1. Lecture Slides CHAPTER 1: Thinking Like an Astronomer
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will, San Diego City College 1

2. What is Astronomy?
Loosely translated – “finding patterns amongst the stars”
Modern definition – the study of the universe

3. What is Astronomy?: Purpose
Astronomy seeks to understand:
Our place in the universe.
Our connection to the universe.
Our astronomical origins.

4. A Few Vocabulary Terms
Universe: everything; the entirety of space and time
Galaxy: a massive system of stars (and other stuff such as interstellar gas and dust) that exists inside the universe
Solar system: a star and its surrounding planets, comets, asteroids, etc… that exists inside a galaxy

5. Our Place in the Universe: The Earth
Earth is a relatively small planet.
Orbiting an ordinary star
Orbiting a supermassive black hole at the center of the Milky Way galaxy.

6. Our Place in the Universe: The Milky Way
The Milky Way is a spiral galaxy.
Contains 100 billion stars
Just one of billions of galaxies
In a universe that is 13.8 billion years old

7. Class Question Class Question: Which of the following ranks the
sizes of the listed objects in the correct order, from
smallest to largest?
Asteroid, Galaxy, Solar System, Universe
Solar System, Galaxy, Asteroid, Universe
Asteroid, Solar System, Galaxy, Universe
Universe, Galaxy, Asteroid, Solar System
Correct Answer: C

8. Our Place in the Universe: How Far Away
The universe is huge!
One of the biggest challenges in astronomy is measuring the distance to stars and galaxies.
We use the travel time of light to measure the vast distances of the universe.

9. Our Place in the Universe: Speed of Light
Speed of light = 300,000 km/s = 186,000 mi/s
Cosmic speed limit!
So fast it could travel around the Earth in 1/7 of a second
Figure 1.2 (a)

10. Our Place in the Universe: Light-Year
A Light-year is a DISTANCE, NOT a TIME!
It’s the distance light travels in one year = 9.5 million million km
=9,500,000,000,000 km
In scientific notation, that’s 9.5 x 1012 km or 9.5 x 1015 m
Don’t believe me? Let’s take a little math break. It won’t hurt, really!
Exponents: 10,000 = 10 to what power?
0.01 = 10 to what power?
The speed of light is constant = 300,000 km/s = 3.0 x105 km/s
So a light year is a distance = speed*time
We know speed. We need time. How many seconds in a year?
T(in year) = days*24 hr/day * 3600 sec/hr = 3.16x107 s
Thus, distance of 1 light year
= speed*time = 3.0 x105 km/s * 3.16x107 s = 9.5 x 1012 km . Yay!
Figure 1.2 (b, c, d)

11. Our Place in the Universe: Light-Year (Cont.)
Figure 1.2 (b) 11

12. Our Place in the Universe: Light-Year (Cont.)
Figure 1.2 (c) 12

13. Our Place in the Universe: Light-Year (Cont.)
Figure 1.2 (d) 13

14. Our Place in the Universe (Cont.)
Figure 1.2 (e) 14

15. Our Place in the Universe (Cont.)
Figure 1.2 (f) 15

16. Our Place in the Universe (Cont.)
Figure 1.2 (g) 16

17. Our Place in the Universe (Cont.)
Figure 1.2 (h) 17

18. Class Question Which of the following is a distance? 500 light-years
Correct Answer: A

19. Class Question Which of the following is a time? 500 light-years
8.3 light-minutes
4 hours
2.5 light-hours
Correct Answer: C

20. Our Astronomical Origins
Stars generate energy by nuclear fusion, creating heavier elements in the process.
Dying stars eject those elements into space.
New stars and planets (and humans) form.
WE ARE STARDUST!

21. Our Connection to the Universe
We study the universe from both the ground and in space.
We use telescopes, spacecraft, physics experiments, etc… to expand our understanding of the universe.

22. Our Connection to the Universe: Science
Science is a way of exploring the physical universe
through the scientific method.
The scientific method is a systematic way of testing
new ideas.

23. Scientific Method Hypothesis: an idea that can explain a phenomenon.
Theory: a hypothesis which tests have failed to disprove.
Physical law: theories that have become very well tested and are of fundamental importance.

24. Our Connection to the Universe:
The Cosmological Principle
The cosmological principle: The testable assumption that the same physical laws that apply here and now also apply everywhere and at all times.
Another way of thinking about it: “There is nothing special about our place in the universe.”

25. Implications of Cosmological Principle
Our view from the Earth is not special or unique.
On a large scale, the universe is the same everywhere.
Matter and energy obey the same physical laws everywhere.
We can learn about distant objects by studying nearby ones. 25

26. Scientific Method The scientific method works like this: Idea
Hypothesis
Prediction
Test

27. Scientific Method: Testing
Scientific theories must be testable and falsifiable.
All scientific knowledge is conditional. Our understanding can change due to additional experimental findings.
=> This is how science makes progress.

28. Scientific Method: Agreement with Nature
Existing, accepted ideas are subject to challenge via the scientific method.
All theories must ultimately agree with nature.

29. Class Question Which of the following represents a scientific way
of looking at nature?
All ideas are equally valuable and should be equally respected.
Well-established ideas should never be checked or tested.
Nature informs us about the usefulness of our ideas.
Correct Answer: C

30. Class Question Which of the following is a valid scientific
Hypothesis?
Parts of the universe will never be discovered.
Stars generate energy by burning coal.
There are processes in nature that we will never understand.
Correct Answer: B

31. Class Question Do you agree with the following statement?
“Evolution cannot be correct because it is just
a theory.”
Yes No
Correct Answer: B

32. Our Connection to the Universe: Patterns in Nature
Science discovers patterns in nature.
Most phenomena work regularly and predictably.

33. Our Connection to the Universe: Language of Patterns
Mathematics is the language of patterns.
It can describe and predict relationships, like the one between distance and time traveled during a car trip.

34. Mathematics can describe and predict phenomena like Earth’s seasons.
Our Connection to the Universe: Language of Patterns (Contd.)
Mathematics can describe and predict phenomena like Earth’s seasons.
Mathematics is a tool to express relationships concisely.

35. Our Connection to the Universe: Ancient Science
Astronomy is an ancient science.
Join the many humans who have thought about the universe.
Build understanding.
Share with others.

36. Chapter Summary Astronomy is the study of the universe.
We use the scientific method and the language of mathematics to help us understand:
Our place in the universe.
Our connection to the universe.
Our astronomical origins.

37. Nebraska Applet
Lookback Time Simulator
Click the image to launch the Nebraska Applet (Requires an active Internet connection)

38. This concludes the Lecture slides for
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will, San Diego City College
This concludes the Lecture slides for
CHAPTER 1: Thinking Like an Astronomer
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