Transcript Diameter of the Milky Way

Chapter 1
Here and Now
Science is not…
• A list of previously known facts about
nature
• A list of equations handed down from
Ancient times
Science
• is a body of knowledge
• is an ongoing human activity
• has beginnings that precede recorded history
What is Science?
The SYSTEMATIC study of the Universe
Gather facts
Modify hypothesis
Guess an explanation
(Guess=hypothesis)
Test hypothesis
The Scientific Attitude
The scientific attitude is one of
• inquiry
• experimentation
• willingness to admit error
The Scientific Attitude
Fact
a close agreement by observers about the same
phenomenon
Hypothesis
• an educated guess presumed to be factual until
supported by experiment
• scientific if there is a test to prove it wrong
The Scientific Attitude
CHECK YOUR NEIGHBOR
Which of these is a scientific hypothesis?
A.
B.
C.
D.
The Moon is made of green cheese.
Atomic nuclei are the smallest particles in nature.
A magnet will pick up a copper penny.
Cosmic rays cannot penetrate the thickness of your Conceptual
Physics Fundamentals textbook.
The Scientific Attitude
CHECK YOUR ANSWER
Which of these is a scientific hypothesis?
.
A
The Moon is made of green cheese.
B.
C.
D.
Atomic nuclei are the smallest particles in nature.
A magnet will pick up a copper penny.
Cosmic rays cannot penetrate the thickness of your Conceptual Physics
Fundamentals textbook.
Explanation:
All are scientific hypotheses! All choices not only have tests for proving
wrongness, but have been proved wrong. Nevertheless, they still pass the test of
being a scientific hypothesis.
The Scientific Attitude
CHECK YOUR NEIGHBOR
Which of these is not a scientific hypothesis?
A.
B.
C.
D.
Protons carry an electric charge.
Undetectable particles are some of nature’s secrets.
Charged particles will bend when moving in a magnetic field.
All of the above are scientific hypotheses.
The Scientific Attitude
CHECK YOUR ANSWER
Which of these is not a scientific hypothesis?
A.
B.
C.
D.
Protons carry an electric charge.
Undetectable particles are some of nature’s secrets.
Charged particles will bend when moving in a magnetic field.
All of the above are scientific hypotheses.
Explanation:
If protons didn’t carry electric charge, they wouldn’t be deflected when crossing a
magnetic field. This would be a test for showing the hypothesis wrong. So both A
and C are capable of being proved wrong, which makes them scientific. Statement
B, however, has no test for wrongness. It is reasonable speculation—but not a
scientific hypothesis.
A scientific theory is a collection of ideas that explain a
phenomenon in a way that is consistent with laws,
observations and experiments.
Understanding the
Universe!!
Viking I Lander Picture from 1976
1999 Picture
from the Mars
Pathfinder
Lander
Note the remotecontrol rover,
Sojourner, next to
a Martian rock
2004 Opportunity Landing Site – and tracks
We did it!! Huygens takes first images
of Titan and survives the crash landing!!
http://www.esa.int/SPECIALS/Cassini-Huygens/index.html
Telescopes gather light
emitted from objects in the
universe
Too Much to Comprehend!
The number of stars in the universe
today is approximately equal to the
number of grains of sand on all the
beaches of Earth!!
The numbers in astronomy are so
large, and small, that astronomers use
scientific notation.
100 = 1
101 = 10
102 = 100
103 = 1000
5.3 x 103 = 5,300
104 = 10,000
8.9 x 104 = 89,000
and, for small numbers
10-1 = 0.1
10-2 = 0.01
2.1 x 10-2 = 0.021
10-3 = 0.001
6.6 x 10-3 = 0.0066
Astronomical distances and sizes are
very very very very large. So,
astronomers use different units.
One “Astronomical Unit” (AU)
average distance between Sun and Earth
• 93,000,000 miles
• 150,000,000 km
• 1.5 x 108 km
Distance Light Travels in One
Year is a “Light-year” (LY)
•9.46 x 1012 km
•63,000 AU or 6.3x 104 AU
•0.307 parsecs (pc)
At the scale of the size of a planet or moon we
describe things in terms of kilometers (km)
At the scale of the Solar System distances are
described in terms of the Astronomical Unit or AU
(ave. distance from Earth to the Sun).
At the scale of the Milky Way Galaxy distances are
described in terms of Light-years which is the
distance light travels in one year.
Thousands of km
Astronomical Unit
A few to about 1,000 Light-years
10,000 to 100,000 Light-years
Millions of Light-years
Billions of Light-years
Guidepost
In this chapter, you will meet three essential questions
about astronomy:
• Where are you in the universe?
• How does human history fit into the time scale of the
universe
• Why should you study astronomy?
Where are You?
To find our place among the stars, we will
zoom out from a familiar scene, to the
largest scales in the universe.
From each frame to the next, we
zoom out by about a factor 100.
A Campus Scene
16 x 16 m
A City View
1 mile x 1 mile
The Landscape of Pennsylvania
100 miles x 100 miles
The Earth
Diameter of the Earth: 12,756 km
Earth and Moon
Distance Earth – Moon: 384,000 km
Earth Orbiting Around the Sun
Distance Sun – Earth = 150,000,000 km
Earth Orbiting Around the Sun (2)
In order to avoid large numbers beyond our
imagination, we introduce new units:
1 Astronomical Unit (AU)
= Distance Sun – Earth =
150 million km
The Solar System
Diameter of Pluto’s orbit: Approx. 100 AU
(Almost) Empty Space Around
Our Solar System
Approx. 10,000 AU
The Solar Neighborhood
Approx. 17 light years
The Solar Neighborhood (2)
New distance scale:
1 light year (ly) =
Distance traveled by light
in 1 year
= 63,000 AU = 1013 km
= 10,000,000,000,000 km
(= 1 + 13 zeros)
= 10 trillion km
Nearest star to the Sun:
Approx. 17 light years
Proxima Centauri, at a
distance of 4.2 light years
The Extended Solar Neighborhood
Approx. 1,700 light years
The Milky Way Galaxy
Diameter of the Milky Way: ~ 75,000 ly
The Local Group:
Our Cluster of Galaxies
Distance to the nearest large galaxies:
several million light years
The Universe on Very Large Scales
Clusters of galaxies are grouped into superclusters.
Superclusters form filaments and walls around voids.
The number 7.14 x 106 is equivalent to:
1.
2.
3.
4.
5.
7,140,000
7,140
0.00000714
714,106
75,684
20%
1
20%
2
20%
20%
3.
4
20%
5
Which of the following descriptions is a
definition of an astronomical unit (AU)?
1.
2.
3.
4.
5.
one ten-millionth of the
distance between the
equator and the North
Pole
the distance between the
king’s nose and the tip of
his middle finger
the distance between the
sun and Proxima Centauri
the average distance
between Earth and the
sun
the distance that light
travels in one year
20%
1
20%
2
20%
20%
3
4
20%
5
What types of distances are typically
listed in astronomical units (AU)?
20%
1.
2.
3.
4.
5.
20%
20%
20%
3
4
20%
Distances to stars
Distances between
galaxies
Distances in the solar
system
The diameter of the
Milky Way Galaxy
The diameter of the
universe
1
2
5
Which of the following lists is in order of
increasing size?
20%
1.
2.
3.
4.
5.
20%
20%
20%
3
4
20%
universe - Milky Way
Galaxy - solar system
Milky Way Galaxy solar system - universe
solar system - Milky
Way Galaxy – universe
universe - solar system Milky Way Galaxy
solar system – universe
- Milky Way Galaxy
1
2
5
Consider the first letter of each of the eight planets’
names. How many entire planet names can be
unscrambled from these eight letters?
25%
1.
2.
3.
4.
25%
25%
2
3
25%
0
1
2
3
1
4