Physics 55Friday, September 2, 2005

Administrative Items

• Observation sessions this Saturday and Sunday, please come (especially on Sat.)

• Read the Physics 55 announcements frequently!

• First quiz a week from today, starts right away at 1:15 pm so be on time.

• PRS transmitter numbers: email to Bo the TA ASAP.

• Next homework available this Monday, will be due a week from Monday.

• Next recitation: Wed won’t work so Thu, Sat, or Sun?

• Start reading Chapter 2 for next week.

Where We Are Heading…

1. Chapter 1: time scales, length scales, speed scales, the expanding universe.

2. Chapter 2 and S1: Understanding the night sky, some celestial navigation.

3. Chapters 4-6, 22: Matter, energy, gravity, dark matter, light, telescopes.

4. Chapter 9: Formation of the solar system

5. Chapter 15: Our Sun

Outline of Today’s Lecture

• How to use the planetarium software SkyGazer to learn about the sky.

• Length, time, and speed scales of the universe (also some astronomical units such as AU and light-year).

• Key components of the universe: planets, asteroids, comets, stars, star systems, nebulas, galaxies, galaxy clusters, the observable universe (see page 4 of text).

PRS Question

The Sun always sets in:1. the west.2. the east.3. the north.4. the south.5. none of the above.

PRS Question: Religion

What is your religious belief?

1. You believe that there is some deity.

2. You believe that there is a God as described by the Bible.

3. You are an agnostic.

4. You are an atheist.

5. Coach Krzyzewski is God.

PRS Question: Most Important Point

The most important point from the first lecture was that

1. astronomy is fun.

2. many crazy exciting discoveries have been made.

3. many ingenious tools have been invented to explore the universe.

4. the universe is comprehensible.

5. the ideas of Newton and Einstein are especially important for understanding the universe.

SkyGazer: Configuration

This will be a show-and-tell, I will run the software in class.

First how to configure1. Select menu Control/Set Location to set longitude and latitude to

Durham and other places.

2. Menu Control/Define Horizon and click Clear to simplify horizon.

3. Select menu Chart/Auto Identify to identify objects with cursor.

4. Select menu Chart/Clear Sky Chart to remove labels.

5. Select menu Chart/Star Symbols/Medium

6. Select menu Chart/Magnitude Limits/Small Town

7. Save configuration via File/Save Settings/Durham so you can quickly recover and use these settings in the future.

SkyGazer: Some things to do

1. See what will be in the sky tonight, using the Time Panel. Zooming out to see the big picture of the sky.

2. How to get information from objects: single left click on object.

3. When and where does the Sun set and rise at different times of year?

4. Zooming in on Venus and Jupiter: center with double click, then “lock” then zoom. Note: useful to make horizon transparent (Control/Define Horizon/Transparent) and turn off night and day (Chart/Natural Sky).

5. Using the Control/Display Panel to turn various visuals on and off, especially asterisms and pictures of asterisms. Great for learning your constellations.

6. Watch retrograde motion of Mars by opening external Demo file.

7. Please explore, you can’t kill the solar system or universe so have fun.

Quantum Mechanics Defines FundamentalLength Scale of Atoms

I show at the whiteboard that an object like an electron or planet in circular orbit around some other object like a proton or star, in principle should be able to have any possible size, i.e., the radius can vary continuously without restriction if all one looks at is Newton’s equation of motion F=ma. But something special happens for atoms: angular momentum “mrv” is quantized and comes in integer multiples of the smallest possible angular momentum hbar. This forces the hydrogen atom and other atoms to have specific average radii and the electron to have specific average speeds.

Unlike atoms, macroscopic objects like planets can have arbitrarily varying speeds and sizes.

Atoms Versus Planets Versus Stars

Planets in our solar system vary in size from .2 to 11 Earth radii, a dynamic range of 100 (but planets can be 10x bigger that Jupiter).

Stars vary in size from 1/100 to 1,000 times the radius of the Sun, dynamic range of 105.

Galaxies vary in size from a billion to 100 billion stars, 100,000 ly

No known limit to size of black holes, main trouble is feeding them