Starlight and What it Tells Us

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Starlight and What it Tells Us
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The Stars in the Sky

• Vary in Brightness
• Distance
• Size
• Vary in Color
• Color Temperature

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Star Names

• Proper star names mostly Arabic
• Greek Letters, Numbers
• Catalog Identifiers
• Faint stars usually have no name

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The Names of Sirius

• Alpha Canis Majoris (Bayer, 1603)
• 9 Canis Majoris (Flamsteed, 1725)
• BD -16 1591 (Bonner Durchmusterung 1859-1903)
• HR 2491 (Harvard Revised Catalog, 1908)
• HD 48915 (Henry Draper, 1918-1924)
• ADS 5423 (Aitken Double Star Catalog, 1932)
• HIP 32349 (HIPPARCOS, 1997)

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The Heavens Are Not Changeless

• The Stars Move
• Most of our constellations would have been
  unrecognizable to Neanderthal Man
• The Solar System Moves
• Very few of our nearby stars would have been
  visible to the first humans
• Stars are Born, Live and Die
• Many of our brightest stars did not exist in the
  days of the dinosaurs

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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, Mars Does Not
• Close Example of pure Black-Body radiation
  Peephole in a pottery kiln

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Black Body Radiation
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Whats The Source of the Light?
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Color Temperature
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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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Sirius and the Pup
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Sirius and the Pup

• Sirius M -1.5 Pup M 8.5
• 10 magnitude difference
• 100 x 100 10,000 times brightness distance
• Sirius and the Pup are same color, therefore same
  temperature (Pup is hotter)
• Pup must have 1/10,000 the apparent area of
  Sirius 1/100 the diameter

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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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Atoms and Radiation
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The Solar Spectrum
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Spectra and Spectral Lines

• Continuous Spectrum Incandescent solids or
  liquids (steel mill) and dense hot gases (Suns
  photosphere)
• Emission Spectrum Thin hot gases (fireworks,
  sodium or mercury vapor lights, Suns
  chromosphere
• Absorption Spectrum Light shining through thin
  gases (Sun and star light)

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How the Chromosphere Works
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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

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Spectral Classification of Stars

• W very hot young stars expelling their outer
  layers
• Main Sequence O, B, A, F, G, K, M (hottest to
  coolest)
• Oh be a fine girl/guy, kiss me
• Subdwarfs L, T, Y (hottest to coolest)
• Chemically Peculiar Stars C, N, R, S
• White Dwarfs D

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Spectral Signatures of Stars

• O Ionized Helium
• B Neutral Helium
• A Strongest Hydrogen Lines
• F Ionized Calcium
• G Strongest Calcium Lines Neutral Metals
• K Neutral Metals Dominate
• M Titanium Oxide

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The Hertzsprung-Russell Diagram
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The Main Sequence O

• 30,000-60,000 K (Blue-white)
• Absolute Magnitude -5
• 1,000,000 times Suns Luminosity
• 16 times Suns Diameter
• 64 times Suns Mass
• Lifetime Less than a million years
• Examples Orion's Belt

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The Main Sequence B

• 10,000-30,000 K (Blue-white)
• Absolute Magnitude -3
• 20,000 times Suns Luminosity
• 7 times Suns Diameter
• 18 times Suns Mass
• Lifetime 10 million years
• Examples Spica

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The Main Sequence A

• Temperature 7500-10,000 K (White)
• Absolute Magnitude 0.5
• 40 times Suns Luminosity
• 2 times Suns Diameter
• 3 times Suns Mass
• Lifetime 600 million years
• Examples Vega, Sirius

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The Main Sequence F

• Temperature 6000-7500 K (Yellow-White)
• Absolute Magnitude 2.5
• 6 times Suns Luminosity
• 1.5 times Suns Diameter
• 1.7 times Suns Mass
• Lifetime 2.5 billion years
• Examples Procyon

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The Main Sequence G

• Temperature 5000-6000 K (Yellow)
• Absolute Magnitude 5
• 1 times Suns Luminosity
• 1 times Suns Diameter
• 1 times Suns Mass
• Lifetime 10 billion years
• Examples Sun, Alpha Centauri A

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The Main Sequence K

• Temperature 3500-5000 K (Orange)
• Absolute Magnitude 6
• 0.4 times Suns Luminosity
• 0.9 times Suns Diameter
• 0.8 times Suns Mass
• Lifetime 10 billion years
• Examples Alpha Centauri B

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The Main Sequence M

• Temperature 2000-3500 K (Red)
• Absolute Magnitude 10 to 15
• 0.04 times Suns Luminosity
• 0.5 times Suns Diameter
• 0.4 times Suns Mass
• Lifetime 5 trillion years
• 75 of all stars
• Examples Barnard's Star, Proxima Centauri

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Sub-Dwarfs

• L 1300-2000 K, Borderline stars with alkali
  metals and metal hydrides
• T 700-1300 K, Substellar, methane in spectra
• Y lt700 K, Substellar, ammonia in spectra
  (predicted)