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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
Copyright © 2015, W. W. Norton & Company
What is Astronomy?
 Loosely translated
– “finding patterns
amongst the stars”
 Modern definition –
the study of the
universe
What is Astronomy?: Purpose
 Astronomy seeks to
understand:
• Our place in the universe.
• Our connection to the
universe.
• Our astronomical origins.
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
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.
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
Class Question
Class Question: Which of the following ranks the
sizes of the listed objects in the correct order, from
smallest to largest?
A.
B.
C.
D.
Asteroid, Galaxy, Solar System, Universe
Solar System, Galaxy, Asteroid, Universe
Asteroid, Solar System, Galaxy, Universe
Universe, Galaxy, Asteroid, Solar System
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.
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
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) = 365.25 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!
Our Place in the Universe: Light-Year (Cont.)
Our Place in the Universe: Light-Year (Cont.)
Our Place in the Universe: Light-Year (Cont.)
Our Place in the Universe (Cont.)
Our Place in the Universe (Cont.)
Our Place in the Universe (Cont.)
Our Place in the Universe (Cont.)
Class Question
Which of the following is a distance?
A. 500 light-years
B. 500 years
Class Question
Which of the following is a time?
A.
B.
C.
D.
500 light-years
8.3 light-minutes
4 hours
2.5 light-hours
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!
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.
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.
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.
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.”
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.
Scientific Method
The scientific method
works like this:




Idea
Hypothesis
Prediction
Test
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.
Scientific Method: Agreement with Nature
 Existing, accepted ideas
are subject to challenge
via the scientific method.
 All theories must ultimately
agree with nature.
Class Question
Which of the following represents a scientific way
of looking at nature?
A. All ideas are equally valuable and should be
equally respected.
B. Well-established ideas should never be checked
or tested.
C. Nature informs us about the usefulness of our
ideas.
Class Question
Which of the following is a valid scientific
Hypothesis?
A. Parts of the universe will never be discovered.
B. Stars generate energy by burning coal.
C. There are processes in nature that we will never
understand.
Class Question
Do you agree with the following statement?
“Evolution cannot be correct because it is just
a theory.”
A. Yes
B. No
Our Connection to the Universe: Patterns in Nature
 Science discovers patterns in nature.
 Most phenomena work regularly and predictably.
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.
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.
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.
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.
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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
wwnpag.es/uou2
Copyright © 2015, W. W. Norton & Company