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 LawScientific LawEmpirical Law

Are theories the end of the discussion?

Claudius Ptolemy (90-168)

• 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?

Earth

Deferent

Epicycle

Mars

Equant

Nicolaus Copernicus (1473-1543)

• New view – Heliocentric• Not better!!!

Johannes Kepler (1571-1630)

• 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.

Focus

Perihelion Aphelion

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 sunP 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 planetsComets orbiting the sunStars orbiting other starsStars orbiting around a galaxyGalaxies orbiting other galaxiesGroups 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 pace2. 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 accelerationForce = mass x accelerationF = m a Mass weightMass = 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

2rGMmF

F = force of gravitynegative = towardsG = constantM= one mass, usually the largerm = the other, usually the smallerr = 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?

• 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

• Radio• Microwave• Infrared• Visible (ROYGBIV)• Ultraviolent• X-ray• Gramma-ray

Ultraviolet

Gamma-ray

How are they different?WavelengthEnergyDANGER!!!!How are they the same?VELOCITY!

Spectra

• Interaction of light and matter• Why?• Light=Energy• Matter made up of atoms• Atoms made up of protons, neutrons, electrons• Electrons influenced by energy

p

e

p

e

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

Hottest

Coolest

But they have already been named/labeled!Just rearrange the order of the labels

Spectral Classification System

• Classifies stars according to TEMPERATURE• Ordered from Hottest to Coolest• Originally included only those visible to eye• Extends to IR types now

OBAFGKMLT• 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

n

+

Deuterium

Helium 3

g

Helium 4

Proton

Proton

What does it all mean?

• Mass Energy• Hydrogen Helium• Evidence?