Astronomy For Dummies

Introduction

Astronomy is the study of the sky, the science of cosmic objects and celestial happenings. It’s nothing less than the investigation of the nature of the universe we live in. Astronomers carry out the business of astronomy by using backyard telescopes, huge observatory instruments, radio telescopes that detect celestial radio emissions, and satellites orbiting Earth or positioned in space near Earth or another celestial body, such as the Moon or a planet. Scientists send up telescopes in sounding rockets and on unmanned balloons, some instruments travel far into the solar system aboard deep space probes, and some probes gather samples and return them to Earth.

Astronomy can be a professional or amateur activity. About 25,000 professional astronomers engage in space science worldwide, and an estimated 500,000 amateur astronomers live around the globe. Many of the amateurs belong to local or national astronomy clubs in their home countries.

Professional astronomers conduct research on the Sun and the solar system, the Milky Way galaxy, and the universe beyond. They teach in universities, design satellites in government labs, and operate planetariums. They also write books like this one (but maybe not as good). Most hold PhDs. Nowadays, many professional astronomers study abstruse physics of the cosmos or work with automated, remotely controlled telescopes, so they may not even know the constellations.

Amateur astronomers know the constellations. They share an exciting hobby. Some stargaze on their own; many others join astronomy clubs and organizations of every description. The clubs pass on know-how from old hands to new members, share telescopes and equipment, and hold meetings where members tell about their recent observations or hear lectures by visiting scientists.

Amateur astronomers also hold observing meetings where everyone brings a telescope (or looks through another observer’s scope). The amateurs conduct these sessions at regular intervals (such as the first Saturday night of each month) or on special occasions (such as the return of a major meteor shower each August or the appearance of a bright comet like Hale-Bopp). And they save up for really big events, such as a total eclipse of the Sun, when thousands of amateurs and dozens of pros travel across Earth to position themselves in the path of totality and witness one of nature’s greatest spectacles.

About This Book

This book explains all you need to know to launch into the great hobby of astronomy. It gives you a leg up on understanding the basic science of the universe as well. The latest space missions will make more sense to you: You’ll understand why NASA and other organizations send space probes to planets like Saturn, why robot rovers land on Mars, and why scientists seek samples of the dust in the tail of a comet. You’ll know why the Hubble Space Telescope peers out into space and how to check up on other space missions. And when astronomers show up in the newspaper or on television to report their latest discoveries — from space; from the big telescopes in Arizona, Hawaii, Chile, and California; or from radio telescopes in New Mexico, Puerto Rico, Australia, or other observatories around the world — you’ll understand the background and appreciate the news. You’ll even be able to explain it to your friends.

Read only the parts you want, in any order you want. I explain what you need as you go. Astronomy is fascinating and fun, so keep reading. Before you know it, you’ll be pointing out Jupiter, spotting famous constellations and stars, and tracking the International Space Station as it whizzes by overhead. The neighbors may start calling you stargazer. Police officers may ask you what you’re doing in the park at night or why you’re standing on the roof with binoculars. Tell ’em you’re an astronomer. They probably haven’t heard that one (I hope they believe you!).

Foolish Assumptions

You may be reading this book because you want to know what’s up in the sky or what the scientists in the space program are doing. Perhaps you’ve heard that astronomy is a neat hobby, and you want to see whether the rumor is true. Perhaps you want to find out what equipment you need.

You’re not a scientist. You just enjoy looking at the night sky and have fallen under its spell, wanting to see and understand the real beauty of the universe.

You want to observe the stars, but you also want to know what you’re seeing. Maybe you even want to make a discovery of your own. You don’t have to be an astronomer to spot a new comet, and you can even help listen for E.T. Whatever your goal, this book helps you achieve it.

Icons Used in This Book

Throughout this book, helpful icons highlight particularly useful information — even if they just tell you to not sweat the tough stuff. Here’s what each symbol means.

remember The Remember icon points out information you should file away for future reference.

technicalstuff This nerdy guy appears beside discussions that you can skip if you just want to know the basics and start watching the skies. The scientific background can be good to know, but many people happily enjoy their stargazing without knowing about the physics of supernovas, the mathematics of galaxy chasing, and the ins and outs of dark energy.

tip This lightbulb puts you right on track to make use of some inside information as you start skywatching or make progress in the hobby.

warning How much trouble can you get into watching the stars? Not much, if you’re careful. But some things you can’t be too careful about. This icon alerts you to pay attention so you don’t get burned.

Beyond the Book

In addition to the book you’re reading right now, be sure to check out the free Cheat Sheet online. It offers a timeline of notable astronomical events and a list of famous female astronomers. To get this Cheat Sheet, simply go to www.dummies.com and enter Astronomy For Dummies in the Search box.

If you want to test your astronomy knowledge, check out the practice quizzes online. Each chapter has a corresponding quiz consisting of multiple choice and true/false questions. I’ve also turned the glossary into flashcards that let you test your knowledge of astronomy terms.

To gain access to the online content, all you have to do is register. Just follow these simple steps:

Register your book or ebook at Dummies.com to get your PIN. Go to www.dummies.com/go/getaccess.

Select your product from the dropdown list on that page.

Follow the prompts to validate your product, and then check your email for a confirmation message that includes your PIN and instructions for logging in.

If you do not receive this email within two hours, please check your spam folder before contacting us through our Technical Support website at http://support.wiley.com or by phone at 877-762-2974.

Now you’re ready to go! You can come back to the practice material as often as you want — simply log on with the username and password you created during your initial login. No need to enter the access code a second time.

Your registration is good for one year from the day you activate your PIN.

Where to Go from Here

You can start anywhere you want. Worried about the fate of the universe? Start off with the Big Bang (see Chapter 16 if you’re really interested).

Or you may want to begin with what’s in store for you as you pursue your passion for the stars.

Wherever you start, I hope you continue your cosmic exploration and experience the joy, excitement, enlightenment, and enchantment that people have always found in the skies.

Part 1

Getting Started with Astronomy

IN THIS PART …

Discover the basic elements of astronomy, check out a list of constellations, and get a crash course on gravity.

Find out about the resources available to help you check out the night sky, including organizations, facilities, and equipment.

Get an introduction to astronomical and artificial phenomena that sweep across the night sky, such as meteors, comets, and artificial satellites.

Chapter 1

Seeing the Light: The Art and Science of Astronomy

IN THIS CHAPTER

check Understanding the observational nature of astronomy

check Focusing on astronomy’s language of light

check Weighing in on gravity

check Recognizing the movements of objects in space

Step outside on a clear night and look at the sky. If you’re a city dweller or live in a cramped suburb, you see dozens, maybe hundreds, of twinkling stars. Depending on the time of the month, you may also see a full Moon and up to five of the eight planets that revolve around the Sun.

A shooting star or meteor may appear overhead. What you actually see is the flash of light from a tiny piece of space dust streaking through the upper atmosphere.

Another pinpoint of light moves slowly and steadily across the sky. Is it a space satellite, such as the Hubble Space Telescope, the International Space Station, or just a high-altitude airliner? If you have a pair of binoculars, you may be able to see the difference. Most airliners have running lights, and their shapes may be perceptible.

If you live in the country — on the seashore away from resorts and developments, on the plains, or in the mountains far from any floodlit ski slope — you can see thousands of stars. The Milky Way appears as a beautiful pearly swath across the heavens. What you’re seeing is the cumulative glow from millions of faint stars, individually indistinguishable with the naked eye. At a great observation place, such as Cerro Tololo in the Chilean Andes, you can see even more stars. They hang like brilliant lamps in a coal black sky, often not even twinkling, like in van Gogh’s Starry Night painting.

When you look at the sky, you practice astronomy — you observe the universe that surrounds you and try to make sense of what you see. For thousands of years, everything people knew about the heavens they deduced by simply observing the sky. Almost everything that astronomy deals with

Is seen from a distance

Is discovered by studying the light that comes to you from objects in space

Moves through space under the influence of gravity

This chapter introduces you to these concepts (and more).

Astronomy: The Science of Observation

Astronomy is the study of the sky, the science of cosmic objects and celestial happenings, and the investigation of the nature of the universe you live in. Professional astronomers carry out the business of astronomy by observing with telescopes that capture visible light from the stars or by tuning in to radio waves that come from space. They use backyard telescopes, huge observatory instruments, and satellites that orbit Earth collecting forms of light (such as ultraviolet radiation) that the atmosphere blocks from reaching the ground. They send up telescopes in sounding rockets (equipped with instruments for making high-altitude scientific observations) and on unmanned balloons. And they send some instruments into the solar system aboard deep-space probes.

Professional astronomers study the Sun and the solar system, the Milky Way, and the universe beyond. They teach in universities, design satellites in government labs, and operate planetariums. They also write books (like me, your loyal For Dummies hero). Most have completed years of schooling to hold PhDs. Many of them study complex physics or work with automated, robotic telescopes that reach far beyond the night sky recognizable to our eyes. They may never have studied the constellations (groups of stars, such as Ursa Major, the Great Bear, named by ancient stargazers) that amateur or hobbyist astronomers first explore.

You may already be familiar with the Big Dipper, an asterism in Ursa Major. An asterism is a named star pattern that’s not identical to one of the 88 recognized constellations. An asterism may be wholly within a single constellation or may include stars from more than one constellation. For example, the four corners of the Great Square of Pegasus, a large asterism, are marked by three stars of the Pegasus constellation and a fourth from Andromeda. Figure 1-1 shows the Big Dipper in the night sky. (In the United Kingdom, some people call the Big Dipper the Plough.)

Photo © Jerry Lodriguss

FIGURE 1-1: The Big Dipper, found in Ursa Major, is an asterism.

In addition to the roughly 30,000 professional astronomers worldwide, several hundred thousand amateur astronomers enjoy watching the skies. Amateur astronomers usually know the constellations and use them as guideposts when exploring the sky by eye, with binoculars, and with telescopes. Many amateurs also make useful scientific contributions. They monitor the changing brightness of variable stars; discover asteroids, comets, and exploding stars; and crisscross Earth to catch the shadows cast as asteroids pass in front of bright stars (thereby helping astronomers map the asteroids’ shapes). They even join in professional research efforts with their home computers and smartphones through Citizen Science projects, which I describe in Chapter 2 and elsewhere throughout the book.

In the rest of Part 1, I provide you with information on how to observe the skies effectively and enjoyably.

What You See: The Language of Light

Light brings us information about the planets, moons, and comets in our solar system; the stars, star clusters, and nebulae in our galaxy; and the objects beyond.

In ancient times, folks didn’t think about the physics and chemistry of the stars; they absorbed and passed down folk tales and myths: the Great Bear, the Demon star, the Man in the Moon, the dragon eating the Sun during a solar eclipse, and more. The tales varied from culture to culture. But many people did discover the patterns of the stars. In Polynesia, skilled navigators rowed across hundreds of miles of open ocean with no landmarks in view and no compass. They sailed by the stars, the Sun, and their knowledge of prevailing winds and currents.

Gazing at the light from a star, the ancients noted its brightness, position in the sky, and color. This information helps people distinguish one sky object from another, and the ancients (and now people today) got to know them like old friends. Some basics of recognizing and describing what you see in the sky are

Distinguishing stars from planets

Identifying constellations, individual stars, and other sky objects by name

Observing brightness (given as magnitudes)

Understanding the concept of a light-year

Charting sky position (measured in special units called RA and Dec)

They wondered as they wandered: Understanding planets versus stars

The term planet comes from the ancient Greek word planetes, meaning wanderer. The Greeks (and other ancient people) noticed that five spots of light moved across the pattern of stars in the sky. Some moved steadily ahead; others occasionally looped back on their own paths. Nobody knew why. And these spots of light didn’t twinkle like the stars did; no one understood that difference, either. Every culture had a name for those five spots of light — what we now call planets. Their English names are Mercury, Venus, Mars, Jupiter, and Saturn. These celestial bodies aren’t wandering through the stars; they orbit around the Sun, our solar system’s central star.

Today astronomers know that planets can be smaller or bigger than Earth, but they all are much smaller than the Sun. The planets in our solar system are so close to Earth that they have perceptible disks — at least, when viewed through a telescope — so we can see their shapes and sizes. The stars are so far away from Earth that even if you view them through a powerful telescope, they show up only as points of light. (For more about the planets in the solar system, flip to Part 2. I cover the planets of stars beyond the Sun in Part 4.)

If you see a Great Bear, start worrying: Naming stars and constellations

I used to tell planetarium audiences who craned their necks to look at stars projected above them, "If you can’t see a Great Bear up there, don’t worry. Maybe those who do see a Great Bear should worry."

Ancient astronomers divided the sky into imaginary figures, such as Ursa Major (Latin for Great Bear); Cygnus, the Swan; Andromeda, the Chained Lady; and Perseus, the Hero. The ancients identified each figure with a pattern of stars. The truth is, to most people, Andromeda doesn’t look much like a chained lady at all — or anything else, for that matter (see Figure 1-2).

© John Wiley & Sons, Inc.

FIGURE 1-2: Andromeda is also known as the Chained Lady.

Today astronomers have divided the sky into 88 constellations, which contain all the stars you can see. The International Astronomical Union, which governs the science, set boundaries for the constellations so astronomers can agree on which star is in which constellation. Previously, sky maps drawn by different astronomers often disagreed. Now when you read that the Tarantula Nebula is in Dorado (see Chapter 12), you know that, to see this nebula, you must seek it in the Southern Hemisphere constellation Dorado, the Goldfish.

The largest constellation is Hydra, the Water Snake. The smallest is Crux, the Cross, which most people call the Southern Cross. You can see a Northern Cross, too, but you can’t find it in a list of constellations; it’s an asterism within Cygnus, the Swan. Although astronomers generally agree on the names of the constellations, they don’t have a consensus on what each name means. For example, some astronomers call Dorado the Swordfish, but I’d like to skewer that name. One constellation, Serpens, the Serpent, is broken into two sections that aren’t connected. The two sections, located on either side of Ophiuchus, the Serpent Bearer, are Serpens Caput (the Serpent’s Head) and Serpens Cauda (the Serpent’s Tail).

The individual stars in a constellation often have no relation to each other except for their proximity in the sky as visible from Earth. In space, the stars that make up a constellation may be completely unrelated to one another, with some located relatively near Earth and others located at much greater distances in space. But they make a simple pattern for observers on Earth to enjoy.

As a rule, the brighter stars in a constellation were assigned a Greek letter, either by the ancient Greeks or by astronomers of later civilizations. In each constellation, the brightest star was labeled alpha, the first letter of the Greek alphabet. The next brightest star was beta, the second Greek letter, and so on down to omega, the final letter of the 24-character Greek alphabet. (The astronomers used only lowercase Greek letters, so you see them written as α, β, … ω.)

So Sirius, the brightest star in the night sky — in Canis Major, the Great Dog — is called Alpha Canis Majoris. (Astronomers add a suffix here or there to put star names in the Latin genitive case; scientists have always liked Latin.) Table 1-1 shows a list of the Greek alphabet, in order, with the names of the letters and their corresponding symbols.

TABLE 1-1 The Greek Alphabet

tip When you look at a star atlas, you discover that the individual stars in a constellation aren’t marked α Canis Majoris, β Canis Majoris, and so on. Usually, the creator of the atlas marks the area of the whole constellation as Canis Major and labels the individual stars α, β, and so on. When you read about a star in a list of objects to observe, say, in an astronomy magazine (see Chapter 2), you probably won’t see it listed in the style of Alpha Canis Majoris or even α Canis Majoris. Instead, to save space, the magazine prints it as α CMa; CMa is the three-letter abbreviation for Canis Majoris (and also the abbreviation for Canis Major). I give the abbreviation for each of the constellations in Table 1-2.

TABLE 1-2 The Constellations and Their Brightest Stars

Astronomers didn’t coin special names such as Sirius for every star in Canis Major, so they named them with Greek letters or other symbols. In fact, some constellations don’t have a single named star. (Don’t fall for those advertisements that offer to name a star for a fee. The International Astronomical Union doesn’t recognize purchased star names.) In other constellations, astronomers assigned Greek letters, but they could see more stars than the 24 Greek letters. Therefore, astronomers gave some stars Arabic numbers or letters from the Roman alphabet, or numbers in professional catalogues. So you see star names such as 61 Cygni, b Vulpeculae, HR 1516, and more. You may even run across the star names RU Lupi and YY Sex. (I’m not making this up.) But as with any other star, you can recognize them by their positions in the sky (as tabulated in star lists), their brightness, their color, or other properties, if not their names.

When you look at the constellations today, you see many exceptions to the rule that the Greek-letter star names correspond to the respective brightness of the stars in a constellation. The exceptions exist because

The letter names were based on inaccurate naked-eye observations of brightness.

Over the years, star atlas authors changed constellation boundaries, moving some stars from one constellation into another that included previously named stars.

Some astronomers mapped out small and Southern Hemisphere constellations long after the Greek period, and they didn’t always follow the lettering practice.

The brightness of some stars has changed over the centuries since the ancient Greeks charted them.

A good (or bad) example is the constellation Vulpecula, the Fox, in which only one of the stars (alpha) has a Greek letter.

Because alpha isn’t always the brightest star in a constellation, astronomers needed another term to describe that exalted status, and lucida is the word (from the Latin word lucidus, meaning bright or shining). The lucida of Canis Major is Sirius, the alpha star, but the lucida of Orion, the Hunter, is Rigel, which is Beta Orionis. The lucida of Leo Minor, the Little Lion (a particularly inconspicuous constellation), is 46 Leo Minoris.

Table 1-2 lists the 88 constellations, the brightest star in each, and the magnitude of that star. Magnitude is a measure of a star’s brightness. (I talk about magnitudes in the later section The smaller, the brighter: Getting to the root of magnitudes.) When the lucida of a constellation is the alpha star and has a name, I list only the name. For example, in Auriga, the Charioteer, the brightest star (Alpha Aurigae) is Capella. But when the lucida isn’t an alpha, I give its Greek letter or other designation in parentheses. For example, the lucida of Cancer, the Crab, is Al Tarf, which is Beta Cancri.

tip If you’re a long-time Astronomy For Dummies reader (possessing at least one of the three previous editions of the book as well as this edition), you may notice some changes in Tab1e 1-2. In 2016, the International Astronomical Union issued a list of official names for bright stars. Seven stars in Table 1-2 were affected, with minor changes in spelling or a whole new name. In one case, a star was named after its constellation: Alpha Pavonis, in Pavo the Peacock, was itself named Peacock.

Identifying stars would be much easier if they had little name tags that you could see through your telescope. If you have a smartphone, you can download an app to identify the stars for you. Just download a sky map or planetarium app (such as Sky Safari, Star Walk, or Google Sky Map) and face the phone toward the sky. The app generates a map of the constellations in the general direction your phone is facing. With some apps, when you touch the image of a star, its name appears. (I describe more astronomy apps in Chapter 2; for the full scoop on stars, check out Chapter 11.)

What do I spy? Spotting the Messier Catalog and other sky objects

Naming stars was easy enough for astronomers. But what about all those other objects in the sky — galaxies, nebulae, star clusters, and the like (which I cover in Part 3)? Charles Messier (1730–1817), a French astronomer, created a numbered list of about 100 fuzzy sky objects. His list is known as the Messier Catalog, and now when you hear the Andromeda Galaxy called by its scientific name, M31, you know that it stands for number 31 in the Catalog. Today 110 objects make up the standard Messier Catalog.

tip You can find pictures and a complete list of the Messier objects at The Messier Catalog website of Students for the Exploration and Development of Space at messier.seds.org. And you can find out how to earn a certificate for viewing Messier objects from the Astronomical League Messier Program website at www.astroleague.org/al/obsclubs/messier/mess.html.

Experienced amateur astronomers often engage in Messier marathons, in which each person tries to observe every object in the Messier Catalog during a single long night. But in a marathon, you don’t have time to enjoy an individual nebula, star cluster, or galaxy. My advice is to take it slow and savor their individual visual delights. A wonderful book on the Messier objects, which includes hints on how to observe each object, is Stephen James O’Meara’s Deep-Sky Companions: The Messier Objects, 2nd Edition (Cambridge University Press).

Since Messier’s time, astronomers have confirmed the existence of thousands of other deep sky objects, the term amateurs use for star clusters, nebulae, and galaxies to distinguish them from stars and planets. Because Messier didn’t list them, astronomers refer to these objects by their numbers as given in other catalogues. You can find many of these objects listed in viewing guides and sky maps by their NGC (New General Catalogue) and IC (Index Catalogue) numbers. For example, the bright double cluster in Perseus, the Hero, consists of NGC 869 and NGC 884.

The smaller, the brighter: Getting to the root of magnitudes

A star map, constellation drawing, or list of stars always indicates each star’s magnitude. The magnitudes represent the brightness of the stars. One of the ancient Greeks, Hipparchos (also spelled Hipparchus, but he wrote it in Greek), divided all the stars he could see into six classes. He called the brightest stars magnitude 1 or 1st magnitude, the next brightest bunch the 2nd magnitude stars, and on down to the dimmest ones, which were 6th magnitude.

Notice that, contrary to most common measurement scales and units, the brighter the star, the smaller the magnitude. The Greeks weren’t perfect, however; even Hipparchos had an Achilles’ heel: He didn’t leave room in his system for the very brightest stars, when accurately measured.

So today we recognize a few stars with a zero magnitude or a negative magnitude. Sirius, for example, is magnitude –1.5. And the brightest planet, Venus, is sometimes magnitude –4 (the exact value differs, depending on the distance Venus is from Earth at the time and its direction with respect to the Sun).

Another omission: Hipparchos didn’t have a magnitude class for stars that were too dim to be seen with the naked eye. This didn’t seem like an oversight at the time because nobody knew about these stars before the invention of the telescope. But today astronomers know that billions of stars exist beyond our naked-eye view. Their magnitudes are larger numbers: 7 or 8 for stars easily seen through binoculars, and 10 or 11 for stars easily seen through a good, small telescope. The magnitudes reach as high (and as dim) as 21 for the faintest stars in the Palomar Observatory Sky Survey and about 31 for the faintest objects imaged with the Hubble Space Telescope.

BY THE NUMBERS: THE MATHEMATICS OF BRIGHTNESS

The 1st magnitude stars are about 100 times brighter than the 6th magnitude stars. In particular, the 1st magnitude stars are about 2.512 times brighter than the 2nd magnitude stars, which are about 2.512 times brighter than the 3rd magnitude stars, and so on. (At the 6th magnitude, you get up into some big numbers: 1st magnitude stars are about 100 times brighter.) You mathematicians out there recognize this as a geometric progression. Each magnitude is the 5th root of 100 (meaning that when you multiply a number by itself four times — for example, — the result is 100). If you doubt my word and do this calculation on your own, you get a slightly different answer because I left off some decimal places.

Thus, you can calculate how faint a star is — compared to some other star — from its magnitude. If two stars are 5 magnitudes apart (such as the 1st magnitude star and the 6th magnitude star), they differ by a factor of 2.512⁵ (2.512 to the fifth power), and a good pocket calculator shows you that one star is 100 times brighter. If two stars are 6 magnitudes apart, one is about 250 times brighter than the other. And if you want to compare, say, a 1st magnitude star with an 11th magnitude star, you compute a 2.512¹⁰ difference in brightness, meaning a factor of 100², or 10,000.

The faintest object visible with the Hubble Space Telescope is about 25 magnitudes fainter than the faintest star you can see with the naked eye (assuming normal vision and viewing skills — some experts and a certain number of liars and braggarts say that they can see 7th magnitude stars). Speaking of dim stars, 25 magnitudes are five times 5 magnitudes, which corresponds to a brightness difference of a factor of 100⁵. So the Hubble can see , or 10 billion times fainter than the human eye. Astronomers expect nothing less from a billion-dollar telescope. At least it didn’t cost $10 billion.

You can get a good telescope for well under $1,000, and you can view the billion-dollar Hubble’s best photos on the Internet for free at hubblesite.org.

Looking back on light-years

The distances to the stars and other objects beyond the planets of our solar system are measured in light-years. As a measurement of actual length, a light-year is about 5.9 trillion miles long.

People confuse a light-year with a length of time because the term contains the word year. But a light-year is really a distance measurement — the length that light travels, zipping through space at 186,000 miles per second, over the course of a year.

When you view an object in space, you see it as it appeared when the light left the object. Consider these examples:

When astronomers spot an explosion on the Sun, we don’t see it in real time; the light from the explosion takes about 8 minutes to get to Earth.

The nearest star beyond the Sun, Proxima Centauri, is about 4 light-years away. Astronomers can’t see Proxima as it is now — only as it was four years ago.

Look up at the Andromeda Galaxy, the most distant object that you can readily see with the unaided eye, on a clear, dark night in the fall. The light your eye receives left that galaxy about 2.5 million years ago. If there was a big change in Andromeda tomorrow, we wouldn’t know that it happened for more than 2 million years. (See Chapter 12 for hints on viewing the Andromeda Galaxy and other prominent galaxies.)

Here’s the bottom line:

When you look out into space, you’re looking back in time.

Astronomers don’t have a way to know exactly what an object out in space looks like right now.

When you look at some big, bright stars in a faraway galaxy, you must entertain the possibility that those particular stars don’t even exist anymore. As I explain in Chapter 11, some massive stars live for only 10 million or 20 million years. If you see them in a galaxy that is 50 million light-years away, you’re looking at lame duck stars. They aren’t shining in that galaxy anymore; they’re dead.

If astronomers send a flash of light toward one of the most distant galaxies found with Hubble and other major telescopes, the light would take billions of years to arrive. Astronomers, however, calculate that the Sun will swell up and destroy all life on Earth a mere 5 billion or 6 billion years from now, so the light would be a futile advertisement of our civilization’s existence, a flash in the celestial pan.

HEY, YOU! NO, NO, I MEAN AU

Earth is about 93 million miles from the Sun, or 1 astronomical unit (AU). The distances between objects in the solar system are usually given in AU. Its plural is also AU. (Don’t confuse AU with Hey, you!)

In public announcements, press releases, and popular books, astronomers state how far the stars and galaxies that they study are from Earth. But among themselves and in technical journals, they always give the distances from the Sun, the center of our solar system. This discrepancy rarely matters because astronomers can’t measure the distances of the stars precisely enough for 1 AU more or less to make a difference, but they do it this way for consistency.

Keep on moving: Figuring the positions of the stars

Astronomers used to call stars fixed stars, to distinguish them from the wandering planets. But in fact, stars are in constant motion as well, both real and apparent. The whole sky rotates overhead because Earth is turning. The stars rise and set, like the Sun and the Moon, but they stay in formation. The stars that make up the Great Bear don’t swing over to the Little Dog or Aquarius, the Water Bearer. Different constellations rise at different times and on different dates, as seen from different places around the globe.

Actually, the stars in Ursa Major (and every other constellation) do move with respect to one another — and at breathtaking speeds, measured in hundreds of miles per second. But those stars are so far away that scientists