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Chapter 2
Origins
Origin of a Galaxy
¾Formation of Universe, Solar System
and Earth
¾Creation of
Oceans
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Formation of the Universe
¾ Huge rotating aggregation of stars, dust,
gas and other debris held together by
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gravity.
Origin of the Solar System
¾Big Bang, 13*109 years ago
¾Formation of elementary particles
¾Gravitational formation of dense
regions
¾1*109 yrs laterÆ
laterÆfirst stars
¾ Rotating cloud of gas from which sun and
planets formed
¾ Initiated by “supernova”
supernova” = exploding star
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Nuclear Fusion: The joining of atoms under tremendous
temperatures and pressures to create atoms of a
heavier element. In the Sun, four hydrogen atoms are
fused to create each helium atom. Two of the
hydrogen's protons become neutrons in the process
Supporting Evidence for the Big Bang
•Edwin Hubble discovered spreading of galaxies.
•Cosmic background radiation (the glow left over from the explosion
itself) discovered in 1964.
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Moderate Size Stars (Our Sun): C & O
Earth, Ocean and Atmosphere accumulated in layers sorted by
density
Large Stars (more, H & He): Fe
The planet grew by the aggregation of particles.
Meteors and asteroids bombarded the surface,
heating the new planet and adding to its growing
mass. At the time, Earth was composed of a
homogeneous mixture of materials.
Supernova: Heavier Elements Formed
Earth lost volume because of gravitational
compression. High temperatures in the interior
turned the inner Earth into a semisolid mass;
dense iron (red drops) fell toward the center to
form the core, while less dense silicates move
outward. Friction generated by this movement
heated Earth even more.
The result of density stratification:
an inner
and
outer core,
a mantle,
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Condensation Theory
A nebula (a large,
diffuse gas cloud
of gas and dust)
contracts under
gravity. As it
contracts, the
nebula heats,
flattens, and spins
faster, becoming a
spinning disk of
dust and gas.
Planets will form in disk.
Warm temperatures allow
only metal/rock “seeds” to
condense in the inner solar
system.
Cold
temperatures
allow “seeds” to
contain
abundant ice in
outer solar
The seeds of
system.
Terrestrial planets are built
from metal and rock.
Solid “seeds”
collide and stick
together. Larger
ones attract others
with their gravity,
growing bigger still.
Terrestrial planets remain in
inner solar system.
Gas giant planets remain in outer
solar system.
“Leftovers” from the
formation process become
asteroids (metal/rock) and
comets (mostly ice).
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How did water and water vapor form on early
Earth?
Star will be born in center.
Hydrogen and
helium remain
gaseous, but other
materials can
condense into solid
“seeds” for building
planets.
and the crust.
gas giant
planets grow
large enough to
attract hydrogen
and helium gas,
making them
into giant,
mostly gaseous
planets; moons
form in disks of
dust and gas
that surround
the planets.
¾ The Sun stripped away Earth’
Earth’s first atmosphere
¾ Gases, including water vapor, released by the
process of outgassing,
outgassing, replaced the first
atmosphere.
¾ Water vapor in the atmosphere condensed into
clouds.
¾ After millions of years, the clouds cooled enough for
water droplets to form.
¾ Hot rain fell and boiled back into the clouds.
¾ Eventually, the surface cooled enough for water to
collect in basins.
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Not to scale
Early Earth
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Sources of Water
¾ Accretion (Gaining material)
¾ Differentiation (Separating based on
density Æ density stratification)
stratification)
¾ Evidence of waterwater- 3.9*109 yrs ago
* Mantle rocks
¾Evidence from meteorites
¾Release through volcanic activity
* Outer space
¾Evidence from Dynamics Explorer
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The evolution of our atmosphere
Age and Time
1 billion = 1,000,000,000 or 109
Earth is 4.6 * 109 years old
Oceans are 4.2 * 109 years old
Oldest rocks date from 3.8 * 109 years ago
First evidence of life dates from 3.6 * 109
years ago
1 million = 1,000,000 or 106
Ocean and atmosphere reach the state we
know today 800 * 106 years ago
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Concentration of
Atmospheric Gases (%)
Methane, ammonia
initial rise of O2 2.7 b. y. ago – but conclusive
evidence is from 2.3 b. y. ago
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Atmosphere
unknown
Nitrogen
50
Water
25
Carbon dioxide
0
Oxygen
3
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Time (billions of years ago)
4.5
1
2
Early atmosphere quite different from today’s
Fig. 2-11,13p. 49
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Radioactive Decay Series
Life probably originated in the ocean
Fossil of a bacteriabacterialike organism (with
an artist’
artist’s
reconstruction) that
photosynthesized
and released oxygen
into the atmosphere.
Among the oldest
fossils ever
discovered, this
microscopic filament
from northwestern
Australia is about
3.5 billion years old.
Parent Isotope
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Billions of years ago
13 Big bang
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5.5
4.6
Today
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Stable Daughter Product
Currently Accepted Half-Life Values
Uranium-238
Lead-206
4.5 billion years
Uranium-235
Lead-207
704 million years
Thorium-232
Lead-208
14.0 billion years
Rubidium-87
Strontium-87
48.8 billion years
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Billions of
years ago
4.6 Earth forms
Millions of
Ocean forms
years ago
First galaxies 4.2
form
3.8 Oldest dated rocks 800 First animals arise
3.6 First evidence
of life
Solar
nebula
begins
to form
Earth
forms
Today
The future of Earth
Past
Millions of years
510 First
ago 66
fishes
End of dinosaurs
Oxygen
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appear
revolution begins
Pangaea 50 First marine
breaks
mammals
Ocean and 210 apart
0.8
atmosphere
End of
reach steady
dinosaurs
state (as today)
Humans appear
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Today
Today
Today
Age and Time –
Past and Future
Future
3.5
The sun swells,
planets destroyed
How long can Earth exist?
¾Our Sun will begin to die in 5 billion years.
¾6 billion years from now the sun will enter the
red giant phase and will engulf the inner planets.
¾At that time, Earth will probably be recycled
into component atoms.
Sun's
output too
low for
liquid-water
ocean
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Billions of years
in the future
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Fig. 2-15, p. 51
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Summary
Most of the atoms that make up Earth, its ocean, and its inhabitants
were formed within stars billions of years ago. Stars spend their lives
changing hydrogen and helium into heavier elements. As they die, some
stars eject the elements into space during cataclysmic explosions. The
sun and planets, including Earth, condensed from a cloud of dust and
gas enriched by the recycled remnants of exploded stars.
Earth formed by accretion – the clumping of small particles into a
large mass. The mass heated as it grew and eventually melted. The
heavy iron and nickel crashed toward Earth’s center to become its
core; the lighter silicates and aluminum compounds rose to the surface
to form a crust. Earth became density stratified – that is layered by
density.
The ocean formed as soon as Earth was cool enough for water to
remain liquid. Life followed soon thereafter.
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Thinking Analytically
1. A light-year is the distance light can travel in one year. Light travels
at 300,000 kilometers per second. Commercial television broadcasting
began in 1939. Television signals travel at the speed of light. How far
away would a space probe have to be before it could no longer detect
those signals?
2. Can you think of any way an astronomer could detect a large planet
orbiting a star without actually seeing the planet? (Hint how would a
star move as the planet orbits it?)
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