Chapter 1 Notes: Olivia Ward

Our Place in the Universe
1.1 Our Modern View of the Universe

What is our place in the universe?

• Our cosmic address: Earth ← Solar System ← Milky Way Galaxy ← Local Group ← Local Supercluster  ← The Universe

• Star:  A large, glowing ball of gas that generates heat and light through nuclear fusion.
• Planet: A moderately large object that orbits a star; it shines by reflecting light. (May be rocky, icy, or gaseous)
• Moon: An object that orbits a planet.
• Asteroid: A relatively small and rocky object that orbits a star.
• Comet: A relatively small and icy object that orbits a star.
• Nebula: An interstellar cloud of gas and/or dust.
• Galaxy: A great island of stars in space, all held together by gravity and orbiting a common center. The center is a black hole.
• Universe: The sum total of all matter and energy.

How did we come to be?

• Birth of the universe: 13,700,000,000 years ago
• The expansion of the universe began with the Big Bang. The universe continues to expand but on smaller scales.
• Galaxies are cosmic recycling plants: The early universe contained only hydrogen and helium. All other elements were made by past generations of stars.
• Life cycles of stars: Stars are born in clouds of gas and dust. They shine with energy released by nuclear fusion (manufactures elements heavier than hydrogen and helium). Massive stars explode when they die, scattering elements they've produced in space.
• Earth and life: By our time the solar system was born, 4.5 billion years ago, 2% of the original hydrogen and helium were converted into heavier elements.
• We are star dust.

How can we know what the universe was like in the past?

• Light travels at 300,000 km per second.
• We see objects as they were in the past.: The farther away we look in distance the further we look back in time.
• Question: When will we be able to see what the universe looks like now?
  
  • Answer: In half a million years.
•  Light-year: The distanced light can travel in 1 year- about 10 trillion km / 6 trillion miles
• Question: Can we see the entire universe?
  
  • Answer: No.
• Question: Why can't we see a galaxy  15 billion light-years away?
  
  • A: No galaxies exist at such great distances.
  • B: Galaxies may exist at that distance but their light would be too fainnt for our telescopes to see.
  • C: Looking 15 billion light-years away means looking to a time before the universe existed. 

1.2 The Scale of the Universe

How big is the Earth compared to our solar system?

• Question: Reduce the size of the solar system by the factor of 10 billion; the sun is now the size of  a large grapefruit (14 cm diameter). How big is the Earth compared to the Sun?
  
  • A: An atom
  • B: The tip of a ball point pen
  • C: A marble
  • D: A golf ball
• Question: How far away are the stars?
  
  • A: 1 mile
  • B: 10 miles
  • C: 100 miles
  • D: 2500 miles (Distance across the United States)
• Question: If you tried to count the 100 billion+ stars in the galaxy at the rate of 1 per second, how long would it take?
  
  • A: A few weeks
  • B: A few months
  • C: A few years
  • D: A few thousand years

How big is the universe?

• The Milky Way is one of about 100 billion galaxies.
• 10^11 stars in a galaxy X 10^11 galaxies = 10^22 stars
• There are as many stars as grains of dry sand on all of Earth's beaches.

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 into 1 year.
  
  • January 1st: The Big Bang
• February: The Milky Way forms
• September 3rd: Earth forms
• September 22nd: Early life on Earth
• December 17th: Cambrian Explosion
• December 26th: Rise of dinosaurs
• December 30th: Extinction of dinosaurs
• December 31st 9 pm: Early hominids evolve
• 11:58 pm: Modern humans evolve
• 25 seconds ago: Agriculture arises
• 11 seconds ago: Pyramids built
• 1 second ago: Kepler and Galileo show Earth orbits the Sun.
• Now

1.3 Spaceship Earth
 How is Earth moving in our solar system?

• We are not sitting still: Earth rotates around its axis once a day. Earth moves at different speeds based on location.
• Our Sun moves randomly relative to the other stars in the local solar neighborhood.
• A typical relative speed of more than 70,000 km/hr
• Stars are so far away, we cannot notice their motion.
• Sun orbits the galaxy every 230 million years.
• Most of the Milky Way's light comes from dish and bulge, but most of the mass is in its halo (dark matter).

How do galaxies move within the galaxies?

•  Galaxies are carried along with the expansion of the universe. Hubble figured out the universe was expanding.
• Example: Bread baking with raisins in it: The bead expands and the raisins spread apart, but the raisins don't expand.
• Hubble discovered that all galaxies are moving away from us and more distant ones move away from s father because they are carried along with expansion.