Astronomy 100 Tuesday, Thursday 2:30 - 3:45 pm Tom Burbine [e-mail] www.xanga.com/astronomy100

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Astronomy 100Tuesday, Thursday 230 - 345
pmTom Burbinetburbine_at_mtholyoke.eduwww.xanga.
com/astronomy100
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Help Desk

• There is an Astronomy Help Desk in HAS 205.  It
  will be open from Monday through Thursday from
  7-9 pm.

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Homework Assignment(Due Today)

• Make up a test question
• Multiple Choice
• A-E possible answers
• 1 point for handing it in
• 1 point for me using it on test
• The question needs to be on material that will be
  on the March 10th exam

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Sample Question

• What is the atomic mass of Krypton which has the
  symbol Fr?
• A) 74.64
• B) 26.34
• C) 87.49
• D) 83.80
• E) None of the above

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Sample Question

• What is the escape velocity of a spacecraft from
  the surface of Saturn?
• A) 1,258,451,118
• B) 1,259,451,118
• C) 1,258,451,118.03
• D) 1,258,451,118.09

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Sample Question

• Earth is which number planet from the Sun?
• A) 4
• B) 2
• C) 7
• D) 3
• E) 8

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Sample Question

• How many days are in a sidereal month?
• A) 29.5
• B) 27.3
• C) 31
• D) 365

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Credit for test question

• I am not giving credit for test questions that
  have no possibility for making it on exam

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Test

• Thursday March 10th
• Will cover Chapters 4, 5, 6, and 7
• Will not cover Supplemental chapters 2 and 3
• If there is a problem with taking the test on
  March 10th, I need to know today
• I will give you all constants you need to know
• But you need to memorize formulas

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Constants (given on top of test)

• c 3 x 108 m/s
• G 6.67 x 10-11 m3/(kg s2)
• h 6.626 x 10-34 joule second
• g 9.8 m/s2

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Formulas (so far)

• E mc2
• KE ½ mv2
• c frequency wavelength
• E hfrequency
• F mass acceleration
• Angular momentum m v r
• Escape velocity square root (2GMplanet/Rplanet)
• F G M1 M2
• distance2

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Homework (due last thursday)

• Calculate acceleration of gravity
• Calculate escape velocity
• Of Mars and Jupiter

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acceleration of gravity

• F M2a G M1 M2 the object is M2
• R2
  M1 is the mass of the planet
• a G M1
  r is the Earths radius
• R2
• a 6.67 x 10-11 m3/(kg?s2) M2
• R2
• Make sure you use kg, meters, seconds

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Escape velocity

• Velocity above this will allow an object to
  escape a planets gravity
• v square root(2 x G x M)/r
• Make sure you use kg, meters, seconds

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Homework (due today)

• In Joules, calculate the typical energy of one
• Gamma ray
• X-ray
• Ultraviolet light
• Visible light
• Infrared light
• Radio
• photon

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Energy of light

• Energy is directly proportional to the frequency
• E h f
• h Plancks constant 6.626 x 10-34 J/s
• since f c/?
• Energy is inversely proportional to the
  wavelength
• E hc/?

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Homework on OWL

• Homework due Monday March 7th at 1159 pm
• 8 questions
• I will divide your number of points by 8 to
  calculate your score

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Another HW (due next Tuesday)

• Pick a telescope (earth-based or in space)
• When it was built
• Where it is located
• Tell me what wavelength (or energy or frequency)
  it observes in
• Tell me a discovery it has had

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So

• If you know the energy of a photon, you can
  calculate its wavelength and frequency
• If you know the wavelength of a photon, you can
  calculate its energy and frequency
• If you know the frequency of a photon, you can
  calculate its wavelength and energy

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So how we learn things about stars

• Composition
• Velocity
• Temperature

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Absorption and Emission lines

• Electrons can only reside in specific energy
  levels around a nucleus
• The energy of that energy level is an energy that
  the electron must have to reside there
• 1 eV 1.6 x 10 -19 Joules

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Electrons

• For an electron to go to a higher energy level,
  it must gain energy
• Either kinetic energy (something hits it)
• Absorbs a photon
• For an electron to go to a lower energy level, it
  must lose energy
• Emits a photon

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Energy emitted or absorbed

• E energy of higher energy level minus energy of
  lower energy level
• E E2 E1 h frequency hc/wavelength

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3.4 eV absorbed
10.2 eV emitted
10.2 eV absorbed
12.1 eV emitted
Doesnt happen
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Emission

• Emission radiation is emitted at characteristic
  wavelengths
• Material is hot so electrons keep on bumping
  into each other
• The bumping causes the electrons to transfer
  kinetic energy to each other
• The electrons have enough energy to jump to a
  specific energy level

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Emission (continued)

• When they jump back down, they emit radiation at
  characteristic energies
• Your telescope will only see light at specific
  energies (or wavelength or frequency)

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Absorption

• Absorption radiation is absorbed at
  characteristic wavelengths
• Radiation passes through the material
• Electrons absorb photons with the energy needed
  to jump to a higher energy level
• Photons that do not have the energy to cause a
  photon to jump to another energy level just pass
  through

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Heated hydrogen gas Emission line spectrum
wavelength
Intensity
White light through cool hydrogen gas Absorption
line spectrum
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Question everybody should be asking
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If electrons are absorbing radiation at
particular energies and then giving off photons
at the same energies, why do we see absorption
lines?Shouldnt the effects cancel out?
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Answer

• We see absorption occurring in just one direction
• But emission is occurring in all directions
• So average emission in our direction is very weak
  
• So absorption will be much stronger than emission
  in our line of sight (our direction)

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Important point

• Each type of atom has energy levels at different
  energies
• So each atom will have emission or absorption
  features at different wavelengths

Intensity
wavelength
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How can you all this to determine velocities?

• Doppler Shift The wavelength of light changes
  as the source moves towards or away from you
• Since you know the wavelength position of
  emission or absorption features
• If the positions of the features move in
  wavelength position, you know the source is moving

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So

• Source moving towards you, wavelength decreases
• Source moving away from you, wavelength increases

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Definitions

• Opaque light is absorbed
• Power rate of energy use Joules/second

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Thermal radiation

• Photons of light emitted inside an opaque object
  tend to bounce around inside the object
• The emitted radiation is called thermal radiation
  since it only depends on temperature

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Animation

• Thermal radiation

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• The thermal radiation spectrum is called a
  blackbody spectrum
• The shape of the blackbody spectrum only depends
  on temperature

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2 Rules

• Rule 1 Hotter objects emit more total radiation
  per unit surface area
• Rule 2 Hotter objects emit photons with a higher
  average energy

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Poker gets brighter when heated More
radiation is emitted
While heated, the poker goes from infrared to red
to white
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Blackbody curves
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Important formula (use on OWL HW)

• Stefan-Boltzman Law
• Emitted power per square meter s T4
• s 5.7 x 10-8 Watt/(m2 Kelvin4)
• Higher temperature, more power emitted

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Important formula (use on OWL HW)

• For a blackbody curve
• Wiens Law
• Wavelength (maximum intensity) 2,900,000 nm
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  T (Kelvin)
• Maximum intensity moves to shorter wavelengths
  with higher temperatures

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Put it all together
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PRS

• When is the next test?
• A) March 8
• B) March 9
• C) March 10
• D) March 11
• E) March 12

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Questions?