, you’ve lived your billionth second. I celebrated with a bottle of champagne when I reached that age. It was a tiny bottle. You don’t encounter a billion very often.

Let’s keep going. What’s the next one up? A trillion: 1012. We have a metric prefix for that: tera-. You can’t count to a trillion. Of course you could try. But if you counted one number every second, it would take you a thousand times 31 years — 31,000 years, which is why I don’t recommend doing this, even at home. A trillion seconds ago, cave dwellers — troglodytes — were drawing pictures on their living room walls.

At New York City’s Rose Center of Earth and Space, we display a timeline spiral of the Universe that begins at the Big Bang and unfolds 13.8 billion years. Uncurled, it’s the length of a football field. Every step you take spans 50 million years. You get to the end of the ramp, and you ask, where are we? Where is the history of our human species? The entire period of time, from a trillion seconds ago to today, from graffiti-prone cave dwellers until now, occupies only the thickness of a single strand of human hair, which we have mounted at the end of that timeline. You think we live long lives, you think civilizations last a long time, but not from the view of the cosmos itself.

What’s next? 1015. That’s a quadrillion, with the metric prefix peta-. It’s one of my favorite numbers. Between 1 and 10 quadrillion ants live on (and in) Earth, according to ant expert E. O. Wilson.

What’s next? 1018, a quintillion, with metric prefix exa-. That’s the estimated number of grains of sand on 10 large beaches. The most famous beach in the world is Copacabana Beach in Rio de Janeiro. It is 4.2 kilometers long, and was 55 meters wide before they widened it to 140 meters by dumping 3.5 million cubic meters of sand on it. The median size of grains of sand on Copacabana Beach at sea level is 1/3 of a millimeter. That’s 27 grains of sand per cubic millimeter, so 3.5 million cubic meters of that kind of sand is about 10 grains of sand. That’s most of the sand there today. So about 10 Copacabana beaches should have about 1018 grains of sand on them.

Up another factor of a thousand and we arrive at 1021, a sextillion. We have ascended from kilometers to megaphones to McDonald’s hamburgers to Cro-Magnon artists to ants to grains of sand on beaches until finally arriving here: 10 sextillion —

the number of stars in the observable universe.

There are people, who walk around every day, asserting that we are alone in this cosmos. They simply have no concept of large numbers, no concept of the size of the cosmos. Later, we’ll learn more about what we mean by the observable universe, the part of the universe we can see.

While we’re at it, let me jump beyond this. Let’s take a number much larger than 1 sextillion — how about 1081? As far as I know, this number has no name. It’s the number of atoms in the observable universe. Why then would you ever need a number bigger than that? What “on Earth” could you be counting? How about 10100, a nice round-looking number. This is called a googol. Not to be confused with Google, the internet company that misspelled “googol” on purpose.

There are no objects to count in the observable universe to apply a googol to. It is just a fun number. We can write it as 10100, or if you don’t have superscripts, this works too: 10^100. But you can still use such big numbers for some situations: don’t count things, but instead count the ways things can happen. For example, how many possible chess games can be played? A game can be declared a draw by either player after a triple repetition of a position, or when each has made 50 moves in a row without a pawn move or a capture, or when there are not enough pieces left to produce a checkmate. If we say that one of the two players must take advantage of this rule in every game where it comes up, then we can calculate the number of possible chess games. Rich Gott did this and found the answer was a number less than 10^(10^4.4). That’s a lot bigger than a googol, which is 10^(10^2). You’re not counting things, but you are counting possible ways to do something. In that way, numbers can get very large.

I have a number still bigger than this. If a googol is 1 followed by 100 zeros, then how about 10 to the googol power? That has a name too: a googolplex. It is 1, with a googol of zeroes after it. Can you even write out this number? Nope. Because it has a googol of zeroes, and a googol is larger than the number of atoms in the universe. So you’re stuck writing it this way: 10googol, or 1010^100 or 10^(10^100). If you were so motivated, I suppose you could attempt to write 1019 zeros, on every atom in the universe. But you surely have better things to do.

I’m not doing this just to waste your time. I’ve got a number that’s bigger than a googolplex. Jacob Bekenstein invented a formula allowing us to estimate the maximum number of different quantum states that could have a mass and size comparable to our observable universe. Given the quantum fuzziness we observe, that would be the maximum number of distinct observable universes like ours. It’s 10^(10^124), a number that has 1024 times as many zeros as a googolplex. These 10^(10^124) universes range from ones that are scary, filled with mostly black holes, to ones that are exactly like ours but where your nostril is missing one oxygen molecule and some space alien’s nostril has one more.

So, in fact, we do have some uses for some very large numbers. I know of no utility for numbers larger than this one, but mathematicians surely do. A theorem once contained the badass number 10^(10^(10^34)). It’s called Skewe’s number. Mathematicians derive pleasure from thinking far beyond physical realities.

Let me give you a sense of other extremes in the universe.

How about density? You intuitively know what density is, but let’s think about density in the cosmos. First, explore the air around us. You’re breathing 2.5 × 1019 molecules per cubic centimeter — 78% nitrogen and 21% oxygen.

A density of 2.5 × 1019 molecules per cubic centimeter is likely higher than you thought. But let’s look at our best laboratory vacuums. We do pretty well today, bringing the density down to about 100 molecules per cubic centimeter. How about interplanetary space? The solar wind at Earth’s distance from the Sun has about 10 protons per cubic centimeter. When I talk about density here, I’m referencing the number of molecules, atoms, or free particles that compose the gas. How about interstellar space, between the stars? Its density fluctuates, depending on where you’re hanging out, but regions in which the density falls to 1 atom per cubic centimeter are not uncommon. In intergalactic space, that number is going to be much less: 1 per cubic meter.

We can’t get vacuums that empty in our best laboratories. There is an old saying, “Nature abhors a vacuum.” The people who said that never left Earth’s surface. In fact, Nature just loves a vacuum, because that’s what most of the universe is. When they said “Nature,” they were just referring to where we are now, at the base of this blanket of air we call our atmosphere, which does indeed rush in to fill empty spaces whenever it can.

Suppose I smash a piece of chalk against a blackboard and pick up a fragment. I’ve smashed that chalk into smithereens. Let’s say a smithereen is about 1 millimeter across. Imagine that’s a proton. Do you know what the simplest atom is? Hydrogen, as you might have suspected. Its nucleus contains one proton, and normal hydrogen has an electron occupying an orbital that surrounds it. How big would that hydrogen atom be? If the chalk smithereen is the proton, would the atom be as big as a beach ball? No, much bigger. It would be 100 meters across — about the size of a 30-story building. So what’s going on here? Atoms are pretty empty. There are no particles between the nucleus and that lone electron, flying around in its first orbital, which, we learn from quantum mechanics, is spherically shaped around the nucleus. Let’s go smaller and smaller and smaller, to get to another limit of the cosmos, represented by the measurement of things that are so tiny that we can’t even measure them. We do not yet know what the diameter of the electron is. It is smaller than we are able to measure. However, superstring theory suggests that it may be a tiny vibrating string as small as 1.6 × 10-35 meters in length.

Atoms are about 10-10 (one ten-billionth) of a meter. But how about 10-12 or 10-13 meters? Known objects that size include uranium with only one electron, and an exotic form of hydrogen having one proton with a heavy cousin of the electron called a muon in orbit around it. About 1/200 the size of a common hydrogen atom, it has a half-life of only about 2.2 microseconds due to the spontaneous decay of its muon. Only when you get down to 10-14 or 10-15 meters are you measuring the size of the atomic nucleus.

Now let’s go the other way, ascending to higher and higher densities. How about the Sun? Is it very dense or not that dense? The Sun is quite dense (and crazy hot) in the center, but much less dense at its edge. The average density of the Sun is about 1.4 times that of water. And we know the density of water — 1 gram per cubic centimeter. In its center, the Sun’s density is 160 grams per cubic centimeter. But the Sun is quite ordinary in these matters. Stars can (mis)behave in amazing ways. Some expand to get big and bulbous with very low density, while others collapse to become small and dense. In fact, consider my proton smithereen and the lonely, empty space that surrounds it. There are processes in the universe that collapse matter down, crushing it until it reaches the density of an atomic nucleus. Within such stars, each nucleus rubs cheek to cheek with the neighboring nuclei. The objects out there with these extraordinary properties happen to be made mostly of neutrons — a super-high-density realm of the universe.

In our profession, we tend to name things exactly as we see them. Big red stars we call red giants. Small white stars we call white dwarfs. When stars are made of neutrons, we call them neutron stars. Stars that pulse, we call them pulsars. In biology they come up with big Latin words for things. MDs write prescriptions in a cuneiform that patients can’t understand, hand them to the pharmacist, who understands the cuneiform. It’s some long fancy chemical thing, which we ingest. In biochemistry, the most popular molecule has ten syllables — deoxyribonucleic acid! Yet the beginning of all space, time, matter, and energy in the cosmos, we can describe in two simple words, Big Bang. We are a monosyllabic science, because the universe is hard enough. There is no point in making big words to confuse you further.

Want more? In the universe, there are places where the gravity is so strong that light doesn’t come out. You fall in, and you don’t come out either: black hole. Once again, with single syllables, we get the whole job done. Sorry, but I had to get all that off my chest.

How dense is a neutron star? Let’s take a thimbleful of neutron star material. Long ago, people would sew everything by hand. A thimble protects your fingertip from getting impaled by the needle. To get the density of a neutron star, assemble a herd of 100 million elephants, and cram them into this thimble. In other words, if you put 100 million elephants on one side of a seesaw, and one thimble of neutron star material on the other side, they would balance. That’s some dense stuff. A neutron star’s gravity is also very high. How high? Let’s go to its surface and find out.

One way to measure how much gravity you have is to ask, how much energy does it take to lift something? If the gravity is strong, you’ll need more energy to do it. I exert a certain amount of energy climbing up a flight of stairs, which sits well within the bounds of my energetic reserves. But imagine a cliff face 20,000 kilometers tall on a hypothetical giant planet with Earthlike gravity. Measure the amount of energy you exert climbing from the bottom to the top fighting against the gravitational acceleration we experience on Earth for the whole climb. That’s a lot of energy. That’s more energy than you’ve stored within you, at the bottom of that cliff. You will need to eat energy bars or some other high-calorie, easily digested food on the way up. Okay. Climbing at a rapid rate of 100 meters per hour, you would spend more than 22 years climbing 24 hours a day to get to the top. That’s how much energy you would need to step onto a single sheet of paper laid on the surface of a neutron star. Neutron stars probably don’t have life on them.