1. Prof. D.C. Richardson Sections 0101-0106
ASTR100 (Spring 2008) Introduction to Astronomy Discovering the Universe
Order E2-01 WORLD GLOBE E2-41 TRANSPARENT CELESTIAL GLOBE
Prof. D.C. Richardson
Sections

2. How can we know what the universe was like in the past?
Light travels at a finite speed (c = 300,000 km/s).
Thus, we see objects as they were in the past:
The farther away we look in distance, the further back we look in time.
Destination
Light travel time
Moon
1 second
Sun
8 minutes
Sirius
8 years
Andromeda Galaxy
2.5 million years
Point out how fast the speed of light is: could circle Earth 8 times in one second….
Also note that the speed of light is always the same…

3. Question: When will be able to see what it looks like now?
Example:
This photo shows the Andromeda Galaxy as it looked about 2½ million years ago.
Question: When will be able to see what it looks like now?
Answer to question: in about 2.5 million years. But point out that for a galaxy, not much will change in that time.
Also worth noting: This photo shows 100,000 years of time, because the galaxy is about 100,000 light-years in diameter. Thus, we see the near side as it was 100,000 years later than the time at which we see the far side.
M31, The Great Galaxy
in Andromeda

4. Definition: A light-year
The distance light can travel in one year.
About 10 trillion km (6 trillion miles).
Distance = Speed x Time
= (300,000 km/s) x (1 yr) x (31,557,600 s/yr)
= 9,500,000,000,000 km!
= 9.5 x 1012 km
Emphasize that a light-year is a unit of distance NOT a unit of time. Remind students of Common Misconception box in text (p. 8). Equation is optional: it is not given in the book, but should be easy for most students to follow.

5. At great distances, we see objects as they were when the universe was much younger.
This slide shows part of Figure 1.4, which you can use to emphasize that we are seeing galaxies as they were in the distant past -- which means when they were YOUNG. (Note: students often equate “past” with “old,” so it’s important to emphasize that galaxies that we see in the past are in fact the ones we are seeing at young ages…)

6. Can we see the entire universe?
This slide shows the rest of Figure 1.4, which you can use to show how the age of the universe limits the extent of our observable universe.

7. Because no galaxies exist at such a great distance.
Why can’t we see a galaxy 15 billion light-years away? (Assume the universe is 14 billion years old)
Because no galaxies exist at such a great distance.
Galaxies may exist at that distance, but their light would be too faint for our telescopes to see.
Because looking 15 billion light-years away means looking to a time before the universe existed.
This is an optional question to ask your students, to see if they have grasped the idea of the observable universe.

8. Because no galaxies exist at such a great distance.
Why can’t we see a galaxy 15 billion light-years away? (Assume the universe is 14 billion years old)
Because no galaxies exist at such a great distance.
Galaxies may exist at that distance, but their light would be too faint for our telescopes to see.
Because looking 15 billion light-years away means looking to a time before the universe existed.
This is an optional question to ask your students, to see if they have grasped the idea of the observable universe.

9. 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.
New Year’s Day: The Big Bang
Milky Way forms
Sun & planets form
Oldest known life (single-celled)
First multi-cellular organisms
Our favorite way to present the scale of time: a modified version of Carl Sagan’s Cosmic Calendar. Worth noting:
Since we are compressing the 14 billion-year history of the universe into one calendar year, 1 month represents about 1.2 billion real years, 1 day represents about 40 million years; 1 second represents about 440 years.
the universe already 2/3 of the way through its history before our solar system even formed.
dinosaurs arose the day after Christmas, died yesterday.
All of (recorded) human history is in the last 30 seconds.
You and I were born about 0.05 seconds before midnight, Dec. 31.

10. 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.
Our favorite way to present the scale of time: a modified version of Carl Sagan’s Cosmic Calendar. Worth noting:
Since we are compressing the 14 billion-year history of the universe into one calendar year, 1 month represents about 1.2 billion real years, 1 day represents about 40 million years; 1 second represents about 440 years.
the universe already 2/3 of the way through its history before our solar system even formed.
dinosaurs arose the day after Christmas, died yesterday.
All of (recorded) human history is in the last 30 seconds.
You and I were born about 0.05 seconds before midnight, Dec. 31.

11. Spaceship Earth

12. How is Earth Moving in Our Solar System?
The Earth rotates around its axis once every day.
Our first motion is ROTATION.
Point out that most of us are moving in circles around the axis at speeds far faster than commercial jets travel, which is why jets cannot keep up with the Sun when going opposite Earth’s rotation…

13. How is Earth Moving in Our Solar System?
Our second motion is ORBIT. Point out the surprisingly high speed of over 100,000 km/hr.
The Earth orbits the Sun (revolves) once every year.

14. How is Earth Moving in Our Galaxy?
Our third and fourth motions are MOTION WITH THE LOCAL SOLAR NEIGHBORHOOD and ROTATION OF THE MILKY WAY GALAXY (about 70,000 km/hr and km/hr, respectively).
The Sun moves randomly relative to other nearby stars, and orbits the galaxy once every 230 million years.

15. Most of Milky Way’s light comes from disk and bulge …
More detailed study of the Milky Way’s rotation reveals one of the greatest mysteries in astronomy…dark matter!
Most of Milky Way’s light comes from disk and bulge …
Although we won’t discuss dark matter until much later in the course, you might wish to mention it now to whet students’ appetites…
…. but most of the mass is in its halo

16. How do Galaxies Move Within the Universe?
Galaxies are carried along with the expansion of the universe.
Describe the raisin cake analogy, and have students work through the numbers with you to make the table. (E.g., “How far away is Raisin 1 at the beginning of the hour? [1 cm] How far is it at the end of the hour? [3 cm] So how far would you have seen it move during the hour? [2 cm] So how fast is it moving away from you? [2 cm/hr]”

17. Part C
The following statements describe ways in which the analogy might apply to the real universe. Which statements are correct?
A. Both the raisin cake and the universe have a well-defined inside and outside.
B. Raisin 1 is near the center of the cake, just as our galaxy is near the center of the universe.
C. The temperature starts low and ends high in both the raisin cake and the universe.
D. The raisins stay roughly the same size as the cake expands, just as galaxies stay roughly the same size as the universe expands.
E. The average distance increases with time both between raisins in the cake and between galaxies in the universe.
F¡. An observer at any raisin sees more distant raisins moving away faster, just as an observer in any galaxy sees more distant galaxies moving away faster.
Enter the letters of all correct statements in alphabetical order (without spaces). For example, if statements C and E are correct, enter CE.
DEF
Correct
Like any scientific model, the raisin cake analogy has limitations, but it gives us a good overall picture of how the universe is expanding.

18. Are we ever sitting still? No!
Earth rotates on axis: > 1,000 km/hr
Earth orbits Sun: > 100,000 km/hr
Solar system moves among stars: ~ 70,000 km/hr
Milky Way rotates: ~ 800,000 km/hr
Milky Way moves
in Local Group
This slide summarizes our motion with spaceship Earth…
Universe
expands

19. Patterns in the Night Sky

20. What are constellations?
A constellation is a region of the sky.
88 constellations fill the entire sky (North & South).
Remind students that we often use the term “constellation” to describe a pattern of stars, such as the Big Dipper or the stars that outline Orion. However, technically a constellation is a region of the sky (and the patterns are sometimes called “asterisms”). A useful analogy for students: a constellation is to the sky as a state is to the United States. That is, wherever you point on a map of the U.S. you are in some state, and wherever you point into the sky you are in some constellation.

21. Thought Question The brightest stars in a constellation…
all belong to the same star cluster.
all lie at about the same distance from Earth.
may actually be quite far away from each other.
You can use this question both to check student understanding of the idea of a constellation and as a way of leading into the concept of the celestial sphere that follows.

22. Thought Question The brightest stars in a constellation…
all belong to the same star cluster.
all lie at about the same distance from Earth.
may actually be quite far away from each other.
You can use this question both to check student understanding of the idea of a constellation and as a way of leading into the concept of the celestial sphere that follows.

23. The Celestial Sphere
Stars at different distances all appear to lie on the celestial sphere.
The ecliptic is the Sun’s apparent path through the celestial sphere.
The illusion of stars all lying at the same distance in the constellations allows us to define the celestial sphere. It doesn’t really exist, but it’s a useful tool for learning about the sky. When discussing this slide, be sure to explain:
North celestial pole
South celestial pole
Celestial equator
Ecliptic
It’s also very useful to bring a model of the celestial sphere to class and show these points/circles on the model.

24. The Celestial Sphere
If you do not have a model of the celestial sphere to bring to class, you might wish to use this slide; you will probably want to skip it if you have a model that you can discuss instead…

25. The Milky Way
A band of light making a circle around the celestial sphere.
What is it?
Our view into the plane of our galaxy.
On the previous slide or your model, you can point out that the celestial sphere is also painted with the Milky Way. Many students may never have seen the Milky Way in the sky (especially if they live in a big city), so the photo here is also worth showing. Key points to emphasize:
We use the term Milky Way in two ways: for the band of light in the sky and as the name of our galaxy.
(2) The two meanings are closely related. We like to use the following analogy: Ask your students to imagine being a tiny grain of flour inside a very thin pancake (or crepe!) that bulges in the middle and a little more than halfway toward the outer edge. Ask, “What will you see if you look toward the middle?” The answer should be “dough.” Then ask what they will see if they look toward the far edge, and they’ll give the same answer. Proceeding similarly, they should soon realize that they’ll see a band of dough encircling their location, but that if they look away from the plane, the pancake is thin enough that they can see to the distant universe.

26. The Milky Way
On the previous slide or your model, you can point out that the celestial sphere is also painted with the Milky Way. Many students may never have seen the Milky Way in the sky (especially if they live in a big city), so the photo here is also worth showing. Key points to emphasize:
We use the term Milky Way in two ways: for the band of light in the sky and as the name of our galaxy.
(2) The two meanings are closely related. We like to use the following analogy: Ask your students to imagine being a tiny grain of flour inside a very thin pancake (or crepe!) that bulges in the middle and a little more than halfway toward the outer edge. Ask, “What will you see if you look toward the middle?” The answer should be “dough.” Then ask what they will see if they look toward the far edge, and they’ll give the same answer. Proceeding similarly, they should soon realize that they’ll see a band of dough encircling their location, but that if they look away from the plane, the pancake is thin enough that they can see to the distant universe.

27. How do we locate objects in the sky?
An object’s altitude (above horizon) and direction (along horizon) specify its location in your local sky.
Now we move from the celestial sphere to the local sky. The local sky looks like a dome because we see only half the celestial sphere. If we want to locate an object:
It’s useful to have some reference points. Students will probably already understand the horizon and the cardinal directions, but explain the zenith and the meridian; a simple way to define the meridian is as an imaginary half-circle stretching from the horizon due south, through the zenith, to the horizon due north.
Now we can locate any object by specifying its altitude above the horizon and direction along the horizon. A good way to reinforce this idea is to pick an object located in your class room, tell students which way is north, and have them estimate its altitude and direction.

28. We measure the sky in angles...
Point out that in general we have no way of judging true (physical) sizes and distances of objects in the sky -- like the illusion of stars lying on the celestial sphere, this is due to our lack of depth perception in space. Thus, we can measure only angular sizes and distances. Use these diagrams as examples. Optional: You can show how angular sizes depend on distance by having students sitting at different distances from you in the class use their fists to estimate the angular size of a ball you are holding. Students in the back will measure a smaller angular size.
blank

29. Angular Measurements Full circle = 360º 1º = 60 (arcminutes)
1 = 60 (arcseconds)
Use this slide if you want to review the definitions of arc minutes and arc seconds.

30. Thought Question
The angular size of your finger at arm’s length is about 1. How many arcseconds is this?
60 arcseconds.
600 arcseconds.
60  60 = 3,600 arcseconds.
This is a quick test of whether students understand what we mean by arcseconds.

31. Thought Question
The angular size of your finger at arm’s length is about 1. How many arcseconds is this?
60 arcseconds.
600 arcseconds.
60  60 = 3,600 arcseconds.
This is a quick test of whether students understand what we mean by arcseconds.

32. Why do stars rise and set?
Earth rotates west to east, so stars appear to circle from east to west.
The answer to the question is very simple if we look at the celestial sphere from the “outside.” But of course, we are looking from our location on Earth, which makes the motions of stars look a little more complex…

33. What moves? The Earth or the sky?

34. Celestial Sphere Zenith: Point directly overhead Horizon: Where
the sky
meets the
ground
SKIPPED THIS 1/31/08

35. Celestial Sphere North Celestial Pole: Point on celestial sphere
above North
Pole
Celestial
Equator:
Line on
celestial
sphere
above Equator
SKIPPED THIS 1/31/08

36. Our view from Earth
Stars near the north celestial pole are circumpolar and never set.
We cannot see stars near the south celestial pole.
All other stars (and Sun, Moon, planets) rise in east and set in west.
A circumpolar star never sets
Now explain the basic motion of the sky seen from Earth. THIS IS FOR THE NORTHERN HEMISPHERE!
Celestial equator
This star never rises
Your horizon

37. (lots of questions during lecture, plus organizational stuff)
Ended Here 1/31/08
(lots of questions during lecture, plus organizational stuff)