Introduction to Astronomy

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Introduction to Astronomy

• Announcements
• HW 1 due Wednesday 06/18/2008
• Course Reserves

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Project Details
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Project 1 Homemade Spectroscope

• Chapter 4 in textbook
• In this project, you will build a simple
  spectroscope from a cardboard tube, aluminum
  foil, and a grating (which will be supplied)
• Construction details can be found at the end of
  Chapter 4 (pg. 144)

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• You will sketch the spectra you see (more on this
  later) from
• Fluorescent light
• Mercury/Sodium vapor streetlight
• Ordinary incandescent light bulbs
• The blue sky (DO NOT LOOK AT THE SUN!)
• Flames (use gas-burning stove, add salt to see
  sodium emission lines and copper wire to see
  green copper emission lines)
• Extra credit for any other sources you want
• This writeup must include a picture of your
  spectroscope. You will keep the real thing.

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Project 2 Moon Observation

• Chapter 6 in textbook
• Look at the Moon on an evening when it is nearly
  full. Make a sketch of the light and dark
  markings that you see on its surface with the
  naked eye.
• Then observe the Moon with binoculars or through
  a telescope (PDO is helpful here) and make an
  enlarged sketch that shows more detail. Mark
  identify a few of the craters you can see.

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• Estimate the diameter of these craters from the
  knowledge that the Moons radius is about 1000
  miles (1700 km). How big is the largest crater
  you can see compared to the size of Logan? Can
  you you see any lunar rays? If so, sketch them
  on your drawing. How long are the rays?
• Can you mark the landing sites where humans have
  touched-down?
• SHOW ALL STEPS OF YOUR WORK!!!

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Project 3 Solar Observation

• Chapter 11 in textbook
• NEVER LOOK DIRECTLY AT THE SUN WITH THE NAKED
  EYE, OR THROUGH BINOCULARS/TELESCOPE!!!!!!!!!
• Measure the diameter of the Sun.
• Take a piece of thin, dark cardboard and put a
  small hole in it. Hold it about 1 meter (3 feet)
  from a piece of white paper so that a small image
  of the sun appears on the paper.
• Carefully measure the distance (d) between the
  cardboard and the piece of paper and the size of
  the Suns image (s) on the paper.
• On a separate piece of paper, draw two straight
  lines that cross with a small angle between them
  (see figure)

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• Draw two small circles between the lines as shown
  in the figure. Convince yourself that if D is
  the distance to the sun (1 AU), and S is the
  Suns diameter, then S/D s/d
• s size of Suns image
• d distance between paper and cardboard
• Look up the value of D, then solve for S
• Does it agree with the value in table 11.1?
• SHOW ALL YOUR WORK!!!

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Light Atoms
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Light Atoms
What is Newton holding? What were the
results of this experiment?
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Properties of Light

• Wave-particle duality
• Light has wave-like properties, and particle-like
  properties, depending on the type of observation
• Weird, right?
• Analogy you are wearing a hat. Two people
  observe you from different positions, but only
  the one wearing glasses sees the hat

Light is a wave on Monday, Wednesday, and
Friday, and a particle on Tuesday, Thursday, and
Saturday. On Sunday, we have to think about it
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• Wave-like
• Interference, diffraction
• Like overlapping
  water waves
• Particle-like
• Photoelectric effect
• Like a game of marbles

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The Schizophrenic Photon
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• Interference cannot be described by the particle
  model, and the photoelectric effect cannot be
  explained by the wave model
• But we have observed both!

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Similarities
This sine-wave goes on forever in both
directions, so it is hard to pinpoint the exact
location of the wave
A particle, on the other hand, is very localized,
so it has a well-defined position
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But adding many different waves gives a
very localized wave-packet and these
wave- packets behave a lot like particles!
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We will usually use the wave model of light from
here on outbut well briefly revisit the
photon model when we talk about CCDs in the next
chapter
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Properties of Light

• Color
• Not physical, all a psychological construct to
  help the brain sort out different wavelengths of
  visible light
• ?red 700 nm
• ?blue 400 nm
• 1 nm 10-9 m

English physicist John Dalton (1766-1844), worked
on colored shadows, color blindness when he
discovered pink flowers appear blue to him
He became obsessed with trying to discover
the cause of color-blindness, so he arranged for
his doctor to REMOVE ONE OF HIS EYES, so Dalton
himself could dissect it to look for blue fluid
inside that would cause his condition!
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Characterizing Light as a Wave

• Self-sustaining electric and magnetic vibrations

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Characterizing Light as a Wave

• Wavelength
• Distance between successive crests or troughs
  of the wave

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Characterizing Light as a Wave

• Frequency
• Imagine you are standing next to a traveling
  light wave (or water wave, if you prefer) that
  passes you
• How many peaks pass you in 1 second?
• Frequency of light
• Speed of Light c Wavelength
  ?
• Speed of Light, c 3.0 x 108 m/s

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Properties of Light

• White light
• Mixture of all visible colors
• Why doesnt mixing paint of all colors produce
  white paint?
• Chemical reactions due to pigment

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The Visible Spectrum
Our eyes are sensitive only to an EXTREMELY
narrow range of light waves ? Visible or
Optical light
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The Electromagnetic Spectrum

• Visible light constitutes a tiny, tiny fraction
  of the whole range of light
• Our eyes are only sensitive to visible light, but
  other types of light are all around us
• Radio waves, TV waves, cellphone signals, body
  heat
• What would the world be like if you could see at
  radio wavelengths?

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The EM Spectrum on Earth

• Radio
• Music, television programs encoded into
  long-wavelength waves
• Wireless bluetooth devices
• Communications
• Infrared (IR)
• Distinguish between hot and cool objects
• Heat lamps at fast-food places cafeterias
• Nerves in skin register this type of light as
  heat

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• Visible
• Everything we can physically see
• Light bulbs
• Reflected sunlight (on Earth)
• Color
• Ultraviolet (UV)
• Suntanning
• Skin cells containing melanin produce Vitamin D
  when they absorb UV light
• Snow blindness
• Blacklights security watermarks

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• X-Ray
• Medical, dental X-rays
• Dock scanning equipment
• By-product of atomic/nuclear detonation
• Gamma-Ray (?)
• Highest energy
• Dock scanning equipment
• Radiation pasteurization
• Some normal perishables (meat, milk, fruits
  vegetables, etc) can be kept fresh
  (unrefrigerated) for weeks with a healthy dose of
  radiation to kill off anything nasty.
• Atomic/nuclear weaponry

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The EM Spectrum in Space

• Radio Pulsars, star remnants
• Microwaves Cold interstellar clouds,
  cosmic background radiation
• IR Young stars, planets, dust
• Visible Stars, the sun
• UV Hot, bright stars
• X-Ray Collapsed stars, black holes
• ?-Ray Active galaxies, GRBs

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The EM Spectrum

• All these different types of light are the SAME
  phenomena
• Self-sustaining vibrations of electric and
  magnetic energy
• The shape of these vibrating energies determines
  if the light is IR, UV, visible, etc
• Energy carried by light wave of wavelength, ?
• Energy hc / ?

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• Which carries more energy?
• Red light or Blue light ?
• Blue light or X-rays ?
• Infrared light or radio waves ?
• Gamma rays or Ultraviolet waves ?

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Properties of Light

• Temperature
• Hot objects emit light (electric stove, an iron
  worked by a blacksmith)
• Hotter objects emit shorter-wavelength light
• Wiens Law (pronounced Veen)
• Cool stove, black element
• A little hotter, red element
• A little hotter, yellow element
• Very hot, white element

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• Wiens Law
• Temperature constant ?max
• One of most important tools for astronomers to
  measure temperature of stars, planets,
  galaxies, etc

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Wiens Law

• Example
• Someone states that because an apple looks red,
  it must be emitting red light. Fortunately, you
  have taken USU 1040 and know that person is full
  of it. How would you show them?
• We can assume the wavelength of the red light is
  700 nm

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• Using Wiens Law, we can calculate the
  temperature that the apple must have in order to
  emit mostly red light
• We get Temperature 7000 F !!!
• Therefore, the apple clearly doesnt EMIT the red
  light, so it must only REFLECT it.

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The Atom
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The Atom

• Protons, Neutrons, and Electrons
• Planetary model of the atom
• Negatively-charged electrons orbit
  positively-charged nucleus
• Electromagnetic force holds atom together
• Typical size 10-10 m 1 ten-billionth of a
  meter
• About X times smaller than the width of a human
  hair
• X 500,000

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• Planetary model is easy way to visualize atoms
• But it is ultimately wrong!
• Accelerated charges radiate photons (light
  energy)
• Therefore, an orbiting electron would constantly
  lose energy (accelerated by centripetal force)
  and move to progressively lower orbits
• Imagine the International Space Station
  in orbit
• Ultimately, it would spiral in to the nucleus and
  the atom would destroy itself.
• Why is this clearly incorrect?

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Quantized Atoms

• Electrons only allowed to orbit at certain,
  discrete distances
• Painter on scaffold
• Developed from theory that even electrons have
  wave-like properties (like light)
• matter waves
• ONLY at small scales
• a person walking through a door does not diffract
  (spread out) into multiple people.
• Ice cubes do not suddenly teleport out of your
  glass and into your pocket

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A fundamental principle of Quantum Mechanics The
electron does not orbit the nucleus. It can be
anywhere in the electron cloud, but we cant
know precisely where until we measure it
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Origin of Light Spectra

• Electrons are not confined to single orbits.
• They can move to higher or lower orbits with
  different energies, under the right
  circumstances.
• Spring analogy
• Imagine proton and electron are connected by a
  spring.
• To move them further apart, must supply energy to
  stretch spring
• To move them closer together, some energy from
  stretched spring is released as the spring
  de-stretches

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• Analogy
• Fast lane slow lane highway
• Merging into fast lane REQUIRES energy
• Merging into slow lane GIVES UP energy
• Same for electrons jumping from one orbit to
  another
• Defines EMISSION ABSORPTION of light.

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• Emission of light energy de-stretching the
  spring
• Absorption of light energy stretching the
  spring
• Conservation of Energy
• Rules the Universe, you will NEVER
  break this law.
• Energy of emitted light difference in
  energy between upper and lower levels
• Difference between energy of upper lower level
  energy of absorbed light ( if NOT equal, NO
  absorption occurs)

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HOW is light emitted?

• The positively-charged nucleus and the
  negatively-charged electrons form a system with
  some amount of stored electrical energy
• Like a battery, positive and negative terminals
• If an electron moves to a lower orbit, closer to
  the nucleus, it creates an electrical disturbance
  in the system

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• A fundamental principle of electromagnetism is
  that an electric disturbance creates a magnetic
  disturbance, and vice versa
• Maxwells Equations
• The electrical disturbance produced by the
  electron moving down to a lower orbit creates a
  magnetic disturbance, which creates an electric
  disturbance, which creates a magnetic
  disturbance, ad infinitum
• Viola! A self-sustaining vibration of electric
  and magnetic energy Light !

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Use in Astronomy

• Because we cannot directly measure astronomical
  sources (with a probe, e.g.), we must analyze the
  light we get from them
• Spectroscopy
• Because the light we receive comes from the very
  hot atoms in a star, we expect that some
  properties of the light can tell us about what
  atom(s) emitted or absorbed it
• Yes, we can tell a whole lot just from light!

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Emission Spectra

• Produced when electrons move from higher energy
  orbits to lower energy orbits
• Emitting light in the process
• Because only certain orbits are allowed, only
  certain transitions are allowed, therefore only
  certain wavelengths of light are observed.

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• Different atoms have different sets of allowed
  electron orbits, so different atoms produce
  different emission spectra.
• Not too long ago, it was thought that all atoms
  emitted the same lighttriumph of quantum
  mechanics that it was able to describe the
  different spectra observed

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Absorption Spectra

• Now, suppose we shine a light through a cloud of
  Hydrogen gas
• The light that matches the energy difference
  between the upper and lower levels of a Hydrogen
  atom will be absorbed by that atom, while other
  wavelengths will pass unaffected.
• This causes the spectrum to contain all normal
  colors, but with dark bands at the absorbed
  wavelengths
• Absorption spectrum

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• Absorption lines appear at the same wavelengths
  as emission lines, for a given element.
• Emission spectra tell us about how hot an object
  is, and what it is made of.
• Absorption spectra tell us about what lies
  between us and an object.

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Announcements

• Homework 1 due tomorrow
• First Project Due 30 June 2008 (Monday)
• Class Website Troubles

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X-ray spectrum of hot gas From exploding star
Radio spectrum of cold gas cloud
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Stellar Classification by Spectra
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The Doppler Shift

• Can determine chemical composition of object by
  emission and absorption spectra, but how?
• Compare observed lines with pure lines measured
  in laboratory
• line catalog
• But any motion of the object will change the
  observed wavelengths of emission and absorption
  lines

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• Analogy
• Firetruck approaches ? high pitch
• Sound waves pile up in front of firetruck,
  moving toward you
• Firetruck recedes ? low pitch
• Sound waves stretch out behind firetruck,
  moving away from you

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• Exact same thing can happen with light waves
• If atom moves toward you when it emits light
• Wavelength decreases blueshifted
• If atom moves away from you when it emits light
• Wavelength increases redshifted

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Doppler Shift

• Physics can get you in trouble with the law
• Photoradar speed-traps use the Doppler effect to
  measure car speeds

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Atmospheric Absorption

• Gases in Earths atmosphere (N2, O2, Ar, CO2)
  absorb light from distant sources
• Sunlight
• Astronomical sources
• Atmospheric Window
• The reason our eyes are sensitive to visible
  light is because it is NOT easily absorbed by the
  Earths atmosphere
• This is also the reason why we need space-based
  telescopes to observe in the IR, UV, X-ray
  regions of the EM spectrum

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NEXT TIME

• Telescopes
• How do we capture all these light waves?