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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
- Veröffentlichungsjahr: 2017
Your updated guide to exploring the night sky Do you know the difference between a red giant and a white dwarf? From asteroids to black holes, this easy-to-understand guide takes you on a grand tour of the universe. Featuring updated star maps, charts, and an insert with gorgeous full-color photographs, Astronomy For Dummies provides an easy-to-follow introduction to exploring the night sky. Plus, this new edition also comes with chapter quizzes online to help your understanding. For as long as people have been walking the earth, those people have looked up into the night sky and wondered about the nature of the cosmos. Without the benefit of science to provide answers, they relied on myth and superstition to help them make sense of what they saw. Lucky for us, we live at a time when regular folks, equipped with nothing more than their naked eyes, can look up into the night sky and gain admittance to infinite wonders. If you know what to look for, you can make out planets, stars, galaxies, and even galactic clusters comprising hundreds of millions of stars and spanning millions of light-years. Whether you're an amateur astronomer, space enthusiast, or enrolled in a first year astronomy course, Astronomy For Dummies gives you a reason to look into the heavens. * Includes updated schedules of coming eclipses of the Sun and Moon and a revised planetary appendix * Covers recent discoveries in space, such as water on the Moon and Pluto's demotion from "planet" status * Collects new websites, lists of telescope motels, sky-watching guides, and suggestions for beginner's telescopes and suppliers * Provides free online access to chapter quizzes to help you understand the content Ever wonder what's out there in the big ol' universe? This is the book for you!
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Astronomy For Dummies®, 4th Edition
Published by:John Wiley & Sons, Inc.111 River StreetHoboken, NJ 07030-5774www.wiley.com
Copyright © 2017 by John Wiley & Sons, Inc., Hoboken, New Jersey
Published simultaneously in Canada
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Library of Congress Control Number: 2017947198
ISBN 978-1-119-37424-4 (pbk); ISBN 978-1-119-37438-1 (ebk); ISBN 978-1-119-37441-1 (ebk)
Astronomy For Dummies®
To view this book's Cheat Sheet, simply go to www.dummies.com and search for “Astronomy For Dummies Cheat Sheet” in the Search box.
Table of Contents
Cover
Introduction
About This Book
Foolish Assumptions
Icons Used in This Book
Beyond the Book
Where to Go from Here
Part 1: Getting Started with Astronomy
Chapter 1: Seeing the Light: The Art and Science of Astronomy
Astronomy: The Science of Observation
What You See: The Language of Light
Gravity: A Force to Be Reckoned With
Space: A Commotion of Motion
Chapter 2: Join the Crowd: Skywatching Activities and Resources
You’re Not Alone: Astronomy Clubs, Websites, Smartphone Apps, and More
Visiting Observatories and Planetariums
Vacationing with the Stars: Star Parties, Eclipse Trips, Dark Sky Parks, and More
Chapter 3: Terrific Tools for Observing the Skies
Seeing Stars: A Sky Geography Primer
Beginning with Naked-Eye Observation
Using Binoculars or a Telescope for a Better View
Planning Your First Steps into Astronomy
Chapter 4: Just Passing Through: Meteors, Comets, and Artificial Satellites
Meteors: Wishing on a Shooting Star
Comets: The Lowdown on Dirty Ice Balls
Artificial Satellites: Enduring a Love–Hate Relationship
Part 2: Going Once Around the Solar System
Chapter 5: A Matched Pair: Earth and Its Moon
Putting Earth under the Astronomical Microscope
Examining Earth’s Time, Seasons, and Age
Making Sense of the Moon
Chapter 6: Earth’s Near Neighbors: Mercury, Venus, and Mars
Mercury: Weird, Hot, and Mostly Metal
Dry, Acidic, and Hilly: Steering Clear of Venus
Red, Cold, and Barren: Uncovering the Mysteries of Mars
Differentiating Earth through Comparative Planetology
Observing the Terrestrial Planets with Ease
Chapter 7: Rock On: The Asteroid Belt and Near-Earth Objects
Taking a Brief Tour of the Asteroid Belt
Understanding the Threat That Near-Earth Objects Pose
Searching for Small Points of Light
Chapter 8: Great Balls of Gas: Jupiter and Saturn
The Pressure’s On: Journeying Inside Jupiter and Saturn
Almost a Star: Gazing at Jupiter
Our Main Planetary Attraction: Setting Your Sights on Saturn
Chapter 9: Far Out! Uranus, Neptune, Pluto, and Beyond
Breaking the Ice with Uranus and Neptune
Meeting Pluto, the Amazing Dwarf Planet
Buckling Down to the Kuiper Belt
Viewing the Outer Planets
Hunting New Planet Number Nine
Part 3: Meeting Old Sol and Other Stars
Chapter 10: The Sun: Star of Earth
Surveying the Sunscape
Don’t Make a Blinding Mistake: Safe Techniques for Solar Viewing
Fun with the Sun: Solar Observation
Chapter 11: Taking a Trip to the Stars
Life Cycles of the Hot and Massive
Star Color, Brightness, and Mass
Eternal Partners: Binary and Multiple Stars
Change Is Good: Variable Stars
Your Stellar Neighbors
How to Help Scientists by Observing the Stars
Star Studies to Aid with Your Brain and Computer
Chapter 12: Galaxies: The Milky Way and Beyond
Unwrapping the Milky Way
Star Clusters: Meeting Galactic Associates
Taking a Shine to Nebulae
Getting a Grip on Galaxies
Joining Galaxy Zoo for Fun and Science
Chapter 13: Digging into Black Holes and Quasars
Black Holes: Keeping Your Distance
Quasars: Defying Definitions
Active Galactic Nuclei: Welcome to the Quasar Family
Part 4: Pondering the Remarkable Universe
Chapter 14: Is Anybody Out There? SETI and Planets of Other Suns
Using Drake’s Equation to Discuss SETI
SETI Projects: Listening for E.T.
Discovering Alien Worlds
Astrobiology: How’s Life on Other Worlds?
Chapter 15: Delving into Dark Matter and Antimatter
Dark Matter: Understanding the Universal Glue
Taking a Shot in the Dark: Searching for Dark Matter
Dueling Antimatter: Proving That Opposites Attract
Chapter 16: The Big Bang and the Evolution of the Universe
Evidence for the Big Bang
Inflation: A Swell Time in the Universe
Dark Energy: The Universal Accelerator
Universal Info Pulled from the Cosmic Microwave Background
In a Galaxy Far Away: Standard Candles and the Hubble Constant
The Fate of the Universe
Part 5: The Part of Tens
Chapter 17: Ten Strange Facts about Astronomy and Space
You Have Tiny Meteorites in Your Hair
A Comet’s Tail Often Leads the Way
Earth Is Made of Rare and Unusual Matter
High Tide Comes on Both Sides of Earth at the Same Time
On Venus, the Rain Never Falls on the Plain
Rocks from Mars Dot Earth
Pluto Was Discovered from the Predictions of a False Theory
Sunspots Aren’t Dark
A Star in Plain View May Have Exploded, but No One Knows
You May Have Seen the Big Bang on an Old Television
Chapter 18: Ten Common Errors about Astronomy and Space
“The Light from That Star Took 1,000 Light-Years to Reach Earth”
A Freshly Fallen Meteorite Is Still Hot
Summer Always Comes When Earth Is Closest to the Sun
The Back of the Moon Is Dark
The “Morning Star” Is a Star
If You Vacation in the Asteroid Belt, You’ll See Asteroids All Around You
Nuking a “Killer Asteroid” on a Collision Course for Earth Will Save Us
The Sun Is an Average Star
The Hubble Telescope Gets Up Close and Personal
The Big Bang Is Dead
Part 6: Appendixes
Appendix A: Star Maps
Appendix B: Glossary
Sky Measures
About the Author
Supplemental Images
Connect with Dummies
End User License Agreement
Guide
Cover
Table of Contents
Begin Reading
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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.
The Remember icon points out information you should file away for future reference.
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.
This lightbulb puts you right on track to make use of some inside information as you start skywatching or make progress in the hobby.
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:
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Find your product (
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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
Understanding the observational nature of astronomy
Focusing on astronomy’s language of light
Weighing in on gravity
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
Letter
Name
α
Alpha
β
Beta
γ
Gamma
δ
Delta
ε
Epsilon
ζ
Zeta
η
Eta
θ
Theta
ι
Iota
κ
Kappa
λ
Lambda
μ
Mu
ν
Nu
ξ
Xi
ο
Omicron
π
Pi
ρ
Rho
σ
Sigma
τ
Tau
υ
Upsilon
φ
Phi
χ
Chi
ψ
Psi
ω
Omega
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
Name
Abbreviation
Meaning
Star
Magnitude
Andromeda
And
Chained Lady
Alpheratz
2.1
Antlia
Ant
Air Pump
Alpha Antliae
4.3
Apus
Aps
Bird of Paradise
Alpha Apodis
3.8
Aquarius
Aqr
Water Bearer
Sadalsuud (Beta Aquarii)
2.9
Aquila
Aql
Eagle
Altair
0.8
Ara
Ara
Altar
Beta Arae
2.9
Aries
Ari
Ram
Hamal
2.0
Auriga
Aur
Charioteer
Capella
0.1
Bootes
Boo
Herdsman
Arcturus
–0.04
Caelum
Cae
Chisel
Alpha Caeli
4.5
Camelopardalis
Cam
Giraffe
Beta Camelopardalis
4.0
Cancer
Cnc
Crab
Al Tarf (Beta Cancri)
3.5
Canes Venatici
CVn
Hunting Dogs
Cor Caroli
2.9
Canis Major
CMa
Great Dog
Sirius
–1.5
Canis Minor
CMi
Little Dog
Procyon
0.4
Capricornus
Cap
Goat
Deneb Algedi (Delta Capricorni)
2.9
Carina
Car
Ship’s Keel
Canopus
–0.7
Cassiopeia
Cas
Queen
Schedar
2.2
Centaurus
Cen
Centaur
Rigil Kentaurus
–0.01
Cepheus
Cep
King
Alderamin
2.4
Cetus
Cet
Whale
Diphda (Beta Ceti)
2.0
Chamaeleon
Cha
Chameleon
Alpha Chamaeleontis
4.1
Circinus
Cir
Compasses
Alpha Circini
3.2
Columba
Col
Dove
Phact
2.6
Coma Berenices
Com
Berenice’s Hair
Beta Comae Berenices
4.3
Corona Australis
CrA
Southern Crown
Alphecca Meridiana
4.1
Corona Borealis
CrB
Northern Crown
Alphecca
2.2
Corvus
Crv
Crow
Gienah (Gamma Corvi)
2.6
Crater
Crt
Cup
Delta Crateris
3.6
Crux
Cru
Cross
Acrux
1.3
Cygnus
Cyg
Swan
Deneb
1.3
Delphinus
Del
Dolphin
Rotanev (Beta Delphini)
3.6
Dorado
Dor
Goldfish
Alpha Doradus
3.3
Draco
Dra
Dragon
Eltanin (Gamma Draconis)
2.2
Equuleus
Equ
Little Horse
Kitalpha
3.9
Eridanus
Eri
River
Achernar
0.5
Fornax
For
Furnace
Alpha Fornacis
3.9
Gemini
Gem
Twins
Pollux (Beta Geminorum)
1.1
Grus
Gru
Crane
Alnair
1.7
Hercules
Her
Hercules
Kornephoros (Beta Herculis)
2.8
Horologium
Hor
Clock
Alpha Horologii
3.9
Hydra
Hya
Water Snake
Alphard
2.0
Hydrus
Hyi
Little Water Snake
Beta Hydri
2.8
Indus
Ind
Indian
Alpha Indi
3.1
Lacerta
Lac
Lizard
Alpha Lacertae
3.8
Leo
Leo
Lion
Regulus
1.4
Leo Minor
LMi
Little Lion
Praecipua (46 Leonis Minoris)
3.8
Lepus
Lep
Hare
Arneb
2.6
Libra
Lib
Scales
Zubeneschamali (Beta Librae)
2.6
Lupus
Lup
Wolf
Alpha Lupi
2.3
Lynx
Lyn
Lynx
Alpha Lyncis
3.1
Lyra
Lyr
Lyre
Vega
0.0
Mensa
Men
Table
Alpha Mensae
5.1
Microscopium
Mic
Microscope
Gamma Microscopii
4.7
Monoceros
Mon
Unicorn
Beta Monocerotis
3.7
Musca
Mus
Fly
Alpha Muscae
2.7
Norma
Nor
Level and Square
Gamma Normae
4.0
Octans
Oct
Octant
Nu Octantis
3.8
Ophiuchus
Oph
Serpent Bearer
Rasalhague
2.1
Orion
Ori
Hunter
Rigel (Beta Orionis)
0.1
Pavo
Pav
Peacock
Peacock
1.9
Pegasus
Peg
Winged Horse
Enif (Epsilon Pegasi)
2.4
Perseus
Per
Hero
Mirfak
1.8
Phoenix
Phe
Phoenix
Ankaa
2.4
Pictor
Pic
Easel
Alpha Pictoris
3.2
Pisces
Psc
Fish
Kullat Nunu (Eta Piscium)
3.6
Pisces Austrinus
PsA
Southern Fish
Fomalhaut
1.2
Puppis
Pup
Ship’s Stern
Naos (Zeta Puppis)
2.3
Pyxis
Pyx
Compass
Alpha Pyxidis
3.7
Reticulum
Ret
Net
Alpha Reticuli
3.4
Sagitta
Sge
Arrow
Gamma Sagittae
3.5
Sagittarius
Sgr
Archer
Kaus Australis (Epsilon Sagittarii)
1.9
Scorpius
Sco
Scorpion
Antares
1.0
Sculptor
Scl
Sculptor
Alpha Sculptoris
4.3
Scutum
Sct
Shield
Alpha Scuti
3.9
Serpens
Ser
Serpent
Unukalhai
2.7
Sextans
Sex
Sextant
Alpha Sextantis
4.5
Taurus
Tau
Bull
Aldebaran
0.9
Telescopium
Tel
Telescope
Alpha Telescopii
3.5
Triangulum
Tri
Triangle
Beta Trianguli
3.0
Triangulum Australe
TrA
Southern Triangle
Atria
1.9
Tucana
Tuc
Toucan
Alpha Tucanae
2.9
Ursa Major
UMa
Great Bear
Alioth (Epsilon Ursae Majoris)
1.8
Ursa Minor
UMi
Little Bear
Polaris
2.0
Vela
Vel
Sails
Suhail al Muhlif (Gamma Velorum)
1.8
Virgo
Vir
Virgin
Spica
1.0
Volans
Vol
Flying Fish
Gamma Volantis
3.8
Vulpecula
Vul
Fox
Anser
4.4
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.
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.
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 need precise measurements over considerable intervals of time to detect their motions across the sky. So 20,000 years from now, the stars in Ursa Major will form a different pattern in the sky. (Maybe they will even look like a Great Bear.)
In the meantime, astronomers have measured the positions of millions of stars, and many of them are tabulated in catalogs and marked on star maps. The positions are listed in a system called right ascension and declination — known to all astronomers, amateur and pro, as RA and Dec:
The RA is the position of a star measured in the east–west direction on the sky (like longitude, the position of a place on Earth measured east or west of the prime meridian at Greenwich, England).
The Dec is the position of the star measured in the north–south direction, like the latitude of a city, which is measured north or south of the equator.
Astronomers usually list RA in units of hours, minutes, and seconds, like time. We list Dec in degrees, minutes, and seconds of arc. Ninety degrees make up a right angle, 60 minutes of arc make up a degree, and 60 seconds of arc equal a minute of arc. A minute or second of arc is also often called an “arc minute” or an “arc second,” respectively.
DIGGING DEEPER INTO RA AND DEC
A star at RA 2h00m00s is 2 hours east of a star at RA 0h00m00s, regardless of their declinations. RA increases from west to east, starting from RA 0h00m00s, which corresponds to a line in the sky (actually half a circle, centered on the center of Earth) from the North Celestial Pole to the South Celestial Pole. The first star may be at Dec 30° North, and the second star may be at Dec 15° 25’12” South, but they’re still 2 hours apart in the east–west direction (and 45° 25’12” apart in the north–south direction). The North and South Celestial Poles are the points in the sky — due north and due south — around which the whole sky seems to turn, with the stars all rising and setting.
Note the following details about the units of RA and Dec:
An hour of RA equals an arc of 15 degrees on the equator in the sky. Twenty-four hours of RA span the sky, and , or a complete circle around the sky. A minute of RA, called a minute of time, is a measure of angle on the sky that makes up of an hour of RA. So you take , or . A second of RA, or a second of time, is 60 times smaller than a minute of time.Dec is measured in degrees, like the degrees in a circle, and in minutes and seconds of arc. A whole degree is about twice the apparent or angular size of the full Moon. Each degree is divided into 60 minutes of arc. The Sun and the full Moon are both about 32 minutes of arc (32’) wide, as seen on the sky, although, in reality, the Sun is much larger than the Moon. Each minute of arc is divided into 60 seconds of arc (60”). When you look through a backyard telescope at high magnification, turbulence in the air blurs the image of a star. Under good conditions (low turbulence), the image should measure about 1” or 2” across. That’s 1 or 2 arc seconds, not 1 or 2 inches.A few simple rules may help you remember how RA and Dec work and how to read a star map (see Figure 1-3):
The North Celestial Pole (NCP) is the place to which the axis of Earth points in the north direction. If you stand at the geographic North Pole, the NCP is right overhead. (If you stand there, say “Hi” to Santa for me, but beware: You may be on thin ice because there’s no land at the geographic North Pole.)
The South Celestial Pole (SCP) is the place to which the axis of Earth points in the south direction. If you stand at the geographic South Pole, the SCP is right overhead. I hope you dressed warmly: You’re in Antarctica!
The imaginary lines of equal RA run through the NCP and SCP as semicircles centered on the center of Earth. They may be imaginary, but they appear marked on most sky maps to help people find the stars at particular RAs.
The imaginary lines of equal Dec, like the line in the sky that marks Dec of 30° North, pass overhead at the corresponding geographic latitudes. So if you stand in New York City, latitude 41° North, the point overhead is always at Dec 41° North, although its RA changes constantly as Earth turns. These imaginary lines appear on star maps, too, as
declination circles.
© John Wiley & Sons, Inc.
FIGURE 1-3: Decoding the celestial sphere to find directions in space.
Suppose you want to find the NCP as visible from your backyard. Face due north and look at an altitude of x degrees, where x is your geographic latitude. I’m assuming that you live in North America, Europe, or somewhere in the Northern Hemisphere. If you live in the Southern Hemisphere, you can’t see the NCP. You can, however, look for the SCP. Look for the spot due south whose altitude in the sky, measured in degrees above the horizon, is equal to your geographic latitude.
In almost every astronomy book, the symbol ″ means seconds of arc, not inches. But at every university, a student in Astronomy 101 writes on a lab report, “The image of the star was about 1 inch in diameter.” Understanding beats memorizing every day, but not everyone understands.
Here’s the good news: If you just want to spot the constellations and the planets, you don’t have to know how to use RA and Dec. Just consult a star map drawn for the current week or month (you can find these on the website of Sky & Telescope or one of the other magazines that I mention in Chapter 2, in the magazines themselves, or using a desktop planetarium program for your home computer or a planetarium app for your smartphone or tablet; I recommend programs, websites, and apps in Chapter 2 as well). But if you want to understand how star catalogs and maps work and how to zero in on faint galaxies with your telescope, understanding the system helps.
And if you purchase one of those snazzy and surprisingly affordable telescopes with computer control (see Chapter 3), you can punch in the RA and Dec of a recently discovered comet, and the scope points right at it. (A little table called an ephemeris comes with every announcement of a new comet. It gives the predicted RA and Dec of the comet on successive nights as it sweeps across the sky.)
Gravity: A Force to Be Reckoned With
Ever since the work of Sir Isaac Newton, the English scientist (1642–1727), everything in astronomy has revolved around gravity. Newton explained gravity as a force between any two objects. The force depends on mass and separation. The more massive the object, the more powerful its pull. The greater the distance, the weaker the gravitational attraction. Newton sure was a smart cookie!
Albert Einstein developed an improved theory of gravity, which passes experimental tests that Newton’s theory flunks. Newton’s theory was good enough for commonly experienced gravity, like the force that made the apple fall on his head (if it really hit him). But in other respects, Newton’s theory was hit or miss. Einstein’s theory is better because it predicts everything that Newton got right, but also predicts effects that happen close to massive objects, where gravity is very strong. Einstein didn’t think of gravity as a force; he considered it the bending of space and time by the very presence of a massive object, such as a star. I get all bent out of shape just thinking about it.
Newton’s concept of gravity explains the following:
Why the Moon orbits Earth, why Earth orbits the Sun, why the Sun orbits the center of the Milky Way, and why many other objects orbit one object or another out there in space
Why a star or a planet is round
Why gas and dust in space may clump together to form new stars
Einstein’s theory of gravity, called the General Theory of Relativity, explains everything that Newton’s theory does plus the following:
Why stars visible near the Sun during a total eclipse seem slightly out of position
Why black holes exist
Why gravitational lensing is found when we observe deep space
Why Earth drags warped space and time around with it as it turns, an effect that scientists have verified with the help of satellites orbiting Earth
How a collision of two black holes produces gravitational waves that shake things up even billions of light-years away
You find out about black holes in Chapters 11 and 13, and you can read up on gravitational lensing in Chapters 11, 14, and 15 without mastering the General Theory of Relativity.
You’ll get smarter if you read every chapter in this book, but your friends won’t call you Einstein unless you let your hair grow, parade around in a messy old sweater, and stick out your tongue when they take your picture.
Space: A Commotion of Motion
Everything in space is moving and turning. Objects can’t sit still. Thanks to gravity, other celestial bodies are always pulling on a star, planet, galaxy, or spacecraft. Some of us are self-centered, but the universe has no center.
For example, Earth
Turns on its axis — what astronomers call
rotating
— and takes one day to turn all the way around.
Orbits around the Sun — what astronomers call
revolving —
with one complete orbit taking one year.
Travels with the Sun in a huge orbit around the center of the Milky Way. The trip takes about 250 million years to complete once, and the duration of the trip is called the
galactic year.
Moves with the Milky Way in a trajectory around the center of the
Local Group of Galaxies,
a couple of dozen galaxies in our neck of the universe.
Moves through the universe with the Local Group as part of the Hubble Flow, the general expansion of space caused by the Big Bang.
The Big Bang
