Starlight and What it Tells Us

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Starlight and What it Tells Us
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Brightness of Stars

• Variations in distance and intrinsic brightness
• Scale based on one by Hipparcos 500 B.C.
• Magnitude Large Numbers Fainter
• One magnitude 2.5 x
• Five magnitudes 100 x

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Magnitudes

• Planet around nearby star 30
• Pluto 13
• Faintest Naked-Eye Star 6
• Big Dipper Stars 2
• Sirius (Brightest Star) -1.6
• Venus -4
• Full Moon -12
• Sun -27

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Absolute Magnitude

• Altair and Deneb are about equally bright as seen
  from Earth
• Altair is 16 l.y. away, Deneb 1600
• Hence Deneb must be about 10,000 times brighter

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Absolute Magnitude

• How bright a star would be at a distance of 32.6
  l.y. (10 parsecs)
• Sun 4.5 (inconspicuous naked-eye star)
• Altair 2.2
• Deneb -7.1 (bright as crescent moon)
• Note Deneb - Altair about 10 magnitudes 100 x
  100 10,000 times

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Black-Body Radiation

• Objects Emit Radiation Because They Are Hot
• Why Black? Because None of the Radiation is
  Reflected from Some Other Source
• The Sun Emits Black-Body Radiation, the Moon Does
  Not

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Why Black-Body Radiation is so Important

• Color is directly related to temperature
• Temperature is the only determinant of color
• Energy per unit area is the same if temperature
  is the same
• If two stars have the same color and distance,
  difference in brightness is due to difference in
  size
• Dwarf and giant stars are literally dwarfs or
  giants

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Spectroscopy

• Different atoms absorb or emit specific
  wavelengths of light
• When light spread into a spectrum, the absorbed
  wavelengths show up as dark (missing) bands
• These spectral lines are indicators of
• Chemical composition
• Physical conditions

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The Solar Spectrum
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Spectral Lines are Affected By

• Electrical and Magnetic Fields
• Number of Electrons Atoms Have Lost (Indicates
  Temperature and Pressure)
• Motion (Doppler Effect)
• Blue-shifted if Motion Toward Observer
• Red-shifted if Motion Away From Observer

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The Doppler Effect
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What the Doppler Effect Tells Us

• Radial Motion
• Rotation of Stars
• Approaching side of star blue-shifted, receding
  side red-shifted
• Unseen Companions (Stars or Planets)
• Star oscillates around center of mass
• Surface and Interior Motions
• Changes in Size
• Interior Oscillations