Lesson 2

• Milky Way galaxy


  A huge disc and spiral shaped aggregation of about at least 100 billion stars and other bodies
• Milky Way galaxy


  • Has a globular cluster or bulge of many, many stars at the center, with a supermassive black hole
  • Is about 100 million light years across
• The solar system revolves around the galactic center once about 240 million years
• Local Group of galaxies

  The Milky Way is part of this group, which in turn is part of the Virgo Supercluster of galaxies
• Based on radioactive dating of meteorites, the Earth and solar system are 4.6 billion years old
• Solar system


  • Much of the mass is concentrated at the center (sun) while angular momentum is held by the outer planets
  • Orbits of the planets are elliptical and in the same plane
  • All planets revolve around the sun
  • Periods of revolution increase with increasing distance from the sun, with innermost planet moving fastest
  • Planets are located at regular intervals from the sun
• Inner terrestrial planets


  • Made of materials with high melting points, such as silicates, iron, and nickel
  • Rotate slower, have thin or no atmosphere, higher densities, and lower contents of volatiles
• Outer gas giant planets

  • Dominated by gases, have larger size
  • Rotate faster, have thick atmospheres, lower densities, and fluid interiors rich in hydrogen, helium, and ices (water, ammonia, methane)
• Except for hydrogen, helium, inert gases, and volatiles, the universe and Earth have similar abundance especially for rock and metal elements
• The sun and the large planets have enough gravity to retain hydrogen and helium, but the Earth's gravity is too low to retain rare inert gases
• The presence of heavy elements such as lead, silver, and uranium on Earth suggests it was derived from remnants of a supernova and that the sun is a second generation star made by recycling materials
• The Earth's origin is known mainly from its compositional differences with the entire Universe, as the planet-making process modified the original cosmic material
• Any acceptable scientific thought on the origin of the solar system has to be consistent with and supported by information about it, and may need to be revised if new data emerges that no longer supports currently accepted ideas
• Nebular Hypothesis


  Proposed in the 1700s by Swedenborg, Kant, and Pierre-Simon Laplace - a rotating gaseous cloud that cools and contracts in the middle to form the sun and the rest into a disc that becomes the planets
• The Nebular Hypothesis failed to account for the distribution of angular momentum in the solar system
• Encounter Hypotheses


  • Buffon's sun-comet encounter
  • Jeans' sun-star encounter
  • Chamberlain and Moulton's planetismal hypothesis
  • Lyttleton's sun's companion star colliding
• Otto Schmidt's accretion theory and Woolfson's capture theory are other proposed hypotheses for the origin of the solar system
• The currently accepted theory on the origin of the solar system relies heavily on information from meteorites
• Current Hypothesis


  1. A slowly rotating gas and dust cloud dominated by hydrogen and helium contracts due to gravity

  2. Most of the mass flows to the center to become a proto-sun, while the remaining materials form a disc that will become the planets

  3. Collisions cause fragments to stick together and form larger proto-planets

  4. High speed collisions destroy much of Mercury's mantle and put Venus in retrograde rotation

  5. Collision of Earth with large object produces the Moon

  6. The proto-Sun's solar wind blasts hydrogen, helium, and volatiles from the inner planets to beyond Mars to form the gas giants
• Recent advancements in the exploration of Mars, the Rosetta comet mission, and the Pluto flyby have provided new information about the solar system
• Solar system


  A set of interconnected components that are interacting to form a unified whole
• Closed system


  A system in which there is only an exchange of heat or energy and no exchange of matter
• Hydrosphere


  The dynamic mass of water that is continuously moving, evaporating from the oceans to the atmosphere, precipitating and returning to the ocean
• 97% of Earth's water is salty, collected in deep valleys along the surface as oceans
• Only 3% of Earth's water is fresh, with two-thirds in the form of ice and one-third in streams, lakes, and groundwater
• Lithosphere


  Contains all the cold, hard, solid land of the planet's crust, the semi-solid land underneath the crust, and the liquid land near the center of the planet
• Atmosphere


  The thin gaseous layer that envelopes the lithosphere, composed of 78% nitrogen, 21% oxygen, 0.9% argon, and trace amounts of other gases
• Biosphere


  Contains all the planet's living things, including microorganisms, plants, and animals, forming ecological communities known as biomes
• Photosynthetic organisms form the base of the food chain, sequestering CO2 from the atmosphere and releasing oxygen as a byproduct
• Sunlight is necessary for life