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Astronomy 170: The Physical Universe
Prof. Jill Bechtold
Graduate TAs: Johanna Teske, Megan Reiter
All Students: Pick up 3 white handouts
Honors: Pick up 4th handout on black cart
The Textbook:
Recommended, Not Required
The Cosmic Perspective
By Bennett, Donahue,
Schneider, & Voit
6th edition
5th edition OK, but there are
always updates
An ebook version is also
available
Do I need to buy the book?
• Class notes will be posted
• Exams will be on lecture material
• If you like to also read the material in a
textbook, get the Bennett et al. book
• New books come with a key to use the
publisher’s web site, Mastering Astronomy,
which has “flash cards”, quiz questions, and
other study aides.
Honors Credit
• If you want honors credit for this class, be sure
you are signed up for Section 3.
• If you are in Section 2 and want honors credit,
please bring a change of class form for me to
sign as soon as possible
• Trip to Sky Center on Mt. Lemmon
• We will have an organizational meeting later
in the semester
What Is Modern Astronomy?
• Science of the nature of everything outside of the
Earth’s atmosphere: planets, stars, interstellar gas,
galaxies, etc.
• What is the content of the Universe?
– What is its history? What is its future?
• How do humans fit in?
"My goal is simple. It is complete
understanding of the universe,
why it is as it is and why it exists at all. "
-- Stephen Hawking
Goals of this course
• Survey of current state-of-the-art
astronomy and astrophysics
• Science literacy in the physical sciences;
current topics
• See how working research scientists think
Our Cosmic Address
STEWARD OBSERVATORY
Ca. 1930
Steward Observatory
THE UNIVERSITY OF ARIZONA
Steward Observatory
The University of Arizona
TUCSON
Steward Observatory
The University of Arizona
Tucson
ARIZONA
Steward Observatory, The University of Arizona
Tucson, Arizona
UNITED STATES, NORTH AMERICA
Steward Observatory, The University of Arizona
Tucson, Arizona
United States, North America
EARTH
EARTH
• Planet
• Diameter = 7900 miles
• Mass = 6x1024 kilograms
• Age = 4.5 billion years
= 4.5 x 109 years
= 4,500,000,000 years
Earth has one moon, which orbits it
The Moon
Earth-Moon distance
= 240,000 miles
or 30 Earth Diameters
Steward Observatory, The University of Arizona
Tucson, Arizona
United States, North America
Earth
SOLAR SYSTEM
(Not to scale)
SUN, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, (Pluto)
Solar system to scale:
SUN
THE SUN
• Star, produces energy by nuclear fusion
(Hydrogen  helium)
• 93 million miles from Earth
• 93 million miles = 1 Astronomical Unit, or A.U.
Visible light image
More on units of distance:
• Instead of miles, kilometers, etc. astronomers often
use light seconds, light minutes, light years
• 1 Light Year = the distance light travels in one year
• 1 Light Year = 5.9 x 1012 miles
THE MOON IS ABOUT 1.3 LIGHT
SECONDS FROM EARTH
THE SUN IS ABOUT 8.3 LIGHT MINUTES
FROM EARTH
Practical matters: Astronomers use scientific
notation to write large & small numbers.
3
10
1000 =
1,000,000,000 =
109
0.001 = 10-3
2,200,000 = 2.2 x 106
1 light minute = the distance light travels in one minute
1 light year = the distance light travels in one year
= almost 10 trillion kilometers or 6 trillion miles
(one trillion = 1012 = 1,000,000,000,000)
Because light takes time to travel through space, when we look
farther away in distance, we look farther back in TIME.
The farthest we can see in space is 13.7 billion light years,
because we think the Universe is 13.7 billion years old.
Orbit of Quaor (Q) and Pluto (P) versus the planets
STARS NEAR THE SUN
Nearest Star to Sun: Alpha Centauri
• Triple star in S. hemisphere
• 4.2 light years from the Sun
• Alpha Centauri A and Alpha Centauri B are orbiting each other
with period 80 years, separation 23 Astronomical Units.
• Proxima Centauri is 13,000 AU from Alpha Centauri A and B.
Space Travel
Speed
Time to
get to
Mars
Time to get
To Jupiter
Time to get
to Saturn
Time to get
to Alpha
Centauri
Freeway speed
75 mph
80 years
600 years
1200 years
38 million
years
Jetliner speed
500 mph 12 years
90 years
180 years
6 million
years
Concorde Speed
(Mach 2)
1350
mph
4.4 years
33 years
66 years
2 million
years
2 months
1.5 years
3 years
70,000 years
Spacecraft Speed
40,000
mph
Back to our COSMIC ADDRESS
Steward Observatory, The University of Arizona
Tucson, Arizona
United States, North America
Earth
Solar System
Milky Way Galaxy:
The Sun is one star of
many in the Milky Way
The Milky Way as seen from Earth
Spiral Galaxy