Chapter 9 Bradt

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Chapter 9Bradts Astronomy Methods

• Outline of Chapter Concepts
• ASTR 402
• Observational Astronomy
• Dr. H. Geller
• Department of Physics and Astronomy
• George Mason University
• Spring 2006

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Concepts Covered in Chapter 9

• Overriding Concept of Information Content of
  Radiation
• Luminosities of Objects
• Determination of Masses
• Determination of Temperatures
• Determination of Distances and Sizes

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Luminosity

• Measure of Luminosity
• At a particular wavelength
• Summed over all wavelengths
• Units
• Watts
• Joules per second

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Luminosity

• Energy flux density (W/m2)
• If radiation is emitted isotropically
• L F 4pr2
• Measurements are of F
• If distance combined with F you get L
• Understand differences between terms
• Brightness or intensity (observed flux density at
  Earth)
• Specific intensity (directional spectral energy
  flux density)
• Surface Brightness (power radiated from unit area
  of surface per unit solid angle in a given
  direction, bandwidth and frequency)

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Determination of Masses

• Application of Keplers Third Law of Planetary
  Motion
• Alternatively
• Apply Newtons Second Law of Motion, and
• Apply Newtons Law of Universal Gravitational
  Attraction

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Determination of Masses

• Mass of spiral galaxy
• Using velocities (Doppler) of stars in spiral
  arms
• Determination of period of revolution about the
  center of the galaxy
• Resultant mass of galaxy within the orbit of the
  stars observed
• Complications due to disk of galaxy
• Spiral galaxy masses not explained by luminous
  matter only
• Leads to existence of dark matter

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Determination of Masses

• Clusters of galaxies
• Application of Virial Theorem
• twice the total kinetic energy of all the
  particles pllus the sum of the potential energies
  equals zero
• Applies to gravitationally bound system of
  discrete objects in stable equilibrium
• Virial mass of a cluster is greater than the
  visible mass
• Dark matter on a larger scale size

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Determination of Temperatures

• Thermal and non-thermal radiation
• Kinetic temperature
• Maxwell-Boltzmann distribution
• Color temperature
• Ultraviolet, blue and visible
• Effective temperature
• Stefan-Boltzmann Law
• Excitation temperature
• Boltzmann formula

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Determination of Temperatures

• Ionization temperature
• Ratio of He I to He II
• Saha equation

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Determination of Distances
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