Transcript 1. Our Place in the Universe

1. Our Place in the
Universe
© 2004 Pearson Education Inc., publishing as Addison-Wesley
1.1 A Modern View of the Universe
Our goals for learning:
• What is our physical place in the Universe?
• Describe our cosmic origins and why we say
that we are “star stuff.”
• Why does looking into space mean looking
back in time?
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Address: Earth
• How would the
Universal post office
find us?
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Star
A large, glowing ball of gas that generates
heat and light through nuclear fusion
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Planet
A moderately large object which orbits a star;
it shines by reflected light. Planets may be
rocky, icy, or gaseous in composition.
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Moon
An object which
orbits a planet.
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Asteroid
A relatively small
and rocky object
which orbits a star.
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Comet
A relatively
small and
icy object
which orbits
a star.
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Solar (Star) System
A star and all the material which orbits it,
including its planets and moons
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Nebula
An interstellar cloud
of gas and/or dust
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Galaxy
A great island of stars in space, all
held together by gravity and
orbiting a common center
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Universe
The sum total of all matter and
energy; that is, everything within
and between all galaxies
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Atom
Microscopic “building blocks” of all
chemical elements
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Where do we come from?
• The first (and simplest) atoms were created during
the Big Bang.
• More complex atoms were created in stars.
• When the star dies, they are expelled into space….
to form new stars and planets!
Most of the atoms in our
bodies were created in
the core of a star!
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Announcement
• (Short) quiz next Weds on material covered thro’ next
Mondays lecture.
• Closed book -please sit spaced apart and no conferring
• Quiz will be graded and we will look at solution sheet
shortly afterwards but you can pick up graded quizzes
the Tues or Thurs of the week after a given quiz - from
Yu Zhou in Physics 422, available Tues & Thurs noon1pm.
•
Reminder ~ 1 week for grade appeals (ie must appeal within 2 weeks of sitting the
quiz)
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Looking back in time
• Light, although fast, travels at a finite speed.
• It takes:
– 8 minutes to reach us from the Sun
– 8 years to reach us from Sirius (8 light-years away)
– 1,500 years to reach us from the Orion Nebula
• The farther out we look into the Universe, the farther
back in time we see!
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Timeout
• Scientific Notation
•
Scientific Notation is a compact and convenient way of expressing very large
and very small numbers using powers of 10. You've all probably encountered
scientific notation before. I hope the examples below are reminders for those
who haven't used it in a while.
• Examples of Scientific Notation:
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The Mass of the Sun:
1,989,000,000,000,000,000,000,000,000,000 kilograms = 1.989x1030
kilograms
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The Size of a Hydrogen Atom:
0.0000000000106 meters = 1.06x10-11 meters
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Timeout
A quick word on powers of 10
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Factor Prefix
103
kilo106
mega109
giga1012 tera10-2 centi10-3 milli10-6 micro10-9 nano-
Examples
kilogram, kilometer, kilobyte
megawatt, megayear, megabyte, megaton
gigayear, gigaton, gigabyte
terawatt, terabyte
centimeter
millimeter, millisecond, milliliter
microsecond, micron
nanosecond, nanometer
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Timeout
A quick word on powers of 10
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Common Examples
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Length:
1 kilometer = 103 meters (1000 meters)
1 centimeter = 10-2 meters (1/100th of a meter)
1 millimeter = 10-3 meters (1/1000th of a meter)
1 micron = 10-6 meters (short for "micrometer")
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Time:
1 nanosecond = 10-9 s (1 billionth of a second)
1 Gigayear = 109 years (1 Billion years)
1 Megayear = 106 years (1 Million years)
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Timeout
• Astronomical Units of Length
•
Meters and kilograms are fine for most terrestrial applications, but when we start talking about the
enormous distances between the planets, or between stars and galaxies, we need to define special
units to keep the numbers from getting too big. The most important of these for our purposes in this
course are:
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The Astronomical Unit (AU):
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1 AU is the Mean Distance from the Earth to the Sun:
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1 AU = 1.496x108 kilometers
The AU is used for expressing the distances between planets.
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In round numbers, you can use "1 AU = 150 Million km" for the purposes of this class.
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The Light Year (ly):
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1 Light Year (ly) is the Distance Traveled by Light in 1 Year:
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1 ly = 9.46x1012 kilometers
The light year is used for expressing the distances between stars.
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Timeout
• Space is BIG
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For example, what is the distance between the Earth and:
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The Moon: 384,000 kilometers
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The Sun: 1 AU (149,600,000 km)
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Alpha Centauri (nearest star): 4.2 light years (266,000 AU)
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Center of the Milky Way Galaxy: 26,000 light years (1.65x109 AU)
As you can see, if you only use kilometers or meters, the numbers would get out of hand
very fast.
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Our goals for learning:
• What does our solar system look like when
viewed to scale?
• How far away are the stars?
• How do human time scales compare to the age
of the Universe?
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How large is the Solar System?
• Let’s view it to scale
– say the Sun is the size of a large grapefruit (13.9 cm)
– then:
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How large is
our Galaxy?
The Sun ~ 26,000 lightyears from the center of
the Milky Way Galaxy,
which is about 80,000 to
120,000 light-years
across (and less than
7,000 light-years thick).
We are located on on one
of its spiral arms, out
towards the edge.
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How far away is the nearest galaxy?
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How large is the Universe?
• Now let’s view the Universe in terms of meters
– Powers of 10 or 10
?
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How old is the Universe?
• The Cosmic Calendar
– if the entire age of the Universe were one calendar year
– one month would be approximately 1 billion real years
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1.3 Spaceship Earth
Our goals for learning:
• Describe the basic motions of “spaceship Earth.”
• How do we know that the Universe is
expanding?
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A Universe in motion
• Contrary to our perception, we are not “sitting still.”
• We are moving with the Earth.
– and not just in one direction
The Earth rotates around
it’s axis once every day!
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The Earth orbits around
the Sun once every year!
The Earth’s axis is tilted
by 23.5º!
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Our Sun moves relative to
the other stars in the local
Solar neighborhood!
Our Sun and the stars of the local
Solar neighborhood orbit around
the center of the Milky Way
Galaxy every 230 million years!
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The Milky Way moves with the expansion of
the Universe!
• Mostly all galaxies
appear to be moving
away from us.
• The farther away they
are, the faster they are
moving.
– Just like raisins in a raisin
cake; they all move apart
from each other as the
dough (space itself)
expands.
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What have we learned?
• What is our physical place in the Universe?
• Earth is a planet in a solar system, which is one of some 100
billion star systems in the Milky Way Galaxy, which is one of
about 40 galaxies in the Local Group, which is part of the
Local Supercluster, which is part of the Universe.
• Describe our cosmic origins and why we say that we
are “star stuff.”
• The Universe began in the Big Bang, which produced only
two chemical elements: Hydrogen & Helium. The rest have
been produced by stars, which is why we are “star stuff.”
© 2004 Pearson Education Inc., publishing as Addison-Wesley
What have we learned?
• Why does looking into space mean looking back
in time?
• Light takes time to travel through space. Thus,
when we look farther away, we see light that has
taken a longer time to reach us.
• What does our solar system look like when
viewed to scale?
• On a scale of 1-to-10 billion, the Sun is about the size
of a grapefruit. The Earth s the size of a ball point and
Jupiter the size of a marble on this scale. The distance
between planets are huge compared to their sizes.
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What have we learned?
• How far away are the stars?
• On the 1-to-10 billion scale, the nearest stars to the Sun
would be thousands of kilometers away. The rest of
the Milky Way must be viewed on a different scale. It
would take thousands of years to count them all.
• How do human time scales 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.
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What have we learned?
• Describe the basic motions of “spaceship Earth.”
• Earth rotates on its axis once each day and orbits around the Sun
once each year. Our Solar System orbits the center of the Milky
Way Galaxy about every 230 million years. Galaxies in the
Local Group move relative to one another, while all other
galaxies are moving away from us with expansion of the
Universe.
• How do we know that the Universe is expanding?
• We observe nearly all other galaxies to be moving away from us,
with more distant ones moving faster.
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