MERCURY

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Big-Bang Theory Nebular Theory
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Big-Bang Theory

• The universe began as hot, dense matter that was
  concentrated into a single place.
• Then, some 15 to 20 billion years ago, an
  explosion sent the matter (and energy) in all
  different directions.
• The fastest moving matter traveled farthest away
  however, background radiation was spread evenly
  throughout the universe.
• After the initial big bang, the force of gravity
  began to affect the matter racing outward.
• The force of gravity began to pull the matter
  into clumps.
• These clumps became the galaxies of the universe.
• Due to nuclear fusion, some of the matter in the
  clumps became stars.

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Nebular Theory

• About 5 billion years ago, a nearby star exploded
  in a huge supernova sending heavy elements
  streaming through space.
• These elements rained down on our nebula, which
  was composed primarily of hydrogen and helium.
• The nebula, which had been slowly spinning, began
  to collapse.
• Gravity pulled matter toward the center.
• The nebula flattened into a huge disk.
• Near the center, a protosun formed.
• Nuclear fusion occurred in the protosun causing
  it to give off light and heat (a star is born).

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Nebular Theory

• Gases and other matter surrounding the newly
  formed sun continued to spin around the sun
  however, the dust and gas particles were not
  spread out evenly.
• Over long periods of time, some of these clumps
  came together to from larger clumps.
• The largest clumps became protoplanets.
• The protoplanets near the sun became so hot that
  most of their lightweight gases boiled away
  (inner rocky planets).
• The protoplanets farther from the sun were less
  affected by the suns heat causing them to grow
  to enormous sizes (gas giants).

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Nebular Theory

• Astronomers believe that one of the satellites
  near Neptune may have broken from that planet
  (Pluto).
• Between Mars and Jupiter, small clumps of matter
  formed asteroids (asteroid belt).
• Farther out in space, other clumps of matter
  formed a huge icy cloud, which became the home of
  comets.

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2-2 Motions of Planets

• Pages 60 - 63

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Earth at the Center?

• Greek scientist Ptolemy placed Earth at the
  center of the universe (2 A.D.)
• All objects travel around an unmoving Earth in
  circular orbits.
• The Greeks thought that the universe was perfect,
  unchangeable, and divine!

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Sun at the Center

• Polish astronomer Copernicus became convinced
  that Earth and all the other planets revolve
  around the sun (1500 1530).
• All planets revolve around the sun in circular
  orbits, the same direction, and a different
  amount of time.

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Elliptical Orbits

• German mathematician Kepler realized that the
  planets move around the sun in an ellipse, or
  oval (16th century).
• Each planet travels in a counterclockwise
  elliptical orbit.
• The more distant from the sun, the longer it
  takes to complete one orbit.

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Inertia and Gravity

• English scientist Sir Isaac Newton explained why
  planets stay in orbit (17th century).
• Inertia causes a planet to move in a straight
  line.
• Gravity pulls the planet towards the sun.

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Motion of the Planets

• Period of Revolution amount of time it takes a
  planet to move around the sun.
• Period of Rotation amount of time it takes a
  planet to spin on its axis.

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