Ch. 1.pdf

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Chapter 1

Our Place in the Universe

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1.1 A Modern View of the

Universe

What is our place in the universe?

How did we come to be?

How can we know what the universe was like inthe past?

Can we see the entire universe?

Our goals for learning:

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What is our place in the universe?

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Star

A large, glowing ball of gas that generatesheat and light through nuclear fusion

The Sun is

our star.

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Planet

A moderately large object that orbits a star;

it shines by reflected light. Planets may be

rocky, icy, or gaseous in composition.

Mars Neptune

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Planet Definition

Orbits a star.

Large enough for

its own gravity tomake it round.

Has cleared most

other objects fromits orbital path.

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Dwarf Planet

Orbits star and is

round in shape, but

has not cleared itsorbital

neighborhood.

Examples:

Ceres in asteroid belt

Pluto in Kuiper belt

Eris Kuiper belt

Pluto and Charon

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Moon (or Satellite)

An object that

orbits a planet.

Ganymede (orbits Jupiter)

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Ida

Asteroid

A relatively small

and rocky objectthat orbits a star.

Small solar system

object.

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Comet

A relatively

small and icyobject that orbits

a star.

Small solarsystem object.

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Solar (Star) System

A star andall the

material

that orbits

it, including

its planets

and moons.

Sometimesmore than

one star.

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Nebula

An interstellar

cloud of gasand/or dust.

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Galaxy

A great island of stars in space, all

held together by gravity and

orbiting a common center.

M31, The Great Galaxy

in Andromeda

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Cluster (or Group) of Galaxies

A collection of galaxies held together by

gravity. Small collections: group

Large collections: cluster

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Supercluster

A gigantic region of space where many

individual galaxies, and groups and

clusters of galaxies are packed closely

together.

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Universe

The sum total of all matter and

energy; that is, everything within

and between all galaxies.

Observable universe: That portion

of the entire universe that can beseen from the Earth, at least in

principle.

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How did we come to be?

H k h t th

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How can we know what the

universe was like in the past? Light travels at a finite speed (300,000 km/s).

Thus, we see objects as they were in the past:

The farther away we loo k in d istance,

the fur ther back we look in t ime.

Destination Light travel time

Moon 1 second

Sun 8 minutesSirius 8 years

Andromeda

Galaxy

2.5 million years

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Example:

We see the

Orion Nebulaas it looked

1,500 years

ago.

M31, The Great Galaxy

in Andromeda

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Example:

This photo shows the Andromeda Galaxy as it looked

about 2.5 million years ago.Question: When will be able to see what it looks like

now?

M31, The Great Galaxy

in Andromeda

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Light-year

The distance light can travel in one

year.

About 10 trillion km (6 trillion miles). About 10x1012 km (1x1013 km or 6x1012

miles).

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At great distances, we see objects as they

were when the universe was much

younger.

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How far is a light-year?

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

= 300,000km

s

365 days

1 yr

24 hr

1 day

60 min

1 hr

60 s

1 min

= 9,460,000,000,000 km

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Can we see the entire universe?

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What have we learned?

What is our physical place in theuniverse?

On Earth

Which is part of the Solar System,

Which is in the Milky Way galaxy,

Which is a member of the Local Group ofgalaxies

Which is located in the LocalSupercluster.

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What have we learned?

How did we come to be? The matter in our bodies came from the

Big Bang, which produced hydrogen andhelium.

All other elements were constructed fromH and He in star and then recycled intonew star systems, including our solarsystem.

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What have we learned?

How can we know what the universewas like in the past?

When we look to great distances, we areseeing events that happened long ago

because light travels at a finite speed. Can we see the entire universe?

No, the observable portion of the universeis about 14 billion light-years in radiusbecause the universe is about 14 billionyears old.

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1.2 The Scale of the Universe

How big is the Earth compared to our solar

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 the age

of the universe?

Our goals for learning:

H bi i E th d t l

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How big is Earth compared to our solar

system?

Lets reduce the size of the solar system by afactor of 10 billion; the Sun is now the size of a

large grapefruit (14 cm diameter).

How big is Earth on this scale?

A. an atom

B. a ball pointC. a marble

D. a golf ball

H bi i E th d t l

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How big is Earth compared to our solar

system?

Lets reduce the size of the solar system by afactor of 10 billion; the Sun is now the size of a

large grapefruit (14 cm diameter).

How big is Earth on this scale?

A. an atom

B. a ball pointC. a marble

D. a golf ball

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The Scale of the Solar System

On a 1-to-10

billion scale: Sun is the

size of alargegrapefruit

(14 cm) Earth is the

size of a ballpoint, 15

metersaway.

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How far away are the stars?

On our 1-to-10 billion scale, its just a

few minutes walk to Pluto.

How far would you have to walk toreach Alpha Centauri?A. 1 mile

B. 10 milesC. 100 miles

D. the distance across the U.S. (2500 miles)

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Answer: D, the distance across the U.S.

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How big is

the Milky Way Galaxy?

The Milky Way Galaxy has a diameter of about100,000 light-years (1x1018 km).

Compare to

Diameter of the Earth: 1.3x104 km

Earth-Sun distance: 1.5x108 km

Distance to Alpha Centauri: 4x1013 km Distance to the Andromeda Galaxy:

2.5x1019 km

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Thought Question

The Milky Way Galaxy has about 100 billionstars.

Suppose you tried to count the stars in our

galaxy, at a rate of one per second, how longwould it take you?

A. a few weeks

B. a few monthsC. a few years

D. a few thousand years

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How big is the Universe? The Milky Way is one of about 100 billion

galaxies. 1011 stars/galaxy x 1011 galaxies = 1022 stars

As many stars as grains of (dry) sand on allEarths beaches

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Now lets step through the universe in powers of

10:

H d lif ti t

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How do our lifetimes compare to

the age of the Universe?

The Cosmic Calendar: a scale on which wecompress the history of the universe into 1 year.

How do our lifetimes compare to

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How do our lifetimes compare to

the age of the Universe?

The Cosmic Calendar: a scale on which wecompress the history of the universe into 1 year.

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What have we learned?

How big is Earth compared to our solarsystem? The distances between planets are huge

compared to their sizeson a scale of 1-to-10billion, Earth is the size of a ball point and the

Sun is 15 meters away.

How far away are the stars? On the same scale, the stars are thousands of

km away.

How big is the Milky Way galaxy? It would take more than 3,000 years to count the

stars in the Milky Way Galaxy at a rate of oneper second, and they are spread across 100,000

light-years.

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What have we learned?

How big is the universe? The observable universe is 14 billion

light-years in radius and contains over100 billion galaxies with a total number of

stars comparable to the number of grainsof sand on all of Earths beaches.

How do our lifetimes compare to the age ofthe universe?

On a cosmic calendar that compressesthe history of the Universe into one year,human civilization is just a few secondsold, and a human lifetime is a fraction of

a second.

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1.3 Spaceship Earth

How is Earth moving in our solar system?

How is our solar system moving in the

Galaxy?

How do galaxies move within the Universe?

Are we ever sitting still?

Our goals for learning:

How is Earth moving in

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How is Earth moving in

our solar system?

We are moving with the Earth in several ways,

and at surprisingly fast speeds

The Earth rotates

around its axis once

every day.

E th bit th S ( l )

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Earth orbits the Sun (revolves) once every

year: at an average distance of 1 AU 150 million km.

with Earths axis tilted by 23.5 (pointing to Polaris) and rotating in the same direction it orbits, counter-

clockwise as viewed from above the North Pole.

Our Sun moves randomly relative to the other

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Our Sun moves randomly relative to the other

stars in the local Solar neighborhood

typical relative speeds of more than 70,000 km/hr

but stars are so far away that we cannot easily noticetheir motion

And orbits the galaxy every 230 million years.

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More detailed study of the Milky Ways rotation

reveals one of the greatest mysteries in

astronomy:

Most of Milky Ways

light comes from

disk and bulge

. but most of the

mass is in its halo

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How do galaxies move within the universe?

Galaxies are carried

along with the expansionof the Universe.

But how did Hubble

figure out that the

universe is expanding?

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Hubble discovered that:

All galaxies outside our Local Group are

moving away from us.

The more distant the galaxy, the faster itis racing away.

Conclusion: We live inan expanding universe.

A i i ill?

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Are we ever sitting still?

Earth rotates on axis: > 1,000 km/hr

Earth orbits Sun: > 100,000 km/hr

Solar system moves among stars: ~ 70,000 km/hr

Milky Way rotates: ~ 800,000 km/hr

Milky Way moves

in Local Group

Universe

expands

Wh t h l d?

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What have we learned?

How is Earth moving in our solar system? It rotates on its axis once a day and orbitthe Sun at a distance of 1 A.U. = 150million km. (A.U. = astronomical unit)

How is our solar system moving in the MilkyWay galaxy?

Stars in the Local Neighborhood moverandomly relative to one another and orbit

the center of the Milky Way in about 230million years.

Wh t h l d?

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What have we learned?

How do galaxies move within the universe?All galaxies beyond the Local Group aremoving away from us with expansion ofthe Universe: the more distant they are,

the faster theyre moving Are we ever sitting still?

No! The Earth is constantly in motion,even though we do not notice it.

1 4 The Human Adventure of

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1.4 The Human Adventure of

Astronomy

How has the study of astronomy affected

human history?

Our goals for learning:

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How has the study of astronomy

affected human history?

Copernican Revolution showed that Earth

was not the center of the universe (Chapter

3).

Study of planetary motion led to NewtonsLaws of motion and gravity (Chapter 4).

Newtons laws laid the foundation of the

industrial revolution.

Modern discoveries are continuing to expand

our cosmic perspective.

Wh t h l d?

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What have we learned?

How has the study of astronomy affectedhuman history?

Throughout history, astronomy hasprovided an expanded perspective on

Earth that has grown hand in hand withsocial and technological developments.