Transcript ASTR 1120 General Astronomy: Stars and Galaxies

ASTR 1200
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http://casa.colorado.edu/~wcash/APS1200/APS1200.html
Explaining the Universe
Science:
Model or Explanation
Test
Correction
Iterate
“Truth” -- All that’s left
Model of the Universe
Myth
Measurement
Modern Science
All That’s Left
Universe Unfolded on Itself in Space and Time
Big Bang and Expanding Universe
about 13 Billion Years ago
Formed Galaxies from gas
Formed Stars and Planets
Earth & Sun about 4.6 Billion Years Old
In this course we will explain why we think these things
about the universe. Plus more….
Question:
Why Are Our Atoms Arranged this Way?
e.g. Why are our atoms on a planet, not in a black hole?
HISTORY IS DESTINY
Make assumption about initial state of matter and then
follow what happens.
eg solar system was a gas cloud that collapsed due to gravity
Sun and Earth formed at same time
Formation of the Solar System
Universe is about 13 Billion Years old (4x1017s)
Milky Way 10 Billion Years old
Sun and Planets – 4.6 Billion Years old
Sun has done about 25 orbits of Milky Way
Stars are being formed all the time -- some young, some old
Stars are formed from clouds of gas
Starts as Thin Cloud
Ensuing High Density
Shocks Is Site of Collapse
Spins Up Like Ice Skater
Swirling Gas
Falls into “Disk Stability”
99.9% Ended in Sun (0.1% in Jupiter)
Probably the Same Around All Stars -- Planets are Common
Shoots Planet-Size
Bullets into Space
Lots of Stars
• 100 Billion Stars in Milky Way
• 100 Billion Galaxies in Visible Universe
• Almost an Avagodro’s Number in
Universe!
Catastrophe Theory
• Theory In Vogue When I was in Grade
School – (1950’s ouch!)
• Two Stars Stray Near Each Other
• Close Orbit Pulled Gas from Sun
• Gas Formed Planets
• Predicts Planets Very Rare
(Collisions Rare)
• Astronomy Advanced Hugely in 20th
Century
Question
Why was the Solar System formed from a
cloud of gas?
a) The Universe started as diffuse gas
b) Old stars explode to create new gas
c) The galaxy makes new gas that must
collapse
d) Stars never collide so they cannot form
solar systems
Galileo
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First Great Modern Scientist
Quantitative Analysis
Tenacious Pursuit of Truth
Challenged Aristotle and Church
Used Telescopes to Demolish Old Solar
System View
• Persecuted In Italy – Celebrated
Elsewhere
Isaac Newton (1643-1727)
• Greatest Scientist of All Time
• Quantitative Prediction
• Mechanics, Optics, Math – Theory & Observation
Two Basic Concepts
Mass – Amount of Matter (Not Weight)
Inertia – Tendency of Objects to Have Unchanged Motion
Motion
• x=v*t
• v=a*t
• x = ½ * a * t2
Motion
Position
X1 marks position in space at time t1
X2 marks position in space at time t2
Velocity
Acceleration
x2  x1
v
t 2  t1
Change of Position
with time
v1 is velocity at time t1
v2 is velocity at time t2
v2  v1
a
t 2  t1
Change of Velocity
with time
Question
A spaceship accelerates from rest at 20gees
(200m/s/s) steadily for 1.2 days (105
seconds). How far has it travelled?
a) 2x107m
b) 5000m
c) 1012m
d) 104m
Newton’s Laws
• Law #1: A body at rest or in motion
remains that way unless acted upon by an
outside force.
• Law #2: a=F/m
• Law #3: For every action there is an equal
and opposite reaction.
Law #1
A body in motion remains that way unless acted upon by an outside force.
No Forces
Contact Forces (electrical)
Gravity
Law #2
F=ma
a=F/m
F = Force
m = mass
a = acceleration
• Have a mass
• Apply a force
• Acceleration Results
Note: Law #1 is special case of Law #2 (F=0
a=0)
Example of Law #2
Push Your Car ---- Large m implies large F to get any a
Let m = 1000kg
If you apply 20N (Newtons is the unit of force)
a=20/1000=.02m/s2 ---- accelerates .02m/s for every second you push
Push for 10 seconds --- car will be moving .02m/sx10s= 0.2m/s
(if on ice so no friction)
(friction is yet another force)
Clicker Question
A force of 250 Newtons is applied to a
100kg object. How fast does it
accelerate?
a) 2.5m/s/s
b) 25m/s/s
c) 0.4m/s/s
d) 4m/s/s
Law #3
For every action there is an equal and opposite reaction.
For every ma there is an equal and opposite ma.
Big one doesn’t move much
Equal Mass Implies Equal Acceleration
But What is F?
a=F/m not enough because we don’t know what F is.
Need a new law to describe forces
There are Four KNOWN Forces
Gravity
Weak Nuclear Force
Electromagnetism
Strong Nuclear Force
In order of strength
Gravity Weakest
Newton’s Law of Universal Gravitation
Gm1m2
F21 
r2
Gm1m2
F12 
r2
(Law #3 Satisfied)
G = 6.7x10-11 in mks units
Value of G is measured. Nobody knows why it is the value it is.
Electrical Force Equation Similar
e1e2
Fe  2
r
Clicker Question
What is the weight of a 100kg person
standing on the surface of the Earth. (in
Newtons) M=6x1024kg. R=6000km
a) 6.7x1021N
b) 1.6x108N
c) 1000N
d) 6.7x10-3N
But What is a Force?
Newtons Laws Give Value, and Show what to do.
Force is an Exchange of Virtual Particles