Physics 55Monday, September 5, 2005

1. Finish discussion of length scales.2. Discuss time and speed scales.3. Begin discussion of Chapter 2: understanding the sky,

Administrative Items

• First quiz this Friday, starts right away at 1:15 pm so be on time. Some suggestions about how to get ready…

• Next recitation: Vote on Blackboard Discussion board.

• Homework returned: graded gently this first time but not the next time.

• Read Chapter 2 (and look at course Announcements page for further information).

Preparing for the First and Later Quizzes

• Read carefully conclusions at end of chapter.• Go over the lecture slides for key points.• Understand the PRS questions.• Go over the reasonable/not-reasonable questions at end of

chapter, questions in Astronomy Place for Chapter 1.• Think actively: in limited amount of time, what are the most

important points that the professor is likely to emphasize?• There will be some multiple choice knowledge questions

(what is a …, list in order of increasing size…), some multiple choice thinking questions (if I change this, what happens), and some calculation questions.

• I will be dropping one quiz grade over the semester.• Keep in mind that you can make up for quiz grades with an

extra credit project.

PRS Question: Light-year

If you look at an object that is 1,000 light-years away, you see it

1. as it was 1,000 years ago.2. as it looked to your ancestors 1,000 years ago.3. as it was 1,000 light-years ago4. as it is right now, but it appears 1,000 times dimmer.

Vocabulary

1. What is a planet?

2. What is a moon?

3. What is a star?

4. What is a star system?

5. What is a nebula?

6. What is a galaxy?

7. What is the universe?

Length Scales

This part of lecture will be at the whiteboard so time for you to take notes. Points discussed will include:

1. What is meant by a “length scale”?2. Comparison of atoms with solar systems: where does

“size” come from? 3. Making models: moon compared to Earth in size and

distance; thicknesses of atmosphere and oceans compared to Earth.

4. Are there biggest and smallest sizes in astronomy? Planets can’t get too big or small, stars also can’t get too big or small, what about galaxies, black holes?

5. Comparisons of hierarchy of atoms with hierarchy of universe: star system to galaxy to clusters to superclusters to universe. Fractals…

6. Astronomical units: AU, light-year

Cosmological Connection Between Ratio of Electrical to Gravitational Forces in H atom?

I will discuss at the whiteboard a neat insight of Paul Dirac in the 1930s: that the ratio of the strength of electrical force to gravitational force between an electron and proton (of a H atom) is so huge that it may be related to the size or age of the universe when measured in some natural units like the radius of a proton.

This suggests a fascinating result: since the universe is expanding, at least one of the basic properties of nature such as the Coulomb constant k, the gravitational constant G, the electric charge e on an electron or proton, or the mass of an electron me would have to change over time. Experiments have tested for this possibility, e.g., by using the fact that looking at a faraway object in space is like looking far back in time, but so far no evidence for such changes. The origin of this huge ratio of electrical to gravitational forces remains a mystery. Another puzzle: this ratio is finely tuned for the existence of life…

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

Relative Size of Planets

“My Very Educated Mother Just Sent Us Nine Pizzas”

Relative Sizes of Some Familiar Stars

Deneb is about 150 times bigger than the Sun, Betelgeuse 900 times!

Some Moons Are Big!

Ida and Dactyl: Non-planets (not round)

35 miles or 56 km

Kuiper Belt and Oort Cloud:The Rest of a Star System

SkyGazer Demo of Distances Between Stars

Light-seconds versus light-minutes versus light-days versus light-years

Typical distance between stars in Milky Way about a light-year or about 256,000 AU. Explains why the human race won’t be getting to the stars soon.

Why is this distance about a ly?!

Our Milky Way

100,000 light-years

The Sun

Answer to Galactic Colonization Time

Fastest human object so far was Pioneer 11 as it slung by Jupiter with speed of about 200,00 km/hr or about .0002 c. So time to travel

to nearest star would be 4/.0002 ~ 20,000 y. Time to travel across entire Milky Way would be about 50,000 ly/.0002 c ~ 250 million years. A fusion-based technology could bring speed up to 0.1 c in

which case colonization time would be about 250,000 y. A long long time on human time scales but short compared to age of life, age of Earth, age of universe.

Also, keep in mind that our Sun is a third generation star, life could have started much earlier than in our solar system, so it is not implausible that a technological species could have colonized much of the Milky Way by this time in the history of the universe.

Pictures of Some Galaxies

Hubble Deep Space Photo of Galaxies

Visual Demo: Distance Between Galaxies

When distance between objects is not muchbigger than size of objects, collisions are likely!Nota bene: average distance is increasing over time because of expansion of universe!

Stars quite unlikely to collide.Galaxies collide a lot, major influence on shape.

Note: Our galaxy is colliding right now with Sagittarius dwarf galaxy!...

20x

PRS Question: Marconi’s Broadcast

The first trans-Atlantic radio broadcast was made by Marconi from Cape Cod to England in 1903. Since radio waves travel at the speed of light, his broadcasts:

1. have not yet left the solar system.2. have traveled to the nearest star, Alpha Centauri.3. have traveled throughout the Milky Way.4. have traveled to the nearest big galaxy, Andromeda.

PRS: How Many Stars Have Heard Marconi?

From what you have learned so far, about how many stars have received Marconi’s 1903 radio message?

1. ~ 10 stars2. ~ 100 stars3. ~ 1,000 stars4. ~ 10,000 stars5. ~ 100,000 stars6. No idea