Transcript Scale of the Universe in space, time, and motion

General Astronomy
Instructor:
Prof. Kaaret
702 Van Allen Hall
E-mail: philip-kaaret [at] uiowa.edu
Phone: 335-1985
Class website:
http://astro.physics.uiowa.edu/~kaaret/genastro11s
Course topics
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Sun, stars
Black holes, neutron stars
Galaxies
Cosmology
Course elements
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Lecture
Homework
In-class exams
Final exam
Laboratory (must pass lab to pass course)
Lectures and Homework
Lectures: MWF 1:30 pm – 2:20 pm, LR70 VAN
Textbook: Foundations of Astrophysics, Ryden and
Peterson
Homework: About once per week, usually due on
Mondays, do in groups of 2-3 if that helps.
Laboratory
Lab: M or T 7:00 pm – 9:00 pm, 666 VAN
Lab will consist of several `canned’ labs and a
research project. Get started on the research
project early. It is possible to get extra credit
for an imaginative or extensive research
project.
Grading
One-hour exams (3 exams, each 100 points)
300
Final exam
250
Homework
200
Laboratory (up to 25 points extra credit)
250
Total
1000
Scales in length, time, and motion
• Astrophysics requires knowledge of the
Universe on the entire range of length scales
from sub-nuclear to cosmological.
• Knowing the typical size and time scales of a
system gives significant insight into that
system.
• Astronomical time scales can be extremely
long.
Sizes are in meters
People
Height of (small) person is about 1 m
People
• If the small person spins around, she can make
one revolution in about 1 second.
• The typical time scale for people, i.e. how fast
they react to some event (how long does it take
you to slam on the brakes if the car in front of
you stops), is of order seconds.
• The typical velocity scale for people is
Velocity = length/time ~ 1 m/1 s = 1 m/s
Typical walking speed is 3 mph = 1.3 m/s
Earth
Radius of earth or R is 6.4106 m
Sun
Radius of Sun or R is 7108 m
Earth to Sun
Distance from Earth to Sun is 1.51011 m
This is one “Astronomical Unit” = 1 A.U.
Earth’s Orbit
• The Earth makes one revolution about the Sun in one
year, or about 3107 seconds.
• A year is the time scale of the Earth’s orbit.
• The velocity scale for the Earth’s orbital motion is
Velocity = 2  1.51011 m/3107 s = 3104 m/s
= 30 km/s = 70,000 mph
• This is much faster than Earth bound speeds
A `speeding bullet’ travels at about 1000 m/s = 2000 mph.
Sun to Nearest Star
Distance from Sun to nearest star is 4.11016 m
Define light-year = ly = 9.46 1015 m
Distance from Sun to Alpha Centauri is 4.3 ly
to Center of Milky Way
Distance to Center of our galaxy is 2.61020 m
or 28,000 ly
Sun’s Orbit
• The Sun makes one revolution about the center of the
Milky Way in 230 million years, or about 71015
seconds.
• This is the `time scale’ of the Sun’s orbit and a reasonable
time scale for interactions of galaxies (how long does a
galaxy take to react to an event like a collision with
another galaxy).
• The velocity scale for the Sun’s orbital motion is
Velocity = 2  2.61020 m/71015 s = 2105 m/s
= 200 km/s
• This is an order of magnitude faster than the orbit of the
Earth around the Sun.
to Nearest (big) Galaxy
Distance to nearest (big) galaxy is 2.41022 m
or 2.6 106 ly
to edge of Observable Universe
Distance to edge of observable universe is
1.31026 m or 1.4 1010 ly
Carl Sagan’s “Cosmic Calendar”
The history of the Universe in one year
Big bang
Milky
Way
forms
Sun and Oldest
planets known
form
life single
cell
First
multicellular
life
Review Questions
• If one made a scale model of the Sun and its closest
neighbor star which could fit in a room, say with a
length of 5 m, what would be the diameter of the Sun?
Of the Earth?
• How long does it take light to travel from the Sun to
the Earth?
• Find Earth’s rotational speed at the equator from the
Earth’s diameter and the length of a day.
• Starting at Earth and driving on an interstellar highway
at 75 mph, how long would it take to reach Alpha
Centauri? The center of the Milky Way?