Introduction

If you spend your evenings pondering the constellations and spotting shooting stars, you're already hooked on astronomy.

If you're one of those people who "gets" how things work, and if you've ever mentioned Newton or Einstein in casual conversation, apologize to your friends because you've got a keen interest in physics and are willing to share it.

Put the two together, and welcome to astrophysics! This field combines the excitement of recognizing planets and stars with the satisfaction of applying mathematics and physics to those same objects. Astrophysics goes beyond cataloging and observing the night sky into performing calculations, making measurements and creating predictions about future behaviors.

As a field, astrophysics covers everything you can see in the sky and then some. From the smallest molecule (no, you can't see these without specialized equipment) to stars and planets, individual celestial bodies have a creation story that mimics that of the universe's evolution. And the cosmos certainly isn't limited to planets and stars!

For example, nebulae are massive dust and gas clouds that often result from the explosive death of a star but, coincidentally (actually not a coincidence, as you'll read in Chapter 5), they're also regions where new stars are born. Galaxies are vast groupings of stars, often containing nebulae and other high-energy astronomical features such as black holes and neutron stars, and they group together to form clusters of galaxies. Nothing in the universe exists in isolation, not even quarks (these are the smallest particles in the universe. Curious? Check out Chapter 15).

And speaking of things that are infinitely tiny, the Big Bang is the both the ending and the starting point of our exploration into astrophysics. The universe began as a single point in space and time, growing explosively over subsequent seconds as it expanded. The forces at play in our current universe (think gravity, electromagnetism, and the nuclear forces) emerged, atoms began to form, matter separated from radiation, and every element in today's world was created. From the cloud of cosmic dust emerged stars, galaxies, dark matter, and all the other pieces of the cosmic puzzle.

But eventually, all good things must come to an end, and unfortunately that includes our universe. Theories abound as to how we'll all meet our demise, but those theories only open the door to more questions. Is this universe the only one to have ever existed? Will another one be created when this one's gone? Or, as some theories suggest, is our universe only one in a series? There are no definitive answers at this point to many of these questions, but studying astrophysics gives you the knowledge you need to start asking those questions, and perhaps even answering those questions for yourself.

About This Book

Welcome to the cosmos! Whether you're curious about how the universe began, want to know more about the science behind eclipses, or are considering becoming an astrophysicist (and if that's the case, we expect to see you at conferences and lectures down the road), this book will get you started.

Astrophysics is a notoriously difficult idea to wrap your head around because it's incredibly vast, yet also extremely detail-oriented. We've broken down the information into digestible chapters so that you can read this book all at once, or can flip through to only the sections that interest you. We've written each chapter as a stand-alone piece with references to other parts of the book as needed. If you have the time and interest, try reading the book cover to cover for a more complete sense of the story: Decipher what you observe in the night sky, understand the science behind it, and get a holistic overview of this universe we call home, from start to finish.

Got questions on unfamiliar terms? Check the glossary at the back of this book for a quick guide. Need a quick refresher on which formula goes with which concept?

Finally, please consider us as resources for any additional questions you may have. Wiley can put you in contact with us directly for follow-up questions, and you can always request additional books and content directly from the publisher. We want to share our love of space science with you! Let us know how we can help.

Foolish Assumptions

We assume you have picked up this book because you think astrophysics sounds interesting - dare we say cool? Maybe you are concerned about the end of the universe. Maybe you want to know what you're looking for at a star party, or perhaps you're looking for a bit of light bedtime reading (hint: maybe save the end-of-the-world chapters for the morning). Any reason to learn about astrophysics is a good one, and we hope this book has fun and thought-provoking information for you.

We don't assume that you are a scientist or have even taken any science classes since high school - or maybe you're a high school student now! Whatever your starting point, this book is for you.

A few key tips before we get started. Scientists, no matter their location, typically use the metric system when running physics or astronomy calculations. The metric system is also known as the International System of units, or SI units. Although you may be more accustomed to the standard American system of feet and pounds (amusingly, this system is actually called the Imperial system!), in this book we provide key quantities in both systems of measurement. Don't worry, though, we won't make you convert the speed of light into furlongs per fortnight! (If a conversion like that is needed, we provide it for you.)

Many of the key words used in this book derive from languages more ancient than modern-day English. Nebula for example, comes from Latin and its plural in Latin is nebulae. We use the language-appropriate plural for these words throughout the book rather than improper Englishizations (you'll not see reference to nebulas). Similarly, scientists prefer standard units of time, and so we refer to CE (common era, starting at the year zero) and BCE (before common era, starting before the year zero).

Icons Used in This Book

Throughout this book, helpful icons can guide you to particularly useful nuggets of wisdom, and also help you see what's fine to skip if your eyes are glazing over. Here's what each symbol means:

The string-tied-on-a-finger icon points out information that will be useful to remember for the future.

This icon indicates technical info; the content next to these icons will typically be information related to the topic you're reading about but with a more in-depth technical explanation. Feel free to skip over these if you're looking for the bigger picture, but use these callouts when you're searching for more detailed information on a subject.

The light bulb graphic highlights particularly useful or interesting tidbits. Scan the page quickly and let your eye be drawn to the Tips for nuggets of information - such as, for example, how stars created light in the first place!

From black holes to solar eclipses to getting tangled up in the math of general relativity, astrophysics can head into complex and dangerous territory. The warning icon calls out areas that may be dangerous (either intellectually or physically!) and require a careful approach.

Beyond the Book

In addition to the book you're reading right now, be sure to check out the free Cheat Sheet. It offers a timeline of astrophysics discoveries, a list of misconceptions, and a list of world record holders, among other things. To get this Cheat Sheet, simply go to www.dummies.com and enter Astrophysics For Dummies in the Search box.

There are also some other books in this series that you might enjoy. Astronomy For Dummies, by Stephen Maran and Richard Fienberg, could be a great starting point if you want to learn more about the observational side of astronomy. You can also brush up on your physics knowledge in Physics I For Dummies or Physics II For Dummies by Steven Holzner.

Where to Go from Here

We're overjoyed to greet you on your journey into astrophysics. If this book were a welcome into our home, you'd be greeted with the aroma of chocolate chip cookies fresh from the oven. We suppose the information contained inside may be a poor substitute, but we've done our best to provide an overview of astrophysics using the written word instead of chocolate.

Feel free to consume this book all at once or sample as you go. Looking for a basic overview of astronomy and physics before you get in too deep? Try out Chapter 2 and Chapter 3 for a quick refresher on everything you wanted to know about astronomy and physics but might have forgotten or been afraid to ask.

How did those stars form? Chapter 5 has you covered. Got a yearning for black holes? Don't worry, Chapter 8 is ready to suck you in (literally? You be the judge). How did it all begin, and how will it end? The Big Bang starts in Chapter 13, then concludes in Chapter 16 with the end of it all.

The universe seems to have had a beginning, a middle (we're in it now), and an end. As you'll learn between these pages, there's a lot going on in between. We hope that by the time you reach the...