ch1 our place in the universe

7/31/2019 CH1 Our Place in the Universe

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Our Place in the Universe

1.1 Our Modern View of the Universe_ What is our place in the universe?_ How did we come to be?_ How can we know what the universewas like in thepast?_ Can we see the entire universe?

1.2 The Scale of the Universe_ How big is Earth compared to oursolar system?_ How far away are the stars?_ How big is the Milky Way Galaxy?_ How big is the universe?

_ How do our lifetimes compare to theage of theuniverse?

1.3 Spaceship Earth_ How is Earth moving in our solarsystem?_ How is our solar system moving inthe Milky WayGalaxy?_ How do galaxies move within theuniverse?_ Are we ever sitting still?

Learning Goals

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1.1 OUR MODERN VIEW OF THEUNIVERSE

What is our place in the universe?

How did we come to be? How can we know what the universewas like in the past?

Can we see the entire universe?

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Earth is a planet orbiting the Sun.

Our Sun is one of more than 100billion stars in the Milky Way

Galaxy. Our galaxy is one ofabout 40 galaxies in the LocalGroup. The Local Group is onesmall part of the Local

Supercluster, which is one small

part of the universe.

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Billions of other galaxies arescattered throughout space. Somegalaxies are fairly isolated, but manyothers are found in groups.

Our Milky Way, for example, is oneof the two largest among about 40galaxies in the Local Group. Groupsof galaxies with more than a fewdozen members are often calledgalaxy clusters.

On a very large scale, observationsshow that galaxies and galaxyclusters appear to be arranged ingiant chains and sheets with hugevoids between them.

The regions in which galaxies andgalaxy clusters are most tightlypacked are called superclusters,which are essentially clusters ofgalaxy clusters.

Our Local Group is located in theoutskirts of the Local Supercluster.

Together, all these structures makeup our universe. In other words, theuniverse is the sum total of all matterand energy, encompassing thesuperclusters and voids andeverything within them.

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The universe began in the Big Bangand has been

expanding ever since,except in localized regions wheregravityhas caused matter to collapse intogalaxies andstars. The Big Bang essentiallyproduced only twochemicalelements: hydrogen and helium. Therest havebeen produced by stars,which is why we are star stuff.

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Telescopic observations of distantgalaxies show that the entireuniverse is expanding, meaningthat the average distancesbetween galaxies are increasing

with time. This fact implies thatgalaxies must have been closertogether in the past, and if we goback far enough, we must reachthe point at which the expansionbegan. We call this beginning the

Big Bang, and from the observedrate of expansion we estimatethat it occurred about 14 billionyears ago.

The universe as a whole hascontinued to expand ever since theBig Bang, but on smaller scales theforce of gravity has drawn mattertogether. Structures such asgalaxies and galaxy clustersoccupy regions where gravity haswon out against the overallexpansion. That is, while theuniverse as a whole continues toexpand, individual galaxies andgalaxy clusters do notexpand.

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Within galaxies like the Milky Way,gravity drives the collapse of cloudsof gas and dust to form stars andplanets. Stars are not livingorganisms, but they nonetheless gothrough life cycles. A star is bornwhen gravity compresses the materialin a cloud to the point where thecenter becomes dense and hotenough to generate energy bynuclear fusion, the process in whichlightweight atomic nuclei smashtogether and stick (or fuse) to makeheavier nuclei.

The star lives as long as it cangenerate energy from fusion anddies when it exhausts its usablefuel.

In its final death throes, a star blowsmuch of its content back out intospace. In particular, massive stars diein titanic explosions calledsupernovae. The returned mattermixes with other matter floatingbetween the stars in the galaxy,eventually becoming part of newclouds of gas and dust from whichfuture generations of stars can beborn. Galaxies therefore function ascosmic recycling plants, recyclingmaterial expelled from dying stars intonew generations of stars and planets.

Our own solar system is a product ofmany generations of such recycling.

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Basic Astronomical Objects

Star - A large, glowing ball of gas that generates heat and light throughnuclear fusion in its core. Our Sun is a star.

Planet - A moderately large object that orbits a star and shines primarilyby reflecting light from its star. According to a definition approved in 2006,an object can be considered a planet only if it (1) orbits a star; (2) is large

enough for its own gravity to make it round; and (3) has cleared most otherobjects from its orbital path. An object that meets the first two criteria buthas notcleared its orbital path, like Pluto, is designated a dwarf planet.

Moon (or satellite) - An object that orbits a planet. The term satellitecan

refer to any object orbiting another object.

Asteroid - A relatively small and rocky object that orbits a star.

Comet -A relatively small and ice-rich object that orbits a star.

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Collections of Astronomical Objects

Solar system - The Sun and all the material that orbits it, including theplanets, dwarf planets, and small solar system bodies. Although the termsolar systemtechnically refers only to our own star system (solarmeans ofthe Sun), it is often applied to other star systems as well.

Star system - A star (sometimes more than one star) and any planets andother materials that orbit it.

Galaxy - A great island of stars in space, containing from a few hundredmillion to a trillion or more stars, all held together by gravity and orbiting acommon center.

Cluster (or group) of galaxies - A collection of galaxies bound togetherby gravity. Small collections (up to a few dozen galaxies) are generallycalled groups, while larger collections are called clusters.

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Collections of Astronomical Objects

Supercluster - A gigantic region of space where many individual galaxiesand many groups and clusters of galaxies are packed more closely togetherthan elsewhere in the universe.

Universe (or cosmos) - The sum total of all matter and energythat is,

all galaxies and everything between them.

Observable universe - The portion of the entire universe that can beseen from Earth, at least in principle. The observable universe is probablyonly a tiny portion of the entire universe.

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Terms Relating to Motion

Rotation - The spinning of an object around its axis. For example, Earthrotates once each day around its axis, which is an imaginary lineconnecting the North Pole to the South Pole.

Orbit (revolution) - The orbital motion of one object around another. For

example, Earth orbits around the Sun once each year.

Expansion (of the universe) - The increase in the average distancebetween galaxies as time progresses. Note that while the universe as awhole is expanding, individual galaxies and galaxy clusters do notexpand.

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How Far is a Light-Year?

One light-year (ly) is defined as thedistance that light can travel in 1 year.This distance is fixed because lightalways travels at the same speedthespeed of light, which is300,000 km/s(186,000 mi/s).

We can calculate the distancerepresented by a lightyear by recallingthat

Distance = speed x time

For example, if you travel at a speed of50 km/hr for 2 hours, you will travel 100km. To find the distance represented by 1

light-year, we simply multiply the speed oflight by 1 year:

1 light-year = (speed of light) x (1 yr)

That is, 1 light-year is equivalent to 9.46trillion km, which is easier to rememberas almost 10 trillion km.

By studying stars of different ages, we have learned that the

early universe contained only the simplest chemical elements:hydrogen and helium (and a trace of lithium). We and Earth aremade primarily of other elements, such as carbon, nitrogen,oxygen, and iron. Where did these other elements comefrom?Evidence shows that these elements were manufacturedby starssome through the nuclear fusion that makes starsshine, and others through nuclear reactions accompanying the

explosions that end stellar lives.

By the time our solar system formed, about 4 billion years ago,earlier generations of stars had converted about 2% of ourgalaxys original hydrogen and helium into heavier elements.Therefore, the cloud that gave birth to our solar system wasmade of about 98% hydrogen and helium and 2% otherelements. That 2% may seem a small amount, but it was more

than enough to make the small rocky planets of our solarsystem, including Earth. On Earth, some of these elementsbecame the raw ingredients of simple life forms, whichultimately blossomed into the great diversity of life on Earthtoday.

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Light takes time to travel

through space, so thefarther away we look indistance, the further back

we look in time. When welook billions of light-yearsaway, we see pieces of theuniverse as they were

billions of years ago.

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Because light takes time to travel through space, we are led to aremarkable fact: The farther away we look in distance, the furtherback we look in time. For example, the brightest star in the nightsky, Sirius, is about 8 light-years away, which means its light takesabout 8 years to reach us. When we look at Sirius, we are seeing it not

as it is today but as it was about 8 years ago.

The Andromeda Galaxy (also known as

M31) lies about 2.5 million light-years

from Earth. Figure 1.3 is therefore apicture of how this galaxy looked about

2.5 million years ago, when early

humans were first walking on Earth.

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No. The age of the universe limits the extent of ourobservable universe. Because the universe is about 14billion years old, our observable universe extends to adistance of about 14 billion light-years. If we tried to look

beyond that distance, wed be trying to look to a timebefore the universe existed.

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1.2 THE SCALE OF THE UNIVERSE

How big is Earth compared to oursolar system?

How far away are the stars?

How big is the Milky Way Galaxy? How big is the universe?

How do our lifetimes compare to theage of the universe?

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On a scale of 1 to 10 billion,the Sun is about the size of agrapefruit. Planets are much

smaller, with Earth the size ofa ball point and Jupiter the sizeof a marble on this scale. Thedistances between planets arehuge compared to their sizes,

with Earth orbiting 15 metersfrom the Sun on this scale.

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Seeing our solar system to scalealso helps put space exploration intoperspective. The Moon, the only otherworld on which humans have ever

stepped, lies only about 4 centimeters(1 inches) from Earth in the Voyagemodel.

The trip to Mars is some 200 times

as far as the trip to the Moon, evenwhen Mars is on the same side of itsorbit as Earth.

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On the 1-to-10-billion scale, it ispossible to walk from the SuntoPluto in just a few minutes. Onthe same scale, the nearest stars

besides the Sun are thousands ofkilometers away.

The nearest star system to ourown, a three-star system called

Alpha Centauri, is about 4.4 light-years away.

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Using a scale on whichthe Milky Way galaxy is thesize of a football field, thedistance to the nearest star

would be only about 4millimeters. There are somany stars in our galaxythat it would take

thousands of years just tocount them.

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The observable universecontains roughly 100billion galaxies, and thetotal number of stars is

comparable to thenumber of grains of drysand on all the beacheson Earth.

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On a cosmic calendarthat compresses thehistory of the universe

into 1 year, humancivilization is just a fewseconds old, and ahuman lifetime lastsonly a fraction of a

second.

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The cosmic calendar compresses the 14 billion-year history of the universe into 1 year, so that eachmonth represents a little more than 1 billion years. This cosmic calendar is adapted from a version

created by Carl Sagan.


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1.3 SPACESHIP EARTH

How is Earth moving in our solarsystem?

How is our solar system moving inthe Milky Way Galaxy?

How do galaxies move within theuniverse?

Are we ever sitting still?

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Earth rotateson its axis once each day and orbitstheSun once each year. Earth orbits at an average distancefrom the Sun of 1 AUand with an axis tilt of 23 to a lineperpendicular to the ecliptic plane.

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As Earth rotates, yourspeed around Earths axis

depends on your location:The closer you are to the

equator, the faster youtravel with rotation.

Notice that Earth rotates

from west to east, which iswhy the Sun appears torise in the east and set inthe west.

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Earth takes a year to complete an orbit of the Sun, but its orbital speed is stillsurprisingly fast. Notice that Earth both rotates and orbits counterclockwise as viewed

from above the North Pole.


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We move randomly relative to other stars in our localsolar neighborhood. The speeds are substantial byearthly standards, but stars are so far away that theirmotion is undetectable to the naked eye. Our Sun and

other stars in our neighborhood orbit the center of thegalaxy every 230 million years, because the entiregalaxy is rotating.

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The small box shows that stars within thelocal solar neighborhood (like the stars of anyother small region of the galaxy) moveessentially at random relative to one another.They also generally move quite fast.


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Our solar system, located about27,000 light-years from thegalactic center, completes oneorbit of the galaxy in about 230million years. Even if you couldwatch from outside our galaxy,this motion would beunnoticeable to your naked eye.However, if you calculate thespeed of our solar system as weorbit the center of the galaxy,you will find that it is close to800,000 kilometers per hour(500,000 miles per hour).

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Stars at different distances from the galactic center orbit at differentspeeds, and we can learn how mass is distributed in the galaxy bymeasuring these speeds. Such studies indicate that the stars in thedisk of the galaxy represent only the tip of the iceberg compared tothe mass of the entire galaxy.

Most of the mass of the galaxy seems to be located outside thevisible disk, in what we call the halo. We dont know the nature ofthis mass, but we call it dark matterbecause we have not detectedany light coming from it.

Studies of other galaxies suggest that they also are made mostly ofdark matter, which means this mysterious matter must significantlyoutweigh the ordinary matter that makes up planets and stars. Aneven more mysterious dark energyseems to make up much of thetotal energy content of the universe.


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This painting shows an edge-on view of the Milky Way Galaxy. Study of galactic rotation shows thatalthough most visible stars lie in the disk and central bulge, most of the mass lies in the halo thatsurrounds and encompasses the disk. Because this mass emits no light that we have detected, wecall it dark matter.


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Galaxies move essentially at random within the LocalGroup, but all galaxies beyond the Local Group aremoving away from us. More distant galaxies are moving

faster, which tells us that we live in an expandinguniverse.

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Two small galaxies (known as the Large and Small MagellanicClouds) apparently orbit our Milky Way Galaxy.

For example, the Milky Way is moving toward the AndromedaGalaxy at about 300,000 kilometers per hour (180,000 milesper hour). Despite this high speed, we neednt worry about a

collision anytime soon. Even if the Milky Way and AndromedaGalaxies are approaching each other head-on, it will bebillions of years before any collision begins.

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When we look outside the Local Group, however, we findtwo astonishing facts recognized in the 1920s by EdwinHubble, for whom the Hubble Space Telescope was

named:1) Virtually every galaxy outside the Local Group is movingawayfrom us.

2) The more distant the galaxy, the faster it appears to be

racing away.

Natural explanation: The entire universe is expanding.

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We are never truly sitting still. We spin around Earths

axis and orbit the Sun. Our solar system moves amongthe stars of the local solar neighborhood while orbiting

the center of the Milky Way Galaxy. Our galaxy movesamong the other galaxies of the Local Group, while allother galaxies move away from us in our expandinguniverse.


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1The Big Picture

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Earth is not the center of the universe but instead is a planet orbiting a ratherordinary star in the Milky Way Galaxy. The Milky Way Galaxy, in turn, is one ofbillions of galaxies in our observable universe.

We are star stuff. The atoms from which we are made began as hydrogen andhelium in the Big Bang and were later fused into heavier elements by massive

stars. Stellar deaths released these atoms into space, where our galaxy recycledthem into new stars and planets. Our solar system formed from such recycledmatter some 41/2 billion years ago.

Cosmic distances are literally astronomical, but we can put them in perspectivewith the aid of scale models and other scaling techniques. When you think about

these enormous scales, dont forget that every star is a sun and every planet is a

unique world.

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1The Big Picture We are latecomers on the scale of cosmic time. The universe was already morethan half its current age when our solar system formed, and it took billions ofyears more before humans arrived on the scene.

All of us are being carried through the cosmos on spaceship Earth. Although wecannot feel this motion, the associated speeds are surprisingly high. Learning

about the motions of spaceship Earth gives us a new perspective on the cosmosand helps us understand its nature and history.

ch1 our place in the universe

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