the scale of the cosmos astronomy: the solar system and beyond 5th edition michael seeds

The Scale of the Cosmos

Astronomy: The Solar System and Beyond

5th edition Michael Seeds


The longest journey

begins with a single step.

- CONFUCIUS

Chapter 1


• You are about to embark on a voyage out to the end of the universe.


• Marco Polo journeyed east and Columbus west. – But you will travel outward away from your home on

Earth, out past the moon, sun, and other planets, past the stars you see in the evening sky, and past billions more that can be seen only with the aid of the largest telescopes.

– You will journey through great whirlpools of stars to the most distant galaxies visible from Earth - and then you will continue on, carried only by experience and imagination as you look for the structure of the universe itself.


• Imagination is your key to discovery – It will be your scientific time machine that

transports you into the past and into the future.

• Go back to watch the birth of the universe, the formation of the first stars, and the origin of the sun and Earth.– Then, rush into the future to see what will

happen when the sun dies and Earth withers.


• Although you will discover a beginning to the universe, you will not find an edge in space. – No matter how far you voyage, you will not run into a

wall or limit beyond which you cannot go. – Rather you will discover evidence that our universe

may be infinite, that it may extend in all directions without limit.

• Such vastness may dwarf our earthly dimensions but not our human curiosity and imagination.


• Astronomy is more than the study of stars and planets. – It is the study of the universe in which we humans

exist.

• You and I live on a small planet circling a small sun drifting through the universe. However, astronomy can take you beyond these boundaries and help you not only see where you are in the universe, but understand what you are. – You have a right to know these things. – Perhaps, you have a duty to know them.


• Do not be humble.– Although astronomical sizes and distance

may dwarf you, remember that you are an intelligent creature, and you are capable of understanding your universe.

– It is, after all, yours.


• Astronomy will introduce you to sizes, distances, and times far beyond your usual experience on Earth.– Your task in this chapter is to grasp the meaning of

these unfamiliar sizes, distances, and times. – Believe it or not, the solution lies in a single word;

scale.– In this chapter, you will compare objects of different

sizes to grasp the scale of the universe.


• Let’s begin with something familiar.– The figure below shows a region about 52 feet

across occupied by a human being, a sidewalk, and a few trees—all objects whose size you can understand.


• Each successive picture in this chapter will show you a region of the universe that is 100 times wider than the preceding picture.– That is, each step will widen your field of view

by a factor of 100.


• In the figure, your field of view widens by a factor of 100, and you can see an area 1 mile in diameter. – The arrow points to the scene shown in the preceding

photograph. – People, trees, and sidewalks

have vanished, but now you can see a college campus and the surrounding streets and houses.


– The dimensions of houses and streets are familiar.

– This is the world you know, and you can relate such objects to the scale of your body.


• You started your adventure using feet and miles, but you should use the metric system of units. – Not only is it used by all scientists around the world,

but it makes calculations much easier.– The photo in the previous figure is 1 mile in diameter.– A mile equals 1.609 kilometers, so you can see in the

photo that a kilometer is a bit over two-thirds of a mile—a short walk across a neighborhood.


• The view in this figure spans 160 kilometers. – In this infrared photo, the green foliage shows up as

various shades of red. – The college campus is now invisible, and the patches

of gray are small cities, with the suburbs of Philadelphia visible at the lower right.


• At this scale, you see the natural features of Earth’s surface.– The Allegheny Mountains of southern

Pennsylvania cross the image in the upper left, and the Susquehanna River flows southeast into Chesapeake Bay.

– What look like white bumps are a few puffs of clouds.


• These features are a reminder that you live on the surface of a changing planet. – Forces in Earth’s crust pushed the mountain ranges

up into parallel folds, like a rug wrinkled on a polished floor.

– The clouds tell you that Earth’s atmosphere is rich in water, which falls as rain and erodes the mountains, washing material down the rivers and into the sea.

– Mountains and valleys are only temporary features on Earth; they are constantly changing.


• As you explore the universe, you will come to see that it—like Earth’s surface—is always evolving.


• Take another look at the red color of the figure. – This is an infrared photograph in which healthy green

leaves and crops show up as red. – Human eyes are sensitive

to only a narrow range of colors.

– As you explore the universe, you will learn to use a wide range of “colors,” from X rays to radio waves, to reveal sights invisible to unaided human eyes.


• At the next step in your journey, you will see our entire planet, which is 12,756 km in diameter. – The photo shows most

of the daylight side of the planet.

– The blurriness at the extreme right is thesunset line.


• Earth rotates on its axis once a day, exposing half of its surface to daylight at any particular moment. – The rotation of Earth

carries you eastwardand, as you cross thesunset line into darkness,you say the sun has set.

– It is the rotation of theplanet that causes thecycle of day and night.


• Earth’s interior is made mostly of iron and nickel, and its crust is mostly silicate rocks. – Only a thin layer of water makes up the oceans,

and the atmosphere is only a few hundred kilometers deep.

– On the scale of the photograph you just saw, the depth of the atmosphere on which life depends is less than the thickness of a piece of thread.


• Enlarge your field of view by a factor of 100, and you will see a region 1,600,000 km wide. – Earth is the small blue dot in the center and the moon,

whose diameter is only one-fourth that of Earth, is an even smaller dot along its orbit 380,000 km from Earth.

– These numbers are so large that it is inconvenient to write them out.


• Astronomy is the science of big numbers, and you will use numbers much larger than these to discuss the universe.

• Rather than writing out these numbers as in the previous slide, it is convenient to write them in scientific notation.

– This is nothing more than a simple way to write numbers without writing lots of zeros.

– In scientific notation, you would write 380,000 as 3.8 x 105.

– The universe is too big to discuss without using scientific notation.


• When you once again enlarge your field of view by a factor of 100, Earth, the moon, and the moon’s orbit all lie in the small red box at lower left. – Now, however, you can

see the sun and two other planets that are part of our solar system.

– Our solar system consistsof the sun, its family ofplanets, and some smallerbodies such as moons and comets.


• Like Earth, Venus and Mercury are planets, small, non-luminous bodies that shine by reflected light. – Venus is about the size of

Earth, and Mercury isa bit larger thanEarth’s moon.

– On this diagram, they are both too small to be seen as anythingbut tiny dots.


• The sun is a star, a self-luminous ball of hot gas, that generates its own energy. – The sun is 109 times

larger in diameter than Earth, but it too is nothing more than a dot in the diagram.


• This diagram has a diameter of 1.6 x 108 km.

• One way astronomers deal with large numbers is to define new units.– The average distance from Earth to the sun is a unit of

distance called the astronomical unit (AU), a distance of 1.5 x 1011 m.


• Using this unit, you can say that the average distance from Venus to the sun is about 0.7 AU. – The average distance from Mercury to the sun

is about 0.39 AU.


• The orbits of the planets are not perfect circles, and this is particularly apparent for Mercury. – Its orbit carries it as close to the sun as 0.307 AU and

as far away as 0.467 AU.– You can see this variation

in the distance from Mercury to the sun.

– Earth’s orbit is more circular, and its distance from the sun varies by only a few percent.


• Your first field of view was only 52 feet (about 16 m) in width.

• After only six steps of enlarging by a factor of 100, you can now see the entire solar system. – Your field of view is 1 trillion

(1012) times wider than in your first view.


– The details of the earlier figure are now lost in the red square at the center of this diagram.

– You see only the brighter, more widely separated objects as you enlarge your view.


• The sun, Mercury, Venus, and Earth lie so close together that you cannot separate them at this scale.

• Mars, the next outward planet, lies only 1.5 AU from the sun.

• In contrast, Jupiter, Saturn, Uranus, Neptune, and Pluto are so far from the sun that they are easy to place in the diagram.– These are cold worlds far from the sun’s warmth.


• Light from the sun reaches Earth in only 8 minutes, but it takes over 4 hours to reach Neptune.

• Notice that Pluto’s orbit is so elliptical that Pluto can come closer to the sun than Neptune does, as it did between 1979 and 1999.


• When you again enlarge your field of view by a factor of 100, the solar system vanishes. – The sun is only a point of light, and all the planets

and their orbits are now crowded into the small red square at the center.

– The planets are too small, and reflect too little light,to be visible so near thebrilliance of the sun.


• Nor are any stars visible except for the sun– The sun is a fairly typical star, and it seems to be

located in a fairly average neighborhood in the universe.

– Although there are many billions of stars like the sun, none is close enough to be visible in the diagram, which shows an area only 11,000 AU in diameter.


• The stars are typically separated by distances about 10 times larger than the diameter of the diagram. – Except for the sun at

the center, this diagram is empty.

– However, you will see stars in the next field of view.


• It is difficult to grasp the isolation of the stars. – If the sun were represented by a golf ball in

New York City, the nearest star would be another golf ball in Chicago.

– Except for the widely scattered stars and a few atoms of gas drifting between the stars, the universe is nearly empty.


• In the diagram, your field of view has expanded to a diameter a bit over1 million AU. – The sun is at the center,

and you can see a few of the nearest stars.

– These stars are so distant that it is not reasonable to give their distances in astronomical units.


• To express distances so large, astronomers define a new unit of distance, the light-year. – One light-year (ly) is the distance that light travels in

one year, roughly 1013 km or 63,000 AU.

– The diameter of your field of view is 17 ly. – The nearest star to the sun, Alpha Centauri, is 4.2 ly

from Earth. – In other words, light from Alpha Centauri takes 4.2

years to reach Earth.


• Here’s more about Alpha Centauri.– It is in the southern sky, so it is invisible from all

but the southernmost parts of the United States where it occasionally peeks above the southern horizon.

– If you ever have the chance, you should locate the sun’s nearest companion in space.

– Later, you will discover that Alpha Centauri is actually three stars orbiting around each other.


• Although stars are roughly the same size as the sun, they are so far away that you cannot see them as anything but points of light, even with the largest telescopes on Earth.– Using indirect methods,

astronomers have found nearly 200 planets orbiting other stars.


• If you expand your field of view by a factor of 100, you see our galaxy. – A galaxy is a great cloud of stars, gas, and dust bound

together by the combined gravity of all the matter. Galaxies range from 1500 to over 300,000 ly in diameter and can contain over 100 billion stars.

– In the night sky, you see our galaxy as a great, cloudy wheel of stars ringing the sky as the Milky Way. Our galaxy is called the Milky Way Galaxy.


• In the figure, the sizes of the dots represent not the sizes of the stars, but their brightness. – This is the custom in astronomical diagrams, and it is

also how star images are recorded on photographs.

– Bright stars make larger spots on a photo than faint stars.

– The size of a star image in a photo informs you not how big the star is, but only how bright it looks.


• In the diagram, you expand your field of view by another factor of 100, and the sun and its neighboring stars vanish into the background of thousands of other stars. – The field of view is

now 1,700 ly in diameter.


– Of course, no one has ever journeyed thousands of light-years to photograph the solar neighborhood, so this is a representative photo of the sky.

– The sun is a relatively faint star that would not be easily located in a photo at this scale.


• What you do not see in the photograph is critically important.– You do not see the thin gas that fills the spaces

between the stars.– Although those clouds of gas are thinner than the best

vacuum on Earth, it is those clouds that give birth to new stars.

– Our sun formed from such a cloud about 5 billion years ago.

– You will see evidence of star formation in your next field of view.


• Of course, no one can journey far enough into space to look back and photograph our home galaxy, so the photo shows a galaxy similar to our own. – Our sun would be invisible

in such a photo.– However, if you could see

it, you would find it in the disk of the galaxy about two-thirds of the way out from the center.


• Our galaxy, like many others, has graceful spiral arms winding outward through the disk.– You will discover that stars are born in great

clouds of gas and dust as they pass through the spiral arms.


• Ours is a fairly large galaxy, roughly 75,000 ly in diameter. – Only a century ago astronomers thought it was

the entire universe—an island universe of stars in an otherwise empty vastness.

– Now, they know that our galaxy is not unique. Indeed ours is only one of many billions of galaxies scattered throughout the universe.


• As you expand your field of view by another factor of 100, our galaxy appears as a tiny luminous speck surrounded by other specks. – The diagram includes a

region 17 million ly in diameter, and each of the dots represents a galaxy.

– Notice that our galaxy is part of a cluster of a few dozen galaxies.


• Galaxies are commonly grouped together in such clusters. – Some of these galaxies have beautiful spiral patterns

like our own galaxy, but others do not. – Some are strangely distorted. – One of the mysteries of modern astronomy is what

produces these differences among the galaxies. – Astronomers have found some important clues and

have developed a fascinating theory.


• If you again expand your field of view, you see that the clusters of galaxies are connected in a vast network. – Clusters are grouped into

superclusters—clusters of clusters—and the superclusters are linked to form long filaments and walls. They outline voids

that seem nearly empty of galaxies.

– These appear to be the largest structures in the universe.


• Were you to expand your field of view another time, you would probably see a uniform fog of filaments and voids. – When you puzzle over the origin of these

structures, you are at the frontier of human knowledge.


• The first hurdle in studying astronomy is keeping a proper sense of scale.

• Remember that each of the billions of galaxies contains billions of stars.


• Later, you will see clear evidence that many of those stars have families of planets like our solar system. On some of those planets, liquid-water oceans and protective atmospheres may have sheltered the spark of life. – It is possible that at least a few other planets are

inhabited by intelligent creatures who share our curiosity, and our wonder, at the scale of the cosmos.

the scale of the cosmos astronomy: the solar system and beyond 5th edition michael seeds

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