Great ideas have their limitations….
Foundations of Modern Astronomy
(and physics)
• Ancient Greeks
• Celestial Models
• Assumptions
– Earth stationary
– All motions around the Earth
– All motions are uniform, circular motions
• Is that such a big problem?
• Evidence for above assumptions?
• Time for some more animation…
Laws, Theories, Hypothesis
What is the difference?
What about
Natural Law
Scientific Law
Empirical Law
Are theories the end of the discussion?
Claudius Ptolemy (90-168)
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Years of previous models, data
Tweaked the models to fit data better
Still not perfect, but the best so far
So that’s all she wrote on that, right?
Epicycle
Mars
Equant
Earth
Deferent
Nicolaus Copernicus (1473-1543)
• New view – Heliocentric
• Not better!!!
Johannes Kepler (1571-1630)
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Finally got it right
Kepler’s 3 Laws of Planetary Motion
What do they tell us?
What don’t they tell us?
Law #1 – The orbits of the planets are ellipses
with the sun at one focus.
Perihelion
Aphelion
Focus
Average distance = (distance from perihelion to aphelion)/2 =?
Average distance = 1 Astronomical Unit
Law #2 – A line from a planet to the sun sweeps
over equal areas in equal intervals of time.
Huh?
Let’s try this – planets move faster when they are
at perihelion than when they are at aphelion.
Velocity changes depending upon their distance
from the sun.
Why?
Law #3 – A planet’s orbital period squared is
proportional to its average distance from the sun cubed.
Mathematically:
P2  a3
P2 = k a3 (k = constant)
Special formula –
If orbiting the sun
P in years, a in AU
P2 = a3
Kepler’s laws – not just for
breakfast anymore!
Actually not just for planets.
Kepler’s laws apply to anything orbiting
Moons orbiting planets
Comets orbiting the sun
Stars orbiting other stars
Stars orbiting around a galaxy
Galaxies orbiting other galaxies
Groups of galaxies orbiting other groups of
galaxies
Galileo Galilei (1564-1642)
• Invented the telescope
• Used the telescope and wrote about it!
• Observations favored Kepler, not Ptolemy
Isaac Newton (1642-1727)
• The ultimate nerd
• Did NOT “invent” gravity
• Did derive the 3 Laws of Motion
Law #1 – A body continues at rest or in uniform motion
in a straight line unless acted upon by some force.
Huh?
Let’s try this 1. Moving Things: straight line, steady pace
2. Non-moving things: well, nothing.
3. The above (1 & 2) remain that way unless
something messes around with the things.
4. Change in motion is caused by a force.
(Change in motion = change in speed, direction)
Law #2 – A body’s change in motion is proportional to
the force acting on it and is in the direction of the force.
Change in motion = acceleration (which can also
be a deceleration)
Force  acceleration
Force = mass x acceleration
F=ma
Mass  weight
Mass = stuff, material, matter
Law #3 – When one body exerts a force on a second
body, the second body exerts an equal and opposite
force back on the first body.
Huh?
Cosmic Karma
“Little brother/sister rule”
Forces are a two way street – you get as good as you
give…
Newton’s Law of Gravity
GMm
F  2
r
F = force of gravity
negative = towards
G = constant
M= one mass, usually the larger
m = the other, usually the smaller
r = distance between centers of masses (usually centers)
Ramifications of gravity
• Limits?
• Composition?
• Changes in motion
– Objects change direction (acceleration)
– Objects change speed (acceleration)
• Distance is squared!
• Feel the effects of gravity?
• Feel the same/different amounts of gravity?
Orbits
• Objects orbit due to gravity
• Change from straight motion to curved path
• Can it explain Kepler’s Laws?
What isn’t explained by Newton?
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How does gravity work?
What is actually causing the change in motion?
Is there “anti-gravity”?
Do Newton’s Laws explain all motions?
Did he invent the fig newton?
• Time to shoot off Newton’s cannon!
Light
• Almost all astronomical data is in this form!
• More than just visible
• Acts like a wave
– Effected by velocity (Doppler effect)
– Measureable wavelength, frequency, velocity
• Acts like a particle
– Photon=particle of energy
– Amount of energy depends on wavelength/frequency
Types of light
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Radio
Microwave
Infrared
Visible (ROYGBIV)
Ultraviolent Ultraviolet
X-ray
Gramma-ray Gamma-ray
How are they
different?
Wavelength
Energy
DANGER!!!!
How are they the
same?
VELOCITY!
Spectra
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Interaction of light and matter
Why?
Light=Energy
Matter made up of atoms
Atoms made up of protons, neutrons, electrons
Electrons influenced by energy
e
p
e
p
Atoms and Light
• Atoms can absorb light depending upon density
– Individual atoms discretely absorb some light
– More atoms (denser) absorb more types of light
– Solid objects absorb most types of light
• Atoms can emit light depending upon density
– Individual atoms discretely emit some light
– More atoms (denser) emit more types of light
– Solid objects emit most types of light
Spectrum
• “Rainbow” but so much more
• Light viewed in detail
– Seen over a range of wavelengths
– Seen over a range of frequencies
– Seen over a range of photon energies
• Type of spectra depends upon conditions
Kirchhoff’s Laws
• Absorption Spectrum
• Emission Spectrum
• Continuous Spectrum
• Type of spectrum reveals information on
– Composition (possibly)
– Temperature
– Density
Do both of these
illustrate the
same thing?
Stellar Spectra
What type of
spectrum is this?
How do you make sense of all of this?
Use basic rules for spectra
• If the spectra shows features for an element,
the element is present.
• If the spectra doesn’t show the feature for the
element, then the element isn’t present.
• Spectra sorted according to elements
presence/absence/unique features.
But…
“Absence of evidence is not evidence of
absence” – Carl Sagan
Are elements present/absent for another reason?
Yes – temperature!
Re-arrange those spectra!
Hottest
Coolest
Coolest
But they have already
been named/labeled!
Just rearrange the order
of the labels
Hottest
Spectral Classification System
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Classifies stars according to TEMPERATURE
Ordered from Hottest to Coolest
Originally included only those visible to eye
Extends to IR types now
• More detail with B0, B1, B2, etc
• Sun = G2
Fusion Confusion
• Not fission
• Energy released by fusing atoms
– Only useful for low mass atoms (<Fe)
– Creates more complex atoms
– Difficult to do
• High Density
• High Temperature
+
Deuterium
n
Helium 3
g
Proton
Helium 4
Proton
What does it all mean?
• Mass  Energy
• Hydrogen  Helium
• Evidence?