Our Place in the Universe

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Chapter 1: Our Place in the Universe


In the first Chapter we will talk about:

1.1 A Modern view of the Universe

1.2 The Scale of the Universe

1.3 Spaceship Earth

In this lecture we introduce the concepts most of which we will study in more depth later this semester…


1.1 A Modern View of the Universe

Our goals for learning:1) What is our physical place in the Universe?2) How did we come to be? 3) How can we know what the Universe was like in

the past? 4) Can we see the entire universe?


1) What is our physical place in the universe?• The Earth is not in the center of Universe…


The Earth: our planet.

Solar system: Sun, eight planets and their moons, asteroids (chunks of rock), comets (balls of ice), interplanetary dust.

Milky Way: our galaxy. Galaxies have from 108 to 1012

stars! Milky way is disk shaped, and solar system is half way towards its edge.

Local Group: 40 galaxies in our local group, bound together by gravity. Groups of Galaxies are called Galaxy Clusters.

Local Supercluster: Universe contains sheets of galaxies and galaxy clusters. Somewhere they are tightly packed, forming Superclusters containing clusters and isolated galaxies, our local group is part of Local Supercluster.

Universe: all matter and energy, in superclusters and voids.


And now, in pictures…

StarA large, glowing ball of gas that generates heat

and light through nuclear fusion


Planet

A moderately large object which orbits a star; it shines by reflected light. Planets may be rocky,

icy, or gaseous in composition.

Mars Neptune

The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit. Pluto-dwarf planet!


Moon (or satellite)

An object that orbits a planet.

Ganymede (orbits Jupiter)


Asteroid

Ida

A relatively small and rocky object that orbits a star.


Comet

A relatively small and icy object that orbits a star.


Solar (Star) System

A star and all the material that orbits it, including its planets and moons

mercuryvenus mars

jupitersaturn

uranus

pluto

neptune


Nebula

An interstellar cloud of gas and/or dust

The Trifid Nebula


GalaxyA great island of stars in space, all held

together by gravity and orbiting a common center

M31, The Great Galaxyin Andromeda


Universe

The sum total of all matter and energy; that is, everything within and between

all galaxies


2) How did we come to be?Big bang and expansion

formation of galaxies

and stars…

Cosmic recycling

Earth, made from “star” material


Big Bang: the beginning. Our telescopic measurement show that universe is expanding → started from very hot, very dense state, 14 billion years ago!

Galaxy formation: on smaller scales gravity has drawn matter together, winning over the force of expansion. Galaxies and stars were formed! Happened few billion years after Big Bang.

Stars: Stars are formed from gas and dust; During their life they produce heavier elements starting from H and He in nuclear fusion; After they die, they explode and blow their content back into galaxy… material gets recycled!

Planets: from these heavier elements, planets (and life) were formed.


3) 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 look in distance,

the further back we look in time.

Destination Light travel time

Moon 1 secondSun 8 minutesSirius 8 yearsAndromeda Galaxy 2.5 million years


Useful definition: a light-year• The distance light can travel in one year.• About 10 trillion km (6 trillion miles).


Example:

This photo shows the Andromeda Galaxy as it looked about 2 1/2 million years ago (when early humans were first walking on Earth).

Question: When we will be able to see what it looks like now? How does the expansion of the Universe affect your answer?

M31, The Great Galaxyin Andromeda

100 000 light years


•At great distances, we see objects as they were

when the universe was much younger.

This is the farthest we can see, when Universe started to “exist”.

Question: Is the following statement sensible: the Universe is between 10 and 16 billion light-years old?


Question: Why can’t we see a galaxy 15 billion light-years away?

(Assume universe is 14 billion years old.)

A. Because no galaxies exist at such a great distance.B. Galaxies may exist at that distance, but their light would

be too faint for our telescopes to see.C. Because looking 15 billion light-years away means

looking to a time before the universe existed.

Question: Is the following statement true: Because nearly all galaxies are moving away from us, we must be located at the

center of the Universe?


What have we learned?• What is our place in

the Universe?• Earth orbits the Sun• There are 100 billion

other stars in the Milky Way

• There are about 40 other galaxies in the Local Group.

• The Local Group is part of the Local Supercluster.• The Local Supercluster is one small piece of the Universe.


What have we learned?

• How did we come to be?• Big Bang starts the expansion of the universe.• Early universe contained only the elements hydrogen

and helium.• All other elements were made in stars and recycled into

new generations of stars within galaxies.• We are “star stuff”


What have we learned?• How can we know what the universe was like in

the past?• Light takes time to travel through space (the speed

of light = c = 300,000 km/s). Thus, when we look farther away, we see light that has taken a longer time to reach us.

• Can we see the entire universe?• No - age limits the size of the observable universe.

For a 14 billion year old universe, our observable universe is 14 billion light-years in radius.


1.2 The Scale of the Universe

Our goal is to get an idea of:• How big is 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?


How big is Earth compared to our solar system?

Let’s reduce the size of the solar system by a factor 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 atomB. a ball pointC. a marbleD. a golf ball


Let’s reduce the size of the solar system by a factor 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 atomB. a ball pointC. a marbleD. a golf ball


The scale of the solar

system


How far away are the stars?

On our 1-to-10 billion scale, it’s just a few minutes walk to Pluto.

How far would you have to walk to reach Alpha Centauri?A. 1 mileB. 10 milesC. 100 milesD. the distance across the U.S. (2500 miles)


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

Question: Explain why it is so difficult to see planets around other stars. How do you think scientists have observed them?

It took Voyager 12 years to exit Solar system, it would take him 100 000 years to arrive to Alpha Centauri!

Closest star, Alpha Centauri is 4.4 light years away.


How big is the Milky Way Galaxy?

The Milky Way has about 100 000 light years in diameter.

On the scale where 1 ly =1mm, Milky way is the size of football field, and Sun is 4 mm far from Alpha Centauri.

Or, lets consider the number of stars…there is 100 billion stars in Milky way…


Suppose you tried to count the more than 100 billion stars in our galaxy, at a rate of one per second…

How long would it take you?

A. a few weeksB. a few monthsC. a few yearsD. a few thousand years


Suppose you tried to count the more than 100 billion stars in our galaxy, at a rate of one per second…

How long you think it would take you?

A. a few weeksB. a few monthsC. a few yearsD. a few thousand years


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

• It would take 100 years to count all the galaxies.

• 1011 stars/galaxy x 1011 galaxies = 1022 stars

As many stars in Universe as grains of (dry) sand on all Earth’s beaches…


• Now let’s step through the Universe in powers of 10:

zooming_26_orders_of_mag.htm


How do our lifetimes compare to the age of the Universe?

• The Cosmic Calendar: a scale on which we compress the history of the universe (14 billion years) into 1 year.

• Each months, a little more than a billion year.• Then…

Big Bang Formation of Milky Way

September: Formation of Solar system (4.6 billion years ago) and first life!

December 26th: First dinosaurs…

…and first humans, in the last 30 seconds!


January 1st: Big Bang.

February: Milky Way formed

Early September: Solar system and Earth formed.Late September: first primitive life.

Mid December: first recognizable life.December 26th: first dinosaurs.December 30th: dinosaurs extinct (65 million years ago).December 31st, 9pm: early hominids began to walk uprightDecember 31st, 11:59:49 : Egyptian pyramids..


What have we learned?• How big is the Earth compared to our solar system?

• On a scale of 1-to-10 billion, the Sun is about the size of a grapefruit. The Earth is the size of a ball point about 15 m away. The distance between planets are huge compared to their sizes.

• 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 one per second. The Milky Way Galaxy is about 100,000 light-years across.


What have we learned?• How big is the universe?

• 100 billion galaxies in the observable Universe.• 14 billion light-years in radius.• As many stars as grains of sand on Earth’s beaches.

• How do our lifetimes compare to the age of the universe?• On a cosmic calendar that compresses the history of

the Universe into one year, human civilization is just a few seconds old, and a human lifetime is a fraction of a second.


1.3 Spaceship Earth

Our goals for learning:

• 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?


How is Earth moving in our solar system?• Contrary to our perception, we are not “sitting still.”• We are moving with the Earth in several ways, and at

surprisingly fast speeds…

The Earth rotatesaround its axis once every day.

Speed at equator1,650 km/hr (faster than commercial jets!).


Earth orbits the Sun (revolves) once every year:• at an average distance of 1 AU ≈ 150 million km.• with Earth’s axis tilted by 23.5º (pointing to Polaris)• and rotating in the same direction it orbits, counter-clockwise as viewed from above the North Pole.

Average speed108,000 km/hr.


Our Sun moves randomly relative to the other stars in the local Solar neighborhood…• typical relative speeds of more than 70,000 km/hrQuestion: Why do the patterns of the stars in our sky look the same from year to year?

… And orbits the galaxy every 230 million years (speed of 800 000 km/ hr)!


More detailed study of the Milky Way’s rotation reveals one of the greatest mysteries in astronomy – dark matter:the stars in the disk of the galaxy represent only the “tip of the iceberg” compared to the mass of the entire galaxy.

Most of the galaxy’s mass lies above and below the diskIn the “dark” halo!


Within the Local Group galaxies moves randomly, for example, Milky way moves toward Andromeda with the speed of 300,000 km/hr.

When we look outside the Local Group we find out astonishing fact…


Hubble discovered that:

• All galaxies outside our Local Group are moving away from us.

• The more distant the galaxy, the faster it is racing away.

Conclusion: We live in an expanding universe!


Are we ever sitting still?

Question: why an expanding Universe implies a beginning (Big Bang)?


What have we learned?• How is Earth moving in our solar system?

• Earth rotates on its axis once each day and orbits around the Sun once each year at an average distance of 1 A.U. (≈150 million km).


What have we learned?• How is our solar system

moving in the Milky Way Galaxy?• Stars in the Local

Neighborhood move randomly relative to each other.

• Our Solar System orbits the center of the Milky Way Galaxy about every 230 million years: the entire Galaxy rotates.


What have we learned?• How do galaxies

move within the universe?• All galaxies beyond

the Local Group are moving away from us with expansion of the Universe: the more distant they are, the faster they’re moving.


What have we learned?

• Are we ever sitting still?• No!

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Our Place in the Universe