Ch 1-2 Notes: Jessica Horn

Chapter 1
1.1 Our Modern View of the Universe
      Our goals for learning
- What is our place in the universe? Our "cosmic address"
-How did we come to be?
-How can we know what the universe was like in the past?
-Can we see the entire universe?
Star: A large, glowing ball of gas that generates heat and light through nuclear fusion
Planet: Moderately large object that orbits a star, shines by reflected light. Planets may be rocky, icy, or gaseous in composition.
Moon (or satellite): An object that orbits a planet (spheres)
Asteroid: A relatively small and rocky object that orbits a star (potato in the sky)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 (black hole)
Universe: Sum total of all matter and energy that is everything within and between all galaxies
Birth of the Universe: 13,700,000,000 years ago
1.

• expansion of universe began with hot, big bang
• expanded in time
• universe continues to expand 
• on smaller scales, gravity has pulled matter together to create galaxies

2. Galaxies: early universe was composed of hydrogen and helium

3.Life cycles of stars: Many generations of stars have lived and died in the Milky Way. 

4. Earth and Life: 2% of hydrogen and helium was converted into heavier elements by the time our solar system was born about four and a half billion years ago.

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

• Light travels at a finite speed (300,000 km/s)
• Destination: Moon. Light travel time: 1 second
• Destination: Sun. Light travel time: 8 mins
• Destination: Sirius. Light travel time: 8 years
• Destination: Andromeda Galaxy. Light travel time: 2.5 million years
• Thus, we see objects as they were in the past: The farther away we look in distance, the further back we look in time.

Light-year: The distance light can travel in 1 year. About 10 trillion kilometers (6 trillion miles). At great distances, we see objects as they were when the universe was much younger. 

*Why can't we see a galaxy 15 billion light-years away? Assume the universe is 14 billion years old. Answer: Looking 15 billion light-years away means looking to a time before the universe existed.

What is our place in the universe?

• Earth is part of a solar system, which is the Milky Way Galaxy, which is a member of the Local Group of galaxies in the Local Supercluster.

How did we come to be?

• The matter in our bodies

1.2

How big is Earth on this scale (solar system is by a factor of 10 billion. The Sun is the size of a grapefruit, 14 cm in diameter)? Answer: The tip of a ballpoint pen. Earth is the size of a tip of a ballpoint pen, 15 m away, on a 1-to-10 billion scale.

On our 1-to-10 billion scale, it's just a few minute's walk to Pluto. How far would you have to walk to reach Alpha Centauri? Answer: The distance across the United States.

Suppose you try to count more than 100 billion stars in our galaxy at a rate of one per second. How long would it take you? Answer: A few thousand years.

How Big is the Universe

• The Milky Way is 1 of about 100 billion galaxies
• 10^11 stars/galaxy X 10^11= 10^22 stars. It has as many stars as grains of dry sand on all Earth's beaches.
• The cosmic calendar: A scale on which we compress the history of the universe into one year.

1.3

• Contrary to our perception, we are not "sitting still"
• We are moving with the Earth in several ways, and at surprisingly fast speeds
• Earth rotates around its axis once every day
• Our sun moves randomly relative to the other stars in the local solar neighborhood
• at typical relative speeds more than 70,000 km/hr
• but stars are so far away that we can't easily notice their motion
• ...and it orbits the galaxy every 230 million years
• Most of the Milky Way's light comes from disk and bulge but most of its mass is in its halo
• Galaxies are carried along with the expansion of the universe. But how did Hubble figure out tat the universe is expanding? 1. Discovered that all galaxies outside our Local Group is moving away from us. 2. The more distant the galaxy, the faster it appears to be racing away. 

Chapter 2

2.1

With the naked eye, we can see more than 2,000 stars, as well as the Milky Way. 

Constellation: A region of the sky. *88 constellations fill the entire sky. *The brightest stars in a constellation...may actually be quite far away from each other

The Celestial Sphere 

The ecliptic is the Sun's apparent path through the celestial sphere.

The 88 official constellations cover celestial sphere

The Milky Way

A band of light that makes a circle aroud the Celestial Sphere. What is it? Our view into the plane of our galaxy.

The Local Sky

An object's altitude (above horizon) and direction (along horizon) specify its location in your local sky. 

Zenith: The point directly overhead

Horizon: All points 90 degrees away from Zenith

Meridian: Line passing through Zenith...

We measure the sky using angles

Angular measurements 

• Full circle = 360 degrees
• 1 degree= 60 arcminutes
• 1 arminute= 60 arcseconds

*The angular size of your finger at arm's length is about...60 X 60 = 3600 arcseconds